Hervé Guyomard

Catherine Laroche

Chantal Le Mouël

Hl1a ! Sloû 'j;I>

As Documentanon

An economic assessment of the Common Market Organisation for bananas in the European Union

Keywords: banana, European Union, tarift quota, welfare. DOCUMENTATION ÉCONOMIE RURALE RENNES

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ECU per tonne for non traditional ACP bananas and 850 ECU per tonne for other suppliers. 66.5 % of the tariff quota is allocated ta traders on the basis of their past trade in dollar and non-traditional ACP bananas (category a traders); 3.5 % is reserved ta newcomers (category c traders); and 30 % is allocated ta traders on the basis of their past trade in EU and ACP traditional bananas (category b traders). The allocation of lieences to operators is determined on the basis of the quantities of bananas marketed, weighted according to the three marketing activ'ties, i.e., primary import (57 %), secondary import (15 %) and ripening (28 %). In addition, following the 50-ca lied Framework Agreement, part of the tariff quota is divided up into specific national quotas allocated to four Latin American countries. Costa Rica received 23.4 % of the quota, while Colombia received 21.0 %, Nicaragua 3 % and Venezuela 2.0 %4. EU producers are guaranteed a minimum income through a deficiency payment up ta 854 000 tonnes. This quantily is divided between the various EU regional suppliersS, A one-time premium of 1000 green ECU per hectare is paid to EU farmers to cease production. Other elements of the regulation are mainly the setting of common quality and marketing standards for ail bananas, and the creation of a Management Committee.

This regulation represents a compromise solution which does not satisfy many actors. Germany argues that the new policy will increase the priee of bananas on the German market. For the five EU member states which applied the 20 % tariff only, it is also possible that the level of protection will increase under the new regime. Latin American producers also have challenged the CMO arguing that the latter implied a significant 1055 of their market share in the EU and that the system of quotas prevented them from increasing their exports in the future"-Furthermore, the y fear that a restrictive policy in the EU means more bananas on the world market and hence, a decrease in the wortd price.

Following the Uruguay Round Agreement, the market share and the "advantages" reserved to ACP countries have been reduced with respect to the proposai of July 1993. ACP producers now fear Ihey are not competitive on the EU market with respect ta dollar bananas and argue that Lomé Convention commitments are not honoured.

The main purpose of this paper is to compare the EU pre-CMO banana policy with the CMO regime in order 10 analyse the various effects of the CMO and ta determine "the likely losers and winners". We quantify the effects of the new EU regulation on world and EU priees, on the structure of EU imports from Latin America, ACP countries and EU regionai suppliers, and on the pattern of consumption in the various EU Member States. The welfare analysis allows us ta determine the likely lasers and winners from the CMO. The model used is a competitive static partial equilibrium model of the world banana market. The adopted modelling framework follows those of existing models of the world banana trade (Malthews, 1992;Borrell and[START_REF] Borrell | Intemational Economies Department, The W0I1d Bank[END_REF][START_REF] Mallhews | The European Community's banana policy after[END_REF][START_REF] Read | The EC internai banana market: the issues and the dilemma[END_REF]. Relative to these models, the main "originality" of our modelling framework lies in the world market clearing mechanism. More precisely, as the preferenlial access provided by the EU to some exporting countries actually plays an active role in determining the equilibrium world banana priee, both before and after the CMO implementation, these favoured trade flows are explicitly taken into account in the market clearing process.

The paper is organised as follows. In section two, background for the results is provided by a graphical analysis in which the effects of the two regimes (i.e., the pre-CMO and the CMO) are examined. Section three outlines the banana trade model used to evaluate the priee, production, consumption, trade and welfare impacts of the CMO policy. Section four analyses the empirical results and section five concludes.

Graphical analysis i) the pre-CMO regime

The consequences of the pre-CMO banana regime in the EU can be studied with the aid of Figure 1. This representalion is an adaptation and an extension from [START_REF] Noichl | European Community trade regulations for bananas and imports from developing countries[END_REF], and [START_REF] Borrell | EC bananarama 1992, the Sequel, the EC Commission Proposai, WPS 958[END_REF]. Three groups of countries export bananas on the world market: EU regional suppliers (Canary Islands, Crete, French overseas territories and Madeira), favoured suppliers (mainly ACP countries) and non-favoured suppliers (mainly Latin American countries). They are noted x, y and z, respectively. The graphical analysis includes four imporling zones within the EU (denoted a, b, c and d, respectively) and the Rest Of the World (ROW). Panel La. reflects the situation in countries of type a (France, Greece, Portugal and Spain) which provide a preferential access to their favoured suppliers at a fixed priee and use a quota to Iimit imports from other sources. The situation in countries of type b (ltaly and the United Kingdom) is depicted on panel 1.b. These eountries provide a preferential aceess to some ACP suppliers (Somalia in the case of Italy, and Belize, Jamaica, Surinam and the Windward Islands in the ease of the United Kingdom) and proteet their market by a quota on dollar bananas in addition to the 20 % common external tariff. Panel 1.c corresponds to eountries of type c (the three Benelux states, Denmark and Ireland) which apply the 20 % tariff on dollar zone imports and otherwise allow for the unrestricted access of bananas. Panel 1.d. represents the situation in Germany, Austria, Finland and Sweden where ail bananas enter free of duty'. The right panel 1.e represents the supply-demand situation in the Rest Of the World where trade is assumed to be free.

The dollar banana export supply function S, is assumed to be elaslic to reflect the high productivity and the possibility to easily expand the production. The EU domestic export supplY function S, and the favoured nation export suppl Y funetion Sy are assumed to be less elaslie and to start at higher priee levels, e and y, respeelively. S~represents the export supply eurve of supplier j to importing country i and D~represents the import demand eurve of country i from exporting country j.

The free-trade equilibrium corresponds to point F on panel 1.e. where the world priee of bananas equals Pw (FT) . At this priee level, exports from EU regional suppliers and ACP eountries are not competitive with respect to those of the dollar zone. As a resull, ail Member States of the EU import dollar bananas only.

The main effeet of the pre-CMO regime in the EU is to deerease the world priee of bananas by restrieting Community imports from non-favoured exporting eountries z. This result ean be shown as follows. In panel 1.a., eountries of type a import the quantity q; from EU regional suppliers x at the fixed priee p•. At this priee, a preferential aeeess for the quantity q; is also given to sorne ACP eountries y and the domestie shortfall is made up by imports from the dollar zone z. By subtracting the export supply funetions of EU regional suppliers and favoured exporting eountries (S; and S;, respeetively) from the domestie import demand eurve D•, one obtains the import demand curve of eountries of type a from non-favoured exporting eountries z. Il is denoted D;. For a fixed priee p., imports of eountries a from the dollar zone are th us equal to q;. In panel 1.b, the situation is similar for type b eountries exeept that favoured suppliers are ACP eountries only and the fixed priee pb is lower than the guaranteed priee p. applied in type •a eountries. In panel 1.e., the corn mon external tariff t is applied to dollar banana imports. This ean be portrayed graphically as a leftward shift of the related imporl demand eurve from D; to lJ;. As a result, eountries of type e import the quantity q; from the dollar zone and the quantity q; from ACP countries. In panel 1.e. depicting the world banana market situation, the pre-CMO regime in the EU induces a leftward shift of the EU import demand curve from the dollar zone, from D,EU (FT) ta D;u (J). World market equilibrium now occurs at point E. The world priee of bananas decreases from P.JFT) ta Pw(l). As a result, consumption in free-trade countries d increases up ta the level q:. Consumption in countries c decreases (with respect ta the free-trade situation) since the positive effect on demand of a lower world price is assumed ta be more than offset by the negative effect of the tariff. For these countries, the demand price increases from Pw{FT) ta pC. In countries b and a, consumers facing a higher price for bananas reduce their consumption. For these countries, the demand priee increases from Pw{FT) ta pb and p., respectively.

With respect ta the free-trade situation, the net effects of the pre-CMO regime are thus ta lower consumption in ail EU Member States but type d countries; ta raise the retum ta preferential exporters, x and y, which can now compete with Latin American producers z, at least on type a, band c markets; and ta lower total exports of dollar banana suppliers ta the EU.

(insert Figure 1)

H) the CMO regime

The consequences of the CMO policy in the EU are described with the aid of Figure 2. ln arder ta simplify the graphical analysis, we first assume that the tariff quota Q is set at the "historical" level, i.e .. that EU imports from the dollar zone in the CMO situation are just equal ta quantities of dollar bananas imported in the pre-CMO regime a The direct consequence of this assumption is that the world price of bananas does not vary with respect ta the pre-CMO case. In panel 1.e, the EU import demand curve ta the dollar zone is then D z EU and the intersection of this demand curve with the supply curve of dollar producers ta the EU at point E determines the world priee of bananas Pw(l). The domestic price in countries of type d increases up ta Pi because imports are now subject ta the fixed levy T. As a result, consumption decreases. In countries of type c, the impact of the CMO on priees and consumption is ambiguous since the 20 % ad-valorem tariff t is replaced by the fixed levy T. In arder ta facilitate the presentation, we make a second simplifying hypothesis assuming that the domestie priee in these type e countries remains unehanged.

As a result, consumption does not vary. In countries of type b and a, the domestie priee Pi is lower than the support priees pb and pa and consumption inereases.

The graphical analysis also shows that the CMO poliey might change the pattern of imports in eaeh group of eountries a, b, e and d. Panel 2.d shows that ACP eountries are now Iikely to be competitive relative to dollar bananas on the market of type d eountries. Panel 2.e. shows that the pattern of imports is unehanged with respect to the pre-CMO situation in type e eountries. Finally, panels 2.b and 2.a show that the domestie priee deerease on type b and a markets should benefil mainly to dollar bananas. In the pre-CMO situation, the support priees pb and pa aeted to improve the competitive position of favoured exporting countries x and y relative to dollar bananas. By deereasing the domestie priee in type'b and a countries, the CMO tends to deter this competitive position and henee, to induee a deerease in the market share of favoured exporting eountries. However, in panel 2.a, we make a third simplifying hypothesis by assuming that the defieieney payments regime where EU regional suppliers are paid the differenee between the EU market priee and the referenee priee does not change the export levels of these domestie produeers whieh therefore remain at q;. ln sum, the CMO poliey should lead to two opposite effeets on ACP and dollar zone exports to the EU. On the one hand, in previously highly proteeted markets (type a and b eountries), the CMO tends to favour dollar bananas to the detriment of other origins, mainly ACP eountries. In panels 2.a and 2.b, imports from the dollar zone inerease from q; to r: and from q: to r:, respeetively, while imports from ACP eountries deerease from q; to r; and from q~to ,;, respeetively. On the other hand, the opposite effeet is Iikely to occur in previously less proteeted markets. In panel 2.d, imports from ACP eountries inerease from 0 to r: and dollar banana imports deerease from q: to r:.

A tariff quota set at the historieal level Q on Figure 2 illustrates the case where the latter is "dollar banana quantity neutral", Le., the situation where, as regards dollar banana exports to the EU, the deerease in country d imports is exaetly eompensated by the inerease in country a and b imports. Let us now assume that the tariff quota is lower than the historieal level. This case is illustrated by a quota fixed at level 0 in Figure 2, panel 2.e. The EU import demand eurve to the dollar zone is now ÎY:u.

-

,
The EU internai priee of bananas is then P.' ft is higher than the one eorresponding to the historieal quota case Cp; > Pi) while the related world price is lower (pJl) < Pw(l). EU imports from the dollar zone are now lower than in the pre-CMO regime since the increase in dollar banana imports in type a and b countries is outweighed by the decrease in corresponding imports in type c and d countries. In sum, a tariff quota more constraining than the historical quota is less favourable to dollar bananas (Latin American countries suffer from a decrease in their market share in the EU and faee a lower world priee for their exports to other markets) and more favourable to ACP exporting countries (a higher domestic price in the EU improves their competitiveness position relative to dollar bananas on EU markets). The opposite case which corresponds to a quota level set above the historical level would benefit dollar bananas and hurt ACP exporting countries.

At this stage of the analysis, it is interesting to underline that the main change between the July 1993 CMO and the revised version of April 1994 lies in the increase• in the tariff quota level (from 2 million tons to 2.2. million tons for the marketing year 1995, for the EU with twelve Member States) and the decrease in the fixed levy (from 100 green ECU per tonne to 75 ECU per tonne). Clearly, the revised version of the CMO tends to favour the dollar zone to the detriment of ACP exporting countries.

ln sum, and whatever the final status of the global quota (i.e., lower, equal or higher than the historical level), the CMO tends to decrease the domestic price in the previously highly protected markets (inducing a welfare gain for corresponding consumers) and to increase the domestic price in other EU markets (causing a loss for corresponding consumers). The changes are mainly at the cost of type d countries, i.e., Germany and the three new Member States. As regards to the budget impact of the CMO, deficiency payments to EU producers increase budget expenditure while budget revenue is raised since ail imports from the dollar zone are now subject to the fixed levy. The net effect on the EU budget depends on the question which factor outweighs the other.

(insert Figure 2)

A policy simulation model of the world banana market

The simulations reported in this paper are carried out with a single commodity, partial equilibrium model of the world banana market. With respect to the graphical analysis presented in the previous section, the model includes a greater number of demanders and suppliers. They are shown in Table 1.

(insert i) modelling the pre-CMO policy

FOllowing the graphical analysis of section 2, three demand markets in the EU have to be distinguished in the pre-CMO regime: quota-protected markets (type a and b countries), tarif(protected mar1<ets (type c countries) and non-protected markets (type d countries).

The highest support price offered by a type a or b importer i to a type x or y preferred supplier } is denoted p;. This support price is fixed and determines the import price for ail bananas on this quotaprotected market i We have th us p; =p', 'd}. Import demand functions of quota-protected countries may then be wrilten as:

(1) D" = a.(r )""

(2) Db = abrpb )'"

For tariff-protected countries in the EU, import demand functions are:

For free markets in the EU (i.e., Germany, Austria, Finland and Sweden) and for the Rest of the World (denoted e), import demand functions are simply:

(4) D d =adrp:)' Id (5) D' = a.(p; )",
According to the graphical analysis, three groups of exporting countries have to be distinguished on the supply side: EU regional suppliers x, ACP producers y and Latin American countries z.

Let us tirst consider the case of an EU regional supplier j which benetits from an export support priee atlevel px~(i.e.• px;. =p; -<) from various importers i,i El(j). where l(j) represents the subset of type a and b importers i which guarantee a tixed support price to this supplier j. At this stage. it is useful 10 order importing countries i according to the price they offer to supplier j such Ihat px; > px; means thatthe highest-price countries come tirs!. Therefore. if at price level pl. imports of country 1 from supplier j (Le., D;) are greater Ihan export supply of country j (Le.• bJp; -c;lJ). then the shortfall on market 1 will be made up by imports from other preferred suppliers (if any), ACP countries (if they are competitive wilh respect 10 dollar bananas al this price level) and/or Latin American suppliers. On the contrary. if at this price level p;, export supply of country j exceeds import demand of country 1 . the latter will import from this preferred supplier only. The "residual" export supply of country j will be exported to other markets. tirst to other pricesupported markets at decreasing prices P:. .... P;!J). and second to non price-supported markets if country j is competitive with respect to dollar banana suppliers. The export supplY function of an EU regional supplier x in the pre-CMO regime may thus be written as (import priees on non pricesupported markets are also ordered, and we assume that p; -c; < p; -c; < p; -c; )9: Finally, the export supply funclion of dollar zone produeers is simply:

(6) S =b (p-I_CI)', when DI ~SI =b (p-I_CI)', X X 1 C X Je X X X X =b (p-2 _ c 2 )', when DI + D 2 > (SI + S2) =b (p-2 _ c 2 )', > DI x x x x x - x x x x x - x = = bJp
(8) S, = bJpx,)"

The world mar1<et equilibrium equation defines the world FOS priee of bananas, Le.,

where S,, Sy and S, are given by equations ( 6), ( 7) and ( 8), respeetively.

The model simultaneously determines the world FOS priee of bananas (equalion (9», the import CIF priees of bananas in type e, d and e importing eountries (via margin equations), the exported quantity by eaeh supplier j (equations (6) to (8», and the imported quantity by eaeh demander i (equations

(1) to (5».

ii) modelling the CMO poliey

We only eonsider the case where the global quota in the EU on dollar banana imports is binding (cf. Under the assumption that the defieieney payment is perceived as eoupled and that il exaetly offsets the support priee deerease, export supply funetions of EU regional suppliers x may be written as" :

(12)S = b (p-I -c')" when D' > Si =b (/'5 1 -Cl)', 3. Although most recent data related to the pre-CMO situation (i.e., 1992) are available, a 1989-1991 average has been chosen for one reason. 1992 figures clearly show that dollar banana imports in the EU increased substantially in 1992 (cf. Table 6 below). Even if the German Unification may explain a part of this increase, some observers have raised the question of whether the dollar ban ana supplies flooded the market in 1992, as it is generally expected when a quota is to be introduced (EuroPA & [START_REF] Europa | Commentary and analysis on Bananarama III, Paper prepared for the CBEA[END_REF][START_REF] Rastoin | Le marché mondial de la banane: Globalisation ou fragmentation. Document de travail présenté au colloque SFER-Unigrains-CIRAD "Globalisalion des économies agricoles et alimentaires. Situation et perspectives[END_REF]. In fact, if import licences are allocated on the basis of past quantities, each importer has an incentive to import more in order to stake claim to future quota rents. Therefore, as 1992 data cannot be considered as "representative" of the pre-CMO market situation due to this "speculative" trade, the 1989-1991 average has been chosen for the base period. However, it is important to note that ail empirical results depend on the choice of the base period. As shown in Figure 2, the impact of the CMO implementation is closely related to the status of the global quota level, Le., lower, equal or higher than the EU dollar banana imports in the pre-CMO situation.

(insert Table 3)

ii) priee e/asticities Supply and demand elasticities used in the simulation exercises are shown in Table 4. Foilowing [START_REF] Borrell | EC bananarama 1992, the Sequel, the EC Commission Proposai, WPS 958[END_REF], the priee elasticity is set at 1.0 for EU regionaJ suppliers and ACP countries while it is set at 2.0 for dollar zone producers. The responsiveness of export supply to priees in the dollar zone is thus assumed to be very high, mainly because i) plantations do not operate at the limit of output capacity, ii) the proportion of fruit rejected on quality grounds can be varied within limits, and i1i) the banana vessels on voyages to export ports in Central and South America can usually make up shortfalls which occur in any one location from adjacent sources of supply [START_REF] Fao | The worfd banana economy 1970-1984. Economie and Social Development Paper 57[END_REF]). In the case of EU regional and ACP suppliers, the availability of land is not sa great and supply elasticities have been set at ha If those of dollar zone producers. Import demand elaslicities range between -0.3 for the Rest of the World ta -1.0 for Italy and the United Kingdom. For some EU Member States (Denmark, France, Germany, Italy and the United Kingdom), it has been possible ta estimate econometrically import price-demand elasticilies. For other countries, elasticity eslimates are derived from the literature. The FAO study [START_REF] Fao | The worfd banana economy 1970-1984. Economie and Social Development Paper 57[END_REF]) estimated priee elaslicities of demand at retail ranging from -0.11 (United States) ta -0.84 (the Netherlands) with a weighted priee elaslicity for the countries analysed in the study of -0.30. Islam and Subramaniam (1988) found priee elaslicities of import demand ranging between -0.3 and -0.4. The Overseas Development Institute (Davenport and Page, 1991) and [START_REF] Kersten | The worfd banana market in transition: a spatial OP rnodel analysis[END_REF] used an EU demand elasticity of -0.5 while Matthews used a slighlly lower value of -0.4.

(insert Table 4)

Simulating the EU Common Market Organisation for bananas

The model was first run to generate base estimates of priee, net export and net import levels under the pre-CMO policy for the base period (baseline scenario). A second run of the model was made ta generate the levels of the same endogenous variables if the CMO policy was enacted (CMO scenario). Welfare changes for producers and consumers may then be calculated by using these simulation results.

The baseline scenario represents an attempt to structurally duplicate the base period priee and quantity data observed under the base period pre-CMO policy regime. In order to save space, the results of this tirst scenario are not presented. They are available from the authors upon request. The important point to note is that the baseline scenario solutions are within Jess than two percent of observed values in the base period. Accordingly. the empirical results of the CMO scenario are directly compared to the observed data for the base period. Rather than going through the details of the various effects of the CMO policy, empirical results will be mainly analysed following the framewor1< of the graphical analysis presented in section 2. Detailed simulation results are shown in Table 5. Panel a depicts the impact of the CMO upon EU Member States, panel b describes the effects upon ACP exporting countries and EU regional suppliers, and panel c shows the impact upon dollar zone producers and the Rest of the Wor1d market.

(insert Table 5)

Il is tirst of interest to analyse the effects of the CMO on the EU market as a whole. The tariff quota of 2.553 million tonnes on dollar and non-traditional ACP bananas would lead the EU banana mar1<et to expand by 57 545 tonnes and average priee in the EU to increase by 5 ECU, total imports increasing by around 1.55 % compared to the base period and the CIF common priee being 1.00 % higher than the average priee in the base period.

However, distribution of import and priee changes vary significantly between Member States.

Consumers in type c and d countries would suffer from a substantial welfare loss to the extent that the common priee under the CMO policy is expected to be mueh greater than corresponding pre-CMO priees. In type c countries. the priee is estimated to inerease by 45.2 % and consumers suffer from the adoption of the regime by around 34 million ECU at ail. The principal loser would be the type d countries, in particular Germany, with the new policy causing priees to increase by 16.9 % and eonsumers' welfare to decrease by 91 million ECU. One type b country, namely lIaly, would also suffer from the CMO policy by around 15 million ECU. This result may be explained as follows. The objective of the pre-CMO policy in Italy was to proteet the market for Somalian exports, but the lallers share was small (6.7 % in the base period) and the bulk of the mar1<et was supplied by dollar bananas. As a result, the pre-CMO CIF import priee in Italy was much lower than in the other type b country (i.e., the United Kingdom) or in type a countries, and the CMO policy leads thus to an increase in the Italian price. As expected, the estimated changes in banana imports in the United Kingdom and in type a countries are positive. As a resull, consumers' welfare increases in these Member States. The principal beneficiary would be France where priees decrease by around 20.9 %, imports increase by 18.3 % and consumers' surplus expands by 79 million ECU. On balance, there would be a small decrease in EU consumers' surplus of 2 million ECU.

The model projects a world FOS priee of bananas of 206.7 ECU per tonne in the dollar zone (+ 0.8 % compared to the base period level). The decrease in ROW imports from the dollar zone is almost negligible (-0.27%). Total exports of dollar bananas increase (from 6.84 million tonnes to 6.95 million tonnes) because the tariff quota in the EU opens imports of dollar bananas more than the total of pre-CMO national policies : the level of 2.553 million tonnes is greater than base period imports of dollar bananas in the EU (2.429 million tonnes). With respect to Figure 2, the analysed situation corresponds thus to the case where the tariff quota is set at a lever higher than Q . Nevertheless, the quota constraint is assumed to be binding ". Il is c1ear that the over-quota tariff is prohibitive and hence, that over-quota imports equal zero.

Secause compensation to EU producers applies to a maximum quantity of bananas much higher than pre-CMO exports of EU regional suppliers (854 000 tonnes to compare with the highest level of shipment over the past ten years, 659 836 tonnes in 1992), EU producers are guaranteed "effective" priees (i.e., priee plus direct aid) similar to what they received in the pre-CMO regime. The quantity exported to the EU by community regional suppliers is almost exactly equal to the quantity exporled in the base period.

The CMO allows traditional ACP bananas to enter the EU duly free up ta 857 700 tonnes. Simulation results presented in table S, panel b, show that ACP countries would not be able to totally exhaust this contingent, traditional ACP exports decreasing from 585 600 tonnes to 518 686 tonnes. In that particular case, the choice of 1991 or 1992 as the base year would not change this conclusion to the ex1ent that observed values for these years are still much lower than the contingent limit (see Table 6).

The new EU banana regime is based on a tariff quota for dollar and non-traditional ACP bananas and on the concept of partnership between trade in EU and ACP bananas on the one hand and dollar bananas on the other hand (European Commission, 1994). Il is generally recognised that ACP countries are less efficient than Latin American suppliers [START_REF] Hallam | Fair trade in bananas ? Agricultural Economies Unit[END_REF].

Furthermore, dollar bananas are commonly perceived as being of better and more regular qua lity than bananas from other sources. A fixed percentage (30 %) of the tariff quota is thus allocated to traders (category b traders) on the basis of their past trade in EU and ACP traditional bananas in order to keep the trade in these bananas alive. Our simulation results suggest that some ACP countries would have difficullies to take full advantage of the new EU regulation in penetrating EU markets, and particularty type c and d country's markets where pre-CMO import shares of ACP producers were almost negligible. This is confirmed, at least partially, by figures shown in Table 6 although traditional ACP banana exports to the EU have considerably increased in 1993 and 1994 compared with the base period levels (+ 203 211 tonnes' and + 167 102 tonnes, respectively), the order of magnitude of increases being much lower if 1992 is used as the comparison year (+ 20 287 tonnes from 1992 to 1994, i.e., + 2.9 %).

(insert Table 6)

Empirical results presented in Table 5 do not take into account the increase in productivity and production observed in some ACP countries after the implementation of the CMO. According to the [START_REF] Fao | The worfd banana economy 1970-1984. Economie and Social Development Paper 57[END_REF], new investments done in 1992 would allow sorne ACP countries (Belize, Cameroon, and Ivory Coast) to fulfil their share of the global ACP contingent. This result may also be explained by complementary factors. First, some ACP producers may have used receipts on selling licences as a "coupled" transfer which has increased the profitability of their exports with respect to dollar bananas on the EU market. Second, the devaluation of the CFA Franc has c1earty improved the competitiveness of some ACP countries, Ivory Coast and Cameroon in particular. The consequences of a productivity increase in some ACP countries may be analysed by reducing exogenously production costs in these countries, i.e., by increasing their relative efficiency with respect to dollar bananas (and hence, also with respect to other ACP countries and EU suppliers). In the model, this possibility is implemented by adding an exogenous shifter in supply equations for the three considered ACP countries 50 that Belize, Cameroon and Ivory Coast could exhaust their contingent share. Results of this scenario, called "CMû scenario + productivity increase in some ACP countries•, are shown in Table 7.

(insert Table 7)

The single EU banana price is eslimated to be 5.4 % lower in the "modified" CMO scenario than in the "initial" CMO scenario (527.6 ECU per tonne and 557.5 ECU per tonne, respectively), while EU consumers' welfare gains are now positive (114 million ECU). Total EU banana imports increase by 134 000 tonnes with respect to the corresponding import levels obtained in the "initial" CMO scenario.

By comparing columns 2 and 3 of Table 7, one easily verifies that total EU imports from ACP countries increase by a smaller amount than export increases of Belize, Cameroon and Ivory Coast.

This result is due to the fact that other ACP countries face now a relative competitive disadvantage compared with the three ACP countries where productivity increases. As a result, exports of other ACP countries diminish in the "modified" CMO scenario with respect to the "initial" CMO scenario.

Conclu ding comments

Along with the realisation of the Single Market, the EU introduced a Common Market Organisation for bananas which suppresses the disparate national policies applied in the pre-CMO regime. The purpose of this paper was then to analyse the principles of the new EU banana regulation and its consequences on prices, imports, exports, and consumers' and producers' welfare.

Background for the analysis is provided by a graphical analysis which clearly shows the key roles played by i) the size of the tariff quota on dollar and tradilional ACP bananas and ii) the capacity of the various ACP countries to exhaust their share of the traditional ACP contingent. The empirical analysis uses a single commodity, parlial equilibrium model of the world banana market. Simulation results suggest that if the quota set at 2.553 million tonnes (for the EU with fifteen Member States) is entirely used, consumption in the EU would increase (with respect to 1989-1991 base period data).

However, the impact of the CMO on EU consumers varies in each Member State according to the national policy applied in the pre-CMO situation, the previously highly protected markets benefiting Simulation results show then that ACP countries' exports to the EU would increase compared with pre-CMO levels. In that case, EU consumers' welfare gain would increase by 116 million ECU with respect to its level in the "initial" CMO scenario. This second scenario highlights the importance of the EU proposai to provide direct aid to ACP countries in order to improve the competitiveness and the quality of ACP bananas with respect to dollar fruits. This proposai has not been yet ratified by the EU Council.

Obviously, the analysis incorporates certain simplifying assumptions and empirical results are subject to several caveats.

To our knowledge, ail the models of the world banana market have assumed perfect competition, essentially because of the tractability of this hypothesis. [START_REF] Hallam | Fair trade in bananas ? Agricultural Economies Unit[END_REF] and [START_REF] Read | The EC internai banana market: the issues and the dilemma[END_REF] take issue with the [START_REF] Borrell | EC bananarama 1992, the Sequel, the EC Commission Proposai, WPS 958[END_REF] estimates of the effects of different scenarios for the EU banana regime arguing that results may be biased because the Borrell and Yang model does not consider the effects of market structure and market power. Unfortunately. they do not provide an alternative model in which perfect competition assumptions would be relaxed. Incorporating imperfect competition into international trade models is a formidable challenge. Even if the importance of capturing market structure in trade policy modelling is now weil recognised. ail empirical work on industrial organisation and trade suffers from the difficulty in modelling oligopoly in a satisfactory way (on this point, see. for example. [START_REF] Mccorriston | Incorporating industrial organization into agricultural trade modelling[END_REF], who discuss the problem of using conjectural variations to measure oligopolistic interactions). The question arises now as to how our results may differ when perfect competition assurnptions are relaxed. On this point, it is particularly important to note that a "non competitive" behaviour of importers has no impact on the EU's banana import market equilibrium under the condition that the tariff quota is constraining. Empirical evidence suggests that the tariff quota is fully used.

The paper does not address the question of the quota rent and of its allocation between exporting countries, traders and importing countries. Il also does not address the problem of licence transfer between operators. The tariff quota is the main policy instrument of the new EU regulation. A pertinent analysis of the quota rent problem does need a careful modelling of ail the operators involved in the banana industry and of ail the aspects of the market structure. Clearly, the market structure is here the crucial point which is important ta correctly represent in arder ta obtain consistent estimates of the rent sharing. There is an important need for further research in this area. The type of an Importer 1 or an exporter .J IS deflned on the basls of ItS status ln the pre-CMO regime (see section 2 of the paper).

Table 2. Notations and variable definitions

Di the quanlity demanded by importer i , (i = a,b,c,d,e,) pi the import CIF priee in country i , l, the ad•valorem tariff applied by country i r. the fixed levy applied by country i , 1'], the priee elasticity of demand in country i , ai the constant parameter of the import demand function for country i Sj the quantity supplied by exporter j, px) the export FOS priee in country j, (j =x,y,z), c) the priee elasticity of supply in country j, b) the constant parameter of the supply function for country j, C; the cost coefficient between imports of country i and exports of country j, i.e., p' = pX j + C;, ' P; the support (import GIF) priee offered by importer i to exporter j, p the common GIF import priee in the GMO regime on EU markets, D; the quantity imported by country i from supplier J, D; the quantity imported by country i from supplier j when the latter benefits from a priee support from country i , S; the quantity exported by supplier j ta country i Q the tariff quota on dollar and non traditional AGP bananas in the GMO regime.

.

  ;-c;)'• when "'[.D>D; ~"'[.S~+S;=bJp;-c;)" ~"'[.D: (p: -c:)" when '\' S' +Sc +Sd +S' = b ( '-c')" ~'\' D' +D c +D d Export supply functions of ACP countries are obtained in a similar way10: ( 7) S =b (p-I_CI)". when DI ~SI =b (p-I_CI)" Y Y Y ) ' . V . l ' Y ) ' ) , = = by(p; -C;l' when LS~+S; +S: +S; =bJp; -C;lx ~LD; +D; +D:

  note 8). The export suppl Y funetion of dollar zone produeers is still given by (8) and the import demand funetion of the Rest of the World remains (5). The supply demand equilibrium equation on non-EU mar1<ets defines then the world FOS priee of dollar bananas, Le., (10) D' +Q = S,, i.e., a e ( pX z + c; r• + Q = b z ( pxzr• Imporl demand funetions in the various EU Member States depend on the eommon demand priee p in the EU, i.e., (11) D' =a,(p)",i=a,b,c,d

=

  =bJp -<l' when '2-D~+ D; ~'2-s~+ s; =b x ( P-c;)', ~'2-D ; :l' when '2-S~+S;+S: =bJp-c;)" ~'2-D;+D; c:/' when LS~=bip-C:/' ~LD,=/The demand-supply equilibrium equation in the EU defines th en the common demand priee p on EU markets, i.e., Value and volume bilateral trade flows (i.e., net exports of supplier j to importer i and net imports of demander i trom exporter j) are based on FAO and EUROSTAT data. FOB and CIF unit values are derived from these volume and value data. Base period data used for the calibration correspond to a 1989-1991 average and are given in Table

  from a price decrease and the North of Europe (Benelux, Denmark and Ireland), Germany and the three new Member States suffering from a substantial price increase. The main 105er would be Germany and the main winner would be France. Consumers welfare gains in previously highly protected markets would be almost 50 high than losses in previously lightly protected countries. As a result, EU consumers as a whole would experience a 2 million ECU welfare loss only (again with respect 10 1989-1991 base period dala). The impact of Ihe CMO on Ihe wor1d FOB price of dollar bananas and on ROW consumers would be almosl negligible.A complemenlary scenario illuslrales Ihe sensilivily of market equilibrium in Ihe EU 10 Ihe abilily of ACP countries 10 exhausllheir ACP conlingent share. According 10 European Commission estimales, three ACP countries (Belize, Cameroon and Ivory Coast) would be able to fulfil their contingent share thanks to new investments done in 1992 and other factors favouring their relative competitiveness.

Table 1 )

 1 3.1. Madel oulline ln very general terms, the model consists in seven demand equations and seven supply equations which are written as constant-elasticity functions. Import CIF prices in importing countries and export FOS priees in exporting countries are Iinked by constant margin equations. The mar1<et-clearing equation guarantees the supply-demand equilibrium on the world mar1<et. Notations are detailed in

	Table 2.
	(insert Tapie 2)

Table 1 .

 1 Importing and exporting zones distinguished in the model

	Importer i		Exporter j
	coun""	type	country	type
	France	a	French overseas	x
			territories
	Greece, Portugal, Spain	a	Canary Islands, Crete,	x
			Madeira
	Italy	b	Somalia	y
	United Kingdom	b	Jamaica, Windward	y
			Islands
	Benelux, Ireland,	c	Ivory Coast, Cameroon	y
	Denmark
	Gerrnany, Austria,	d	Other ACP countries	y
	Finland, Sweden
	Rest Of the World	e	Other countries, i.e.,	z
			dollar zone

Table 3 .

 3 Base period data used for the model calibration importer i

		France	South of	Italy	United	Benelux,	Germany	Austria,	Rest of the	Total	FOB unit
			the EU		Kingdom	Denmark,		Finland.	World		value
	exporter j		(except			Ireland		Sweden
			Italv)
	French overseas territories	291 944	12	261	1 323	41	81	0	0	293662	391
	Canary Islands, Madeira,	45	414 348	1	11	0	480	0	0	414884	560
	Crete
	Cameroon, Ivory Coast	170226	107	7953	2759	290	92	0	0	181 427	249
	Somalia	a	12897	28763	a	124	a	a	a	41 783	264
	Jamaica, Windward Islands	179	7269	23600	276395	105	125	a	a	307672	422
	Other ACP countries	0	122	a	51 697	58	a	2842	0	54718	289
	Other countries	15 325	98566	371 874	60 601	364589	1 162468	355 118	4408705 6837246	205
	Total	477 719	533319	432452	392786	365208	1 163245	357959	4408705
	CIF unit value (20 % taritt on	709	624	521	648	460	477	511	292
	dollar bananas included)
	Note: quantities in tonnes, priees in ECU per tonne,

Table 4 .

 4 lm port demand and export supply elasticities used in the simulation exercises

		Demand	Su lply
	country	elasticitv	country	elasticity
	France	-0.7 (-0.70 [22.08])	French overseas	1
			territories
	South of Europe 3	-0.7
	(Greece, Portugal,		Canary Islands, Crete,	1
	Spain)		Madeira
	Ilaly 1	-1.0 (-1.07 [5.31])	Somalia	1
	United Kingdom 1	-1.0 (-1.09 [5.13])	Jamaica, Windward	1
			Islands
	Benelux, Denmark,	-0.4 (-0.73 [3.53])	Ivory Coast, Cameroon	1
	Ire land 1.2
	Germany, Austria j Finland, Sweden	-0.4	Other ACP countries	1
	Rest Of the World 3	-0.3	Other countries, Le.,	2
			dollar oroducers
	1 ln brackets, econometnc estlmates wlth the assoclated t -Student.
	2 Estimation for Denmark only.
	3 Assumed.

Table 5 .

 5 Main effeets of the Common Market Organisation for bananas in the European Union (tariff quota of 2.553 million tonnes, EU with fifteen Member States) Panel a. Impact upon EU importing countries Impact upon dollar banana producers and the Rest Of the World

	base data		simulation results
	imports	CIF import	imports	CIF common	welfare
	(tonnes)	average priee	(tonnes)	priee in the	change
		(ine. tariff)		EU (ine. tariff) (million ECU)
		(ECUltonne)		(ECU/tonne)'

Panel c.

Table 7 .

 7 Impact of a productivity increase in some ACP countries on CMO scenario simulation CMO scenario + productivity increase in sorne ACP countries, i.e., Belize, Cameroon and Ivory Coast.

		results
			base year	"initial" CMO	"modified"
			(average	scenario	CMO
			1989-90-91 )		scenario(1)
		exports (in million tonnes) to the EU trom
		the dollar zone	2.429	2.553	2.553
		ACP countries	0.586	0.519	0.653
		Ivory Coast, Cameroon	0.181	0.144	0.310
		Jamaica and Windward Islands	0.308	0.244	0.222
		Somalia	0.042	0.897	0.052
		other ACP countries (inciuding Belize)	0.055	0.073	0.067
		total EU imports (in million tonnes)	3.723	3.780	3.914
		EU consumers' weltare change (million ECU)	-	-2	+ 114
		w.r.t. the base year
		CIF import price in the EU (ECU/tonne)	552	557.5	527
		FOB export price trom the dollar zone	205	206.7	206.7
	1	(ECU/tonne)
		(1)

 3).

(1) The CMO came into effect on 1 July 1993. Imports from the dollar zone were equal to 1.18 million tonnes for the first semester of 1993; the quota for the second semester was set at 1.1 million tonnes but imports were slightly lower (966 000 tonnes) than this quantity.

(2) The tariff quota is set at 2.1 million tonnes for 1994 and 2.2 million tonnes for 1995 for the EU with 12 Member States. •t -l'

, " , , , , , ,"

" r q: q: q; qtl q; q: ql> q; q: q' q, ---T--!t1t~\~~~~~~~~-::---S:

DE"

, This regime was the one applied in Austria, Finland and Sweden as weil before they join the European Union.

2 Protocol 5 of the Lomé Convention states that ..... no ACP country shall be placed, as regards access to its traditional markets and its advantages on those markets, in a less favourable situation than in the past or at present...

3 Ivory Coast 155 000 tonnes, Cameroon 155 000 tonnes, St. Lucia 127 000 tonnes, Jamaica 105 000 tonnes, St. Vincent and the Grenadine 82 000 tonnes, Dominica 71 000 tonnes, Somalia 60 000 tonnes, Belize 40 000 tonnes, Surinam 38 000 tonnes, Grenada 14 000 tonnes, Madagascar 5 900 tonnes, and Cape Verde 4 800 tonnes.

4 ln return, these four suppliers have agreed to take no further action on a GATT panel which found against the EU banana regime.

5 Canary Islands 420 000 tonnes, Guadeloupe 150 000 tonnes, Martinique 219 000 tonnes, Madeira 50 000 tonnes, and Crete 15 000 tonnes.

6 The CMO is also of interest for the US mainly because several of the major suppliers of Latin American bananas are US multinational companies.

7 Austria, Finland and Sweden became EU Member States during the CMO implementation lime period. Accordingly, the analysis considers the EU with 15 Member States in both the pre-CMO and the CMO regime.

8 We will relax this assumption in the empirical analysis since evidence suggests that the tariff quota on dollar bananas is binding.

10 Obviously P~= 15', 'di, and p; = pk, 'dk = c,d,e.

" ln order to simplify the presentation, we assume the same priee order as in the pre-CMO regime, 12 As noted before, empirical results depend on the choice of the base period for comparison. If the model was calibrated with 1991 or 1992 data, pre-CMO imports in the EU from the dollar zone wou Id be higher than the tariff quota. The choice of 1991 or 1992 as base year corresponds to a quota set at the level Qin Figure 2.


Jean-François Baroiller

Alexis Fostier

Chantal Cauty

Xavier X Rognon

Bernard Jalabert

Effects of high rearing temperatures on the sex ratio of progeny from sex reversed males of Oreochromis niloticus

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.


Aboubacar Jean-François Baroiller

Comparative effects of a natural androgen, 11β hydroxyandrostenedione, and a synthetic androgen, 17α-methyltestosterone, on the sex ratios of Oreochromis niloticus


Basis of the sexual and territorial behavior in males of Oreochromis niloticus and Oreochromis mossambicus

Yves Rouger

To cite this version:

Yves Rouger. Basis of the sexual and territorial behavior in males of Oreochromis niloticus and Oreochromis mossambicus. Le troisième symposium international sur le tilapia en aquaculture, 41, ICLARM, 630 p., 1996, ICLARM Conference Proceedings, 971-8709-88-6. hal-02842693


Y Rouger

Basis of the Sexual and Territorial Behavior in Males of Oreochromls nllotlcus and Oreochromls mossamblcus

This study compared the social hlerarchy of three groups of five males of two tilapia species:

Oreochromls nlloticus and Oreochromls mossambicus. Measurements of clrculating steroid (testosterone, 11-KT and 17 .20P) showed hlgh testosterone levels in dominant males of O. niloticus, whereas no slgniflcant dlfference was observed in the levels of testosterone between dominant and domlnated males of O. mossambicus. ln O. niloticus, highly aggress.ive and territorial behaviors are correlated whereas O. mossambicus has low endocrine levels and does not have a highly hierarchlcal social behavior. ln O. niloticus, the color of the dominant male ls testosterone-dependent and has an appeasing function: pre-spawning females adopt this color after a sharp lncrease ln testosterone and are thus able to enter the nest guarded by the male.

The use of dlfferent sterold Implants confirms the rote of testosterone in the aggresslve behavîor of the tilapla. allowing some fish to rise in the social hierarchy. The effec.ts. of t t •KT are more obvious for nest-digging activities than for chasing behavlor.

Introduction

Previous studîes have shown a correlation between gonadal cydes, circulating steroid levels and sexual behavîor in many fish, especially in salmonîds [START_REF] Liley | Hormones. pheromones and reproductive behavlor ln flsh[END_REF].

ln clchlids, [START_REF] Aronson | Influence of the stimuli provided by the male cichlid flsh Tilapiâ milcrocephala on the spawnlng frequency of the female[END_REF][START_REF] Aronson | Factors influencing the spawning frequen<::y in the female cichlid fü;h Tllapla macrocepha/a[END_REF] desc.ribed the role of externat stimuli (visual stimuli, sound emîsslon, contact between the lateral lines and chemîcal communication) produced by other mates on spawning frequency. In Oreochromis mossamblcus, Silverman ( 1978aSilverman ( , 1978b) ) suggested an effect comparable to pheromones as shown by Salomon ( 1977) in other species. ln O. mossam~ bleus, males as well as females can increase spawning frequency.

This study compares the social behavior and the endocrine levels of two species of tilapia: Oreochromis nl/oticus and O. mossambicus.

Materlals and Methods

Flsh

The fish used in the experiments were O. niloticus from a laboratory straln originally coming from Bouake (Côte d'ivoire). Ali animais came from the same culture batch and were one year old. They measured 14.5±1.29 cm and weighed 96.'.h3.5 g (mean body weight). Experiments were performed every year between April and September from 1989 to 1992.

for O. mossamblcus, the animais came from a strain developed in Louvain-la-Neuve (Belgium) by Ursula Falter. ln this experiment, 10 females of O. mossamblcus and eight females of O. nHotlcus were used as well as t 5 males of each species.

Culture Conditions

Outside experlmental periods, males and females were separated and placed in tanks containing 0.5 m' recirculated water kept at 2 7°C. These tanks contained only tilapias. During experimental periods, the animais were placed in 300-1 glass tanks containing adsorbing sand at the bottom and were transferred during tests to an 800-1 tank also containing adsorbing sand at the bottom.

Experlmenta/ Protoco/s and Ethograms

Males were dlvided into groups of five animais. An ethogram was used to establish the social hierarchy for each of these groups and to visualize the social relationships within the group. For easy identification, the animais were tagged by fixing a nylon string with different colored beads formlng a determined pattern on the posterior part of the head. The fish were anesthetlzed and a blood test was done before and after a series of behavioral tests conducted over a five-day period to evaluate their social hierarchy. These tests were triplicated for each groups.

On the ethogram where the five fish are represented in a circle (see fig. f), the dominant male, i.e., that which directs the greatest number of activities towards its conspecifics, is placed at the top. The dominated animal, i.e., that which sustains the greatest number of aggressive actions wîthout reciprocation, is placed at the bottom. lnterac-tions between animais are represented by arrows oriented in the direction of the action. The thicker the line, the hlgher the number of aggressive actions between fish. The number of fish is indicated in drdes.

Social and Sexua/ Behal'/or

The different elements of the social and sexual behavior were recorded during a series of 10-minute tests every hour, eight times a day. This behavlor was characterlzed by pursuits, mouthto-mouth contacts, mouth-to-flank contacts, parallel swlmming, erectlon of the dorsal fin and nest-digging with removal of sand.

E.ach group studied was composed of five males of slmilar size and body weight.

B/ood Tests

The animais were anesthetized individually wlth a solution of phenoxyethanol at 2 ml• l-1 of water poured in a t 0-1 water tank. Blood was taken ( tt.5 ml) between 0900 and t 1 OO hours using a 2-ml heparinized syringe, and preserved in an ice box before centrifugation of ail samples in a refrigerated centrifuge (4°C} for 10 minutes at 2,500 revolutions per minute.

Hormone Assay

Hormone concentrations were determined by radio-immunology. Testosterone and 11-KT were measured using the method developed by fostler et al. (1982). giving a coefficient of variation of 8. t 2% ( 18.4 ng•ml-1 , n= t 5) for testosterone and 6. 3 7% ( t 7 .5 ng•m1• 1 , n= t 5) for 11-KT. The method described by fostler et al. ( t 981} was used to measure 1 7 .2P with a coefficient of variation of S. 74% (9.25 ng, n= 15).

Statlstlcal Analysls

Paired comparison was done after an analysis of variance and Barlett's test. Data were compared using a t-test. Unless otherwise indkated, the level of significance was P=0.05.

Results

Ethogr11ms oF the Male Graups

The study of the three groups of flve males clearly showed the presence of a dominant male in two groups, whereas social relationships in the third group were much more homogeneous although a dominant male could be recognized. Interactions between the fish also showed a second dominant animal. This was confirmed by the withdrawal of the dominant animal. Fig. t shows three diagrams summarizing ail activities directed to the other fish within the groups: pursuits, mouth-to-flank and mouth-tomouth contacts, and erection of the dorsal fin when approaching another fish. mossambicus, no significant difference was observed. The same differences were observed, although somewhat reduced,

Hormone levels and

-E o. c:
., in the levels of the two other steroids (11-KT and 17.20P).

Hormone LeJ'e/s and Sound Emission ln Males

ln a collaborative study with U. Falter and Olivier Dufayt from the University of Louvain-la-Neuve (Belgium). we have for the first time established a correlation between sound emissions in males of these two species and other behavior patterns.

The male defending its territory chases the other fish (other males or females that are not ready to spawn) and emits a brief sound for less than half a sec-ond. The frequency of this sound emission is high: 400-700 Hz in O. niloticus and approximately 250 Hz in O. mossamblcus.

These emissions which are charactetistics in dominant males are, like in the defense of the territory, dependent on plasma testosterone. Only the dominant males can emit these sounds.

Hormone Lel'els and Acceptance oF the Female by Territorial Males

ln O. nllot/cus, the color taken by the pre-spawning females (which is identical to that of the dominant male) is related to the high level of aggressive behavior in this species. This color pattern inhibits the aggressive behavior of the male that accepts the female of the same col or. We have observed that some males with strong color patterns are sometimes accepted in the nest of the territorial male. The analysis of the female hormone levels (Fig. 3) showed that the change in color is correlated to an important testosterone level, particularly when the female is ready to spawn. ln O. mossambicus, although the same high testosterone level has been observed in the pre-spawning females, changes in the color pattern cannot be perceived by the human eye. The other steroids, circulating at relatively low levels, involve very few changes regardless of the species.

EFFect of Sterold Implants on the Social Behavlor of O. nllotlcus

In order to complete our study and to confirm the action of steroids on the social behavior of fish, we used implants with various quantities of several steroids in the flsh and studied their effect by observing the corresponding ethogram. Three animais out of a group of five received steroid implants while the two others were used as control.

First, we studied the dynamics of release of pure testosterone from the implants at O. 5 mg• t OO mg-1 implant 50 m1-1 saline solution. Between 6 and 36 hours, the quantity of hormone released was relatively stable for 1. 75± 0.25 ng•ml-1 • The animais receiving testosterone implants showed increased aggressiveness, thereby rising in the social hierarchy (Fig. 4), the number of pursuits increasing four hours after the implantation of steroids. The nest-digging behavior did not vary significantly in the three days following the implantation (Fig. 5).

411

The controls' aggressive behavior decreased signlficantly in terms of pursuits and nest-digging activities. This reduced activity can be explained by increased activity and aggresslveness of the animais with implants living with them (Fig. 6).

There was also a significant increase in nest-digging activities in the animais with implants of t 1-KT after implantation (Fig. 7). ln contrast, the implantation of 17.ZOP did not cause any significant change in behavior aside from a general reduced level of activity (Fig. 8).

Conclusion

These results show the significant role of testosterone in the territorial behavior of males and in the species' level of aggressiveness. The sounds emitted by dominant males during pursuits against conspeciflcs, or to accompany and guide the female ready to spawn to the nest, are testosterone-dependent. This hormone is also responsible for the change in color pattern ln the dominant male. This phenomenon is also observed in pre-spawning females which can thus enter the nest. The adoption of this malespecific color by the female has an lnhlbitory effect on the aggressive behavlor of the territorial male. ln O. mossambicus, the aggressive behavior is practically nonexistent and endocrine levels are also much lower. The female does not need to take on the male-specific color pattern to be accepted by the male.

Ketotestosterone has a more obvious effect on nest-digging actlvlty. Hence, the various hormones have each a specific action on social and reproductive behaviors. However, the steroid t 7 .ZOP does not seem to have any direct action on male behavior when implanted. Yet, studies by [START_REF] Stacey | Function and evolution of fish hormonal pheromones[END_REF] have shown that this steroid may act as a pheromone in goldfish ( Carassius auratus). The different steroids thus appear to have a specific raie in the different sequences of behavior involved in the defense of the territory and in the development of the reproductive behavior.

Fig. t .

 t Fig. t. Ethograms for three groups of five males of Oreochromls niloticus. The numbers ln clrcles indicate individuai flsh. Numbers along arrows lndlcate the numbers of aggresslve actions (see text).

Fig. 2 .

 2 Fig. 2. Sterold hormone levels and social hierarchy in Oreochromis ni/oticus and Q_ mossamb/cus. On the ethograms, * ~dominant. N and M refer to the species used, and D ~ dominated (see text for discussion of comparisons and slgnlflcance of differences).

Fig. 3 .Fig. 4 .

 34 Fig. 3. Comparative study of sterold levels ln female of Oreochromis nlloticus and O. mossamblcus.

Fig. 5 .

 5 Fig. 5. Hfect of implants (testosterone 0.25 mg) on the social behavlor of Oreochromis nl/otkus.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Effect of implants (control) on the social.behavlor of Oreochromis ni/otlcus.

Fig. 8 .

 8 Fig. 8. Effect of implants ( 1 7 .ZOP 0.25 mg) on the social behavlor of Oreochromls nllotlcus.


J F Baroillf

Toguyeni

Comparative Effects of a Natural Androgen, 11 B-Hydroxyandrostenedione, and a Synthetic Androgen, 17a-Methyltestosterone, on the Sex Ratios of Oreochromls nllotlcus

Synthetic androgens are often considered to be more potent than natural androgens For sex reversai treatments in fish. Preliminary experiments on the natural androgen. 11 Bhydroxyandrostenedione ( 11 B-OHti.4), recently identified in the gonads of fry of O. nllot/cus ln the early stages of testicular ontogenesis, showed a high masculinizing potential. Fry of 10-14 days post-fertilization (dPF) were produced from single pair matings using untreated females and either normal or sex reversed males, to assess any significant deviation in sex ratio compared to the controls. The fry were then treated during a minimum perlod of 21 days either wlth the natural or synthetic androgen added to the feed at different concentrations. Complete sex reversai was achleved with both steroids. Ali-male populations were produced ln elght groups treated with 1 0-35 µg 1 1 B-OHti.4• g• 1 of feed and in two groups treated with 1 7 u-methyltestosterone ( 17 u-MT) at 5 and 20 µg•g•' of feed, respectlvely. There was no significant difference in the potency of the two androgens for concentrations higher or equal to 10 µg•g••. However. at 5 µg• g••. only 17u-MT produced 100% male populations whereas 11 B-OHti.4 produced 88.9%. ln contrast, the lowest concentration of 1 1 B-OHti.4 ( 1 µg• g• 1 ) significantly deviated sex ratios whereas 17u-MT had no effect compared to the controls.

The natural androgen 11 B-OHti.4 can therefore constltute an alternative treatment to synthetlc sterolds. Low concentration treatments can be optimized by increasing the du ration of treatment.

Introduction

Today, with an annual global production of approximately 500,000 tons [START_REF] Lazard | Transfert de poissons et développement de la production piscicole. Exemple de trois pays d'Afrique sub-saharienne[END_REF]. tilapias ( Oreochro-mis, Sarotherodon and Ti/apla) constitute, with the cyprinids and salmonids, one of the three most important groups of freshwater fish for aquaculture. However, in culture conditions where competition for food is high, the highly efficient reproduction of the species of Oreochromis [START_REF] Baroiller | Contribution of research ln reproductive physiology to the culture of tllaplas[END_REF] and their early sexual maturity lead to overcrowding and stunting, resulting in limited economic yîelds for fish farms. A solution to this problem consists in producing monosex populations. Alimale populations are preferred because theîr growth performance is better compared to females (Pruginin 1967[START_REF] Hickling | Fish hybridizatlon[END_REF][START_REF] Hanson | Growth comparlson of monosex tilapia produced by separation of sexes, hybridization and sex reversai[END_REF].

Currently, two techniques are used to produce monosex populations for fish culture (Baroiller and Jalabert t 989):

Ma.nuai sexing. Based on the sexual dimorphism observed in the urogenital papilla, this technique entails the elimination of ail females, i.e., approximately half of the initial population as soon as possible (after a two to three month nursing period). Manual sexing, which is used in Africa, is time-consuming: it also requires qualified personnel, and includes 3-10% errors (Lazard 1980: Chervinski andRoth bard 1982) .. ln addition, this technique requires rearing a population of fry for two to three months after which half (the females) will be eliminated. Although manual sexing is simple. it is expensive in terms of time and tabor and implies the underutilization of farming infrastructures and lower feed productivity.

Hormonal sex reversai. This technique consists in masculinizing the entire population of fry by adding a steroid to the feed for a short period (Guerrero 1982: Hunter andDonald son 1983;[START_REF] Varadaraj | Masculinization of Oreochromls mossamb/cus by administration of 17a.-methyl-5androsten-3&-17&-dlol through rearing water[END_REF][START_REF] Baroiller | Contribution of research ln reproductive physiology to the culture of tllaplas[END_REF]. Hormonal sex reversai has been commonly used for several decades by a number of tilapia producing countries such as Israel, Taiwan and the Philippines. This technique requires the systematic treatment of every new population of fry. However, the use of hormones in the production of animais for 239 human consumption is still prohibited in many countries (France and the United Kingdom, for example) whlch consider that the treatment and the effects of the synthetic steroid waste products are stîll insufficiently understood, especially their ecological impact. ln tilapias, as in ait teleost fish, no definite physiological proof supports the hypothesis of [START_REF] Yamamoto | Sex differentlation[END_REF] that steroids are the natural inducers of differentiation [START_REF] Adkins-Regan | Hormones and sexual differentiation[END_REF]. The modification of the natural process of sexual differentiation by exogenous steroids could be due to pharmacological effects (Reinboth 1970). ln fact, few studies have investigated early steroidogenesis during the gonadal sex differentiation of gonochoristic flsh (van den Hurk et al. 1982[START_REF] Rothbard | Changes in steroid concentrations during sexual ontogenesis in tilapia[END_REF]Baroiller 1988a and b;[START_REF] Baroiller | Precocious steroidogenesis in the gonads of Oreochromis niloticus du ring and after sexual differentiation[END_REF].

ln Oreochromis niloticus, early steroidogenous potential in male and female gonads has been analyzed during the first three months of their life; this period covers the entire process of testicular and ovarian differentiation (Baroiller et al.. in press). Testosterone can be synthesized by the gonads of both sexes whereas oestradiol is exclusively produced by the ovaries (Baroiller 1988b). ln contrast, certain androgens like t 1 Bhydroxyandrostenedlone ( 1 1 B-OH~4) and adrenosterone prove male sex-specific during the same period (Baroiller 1988a and b) and show masculinizing potentialities (Baroiller 1988b).

Artificial steroids are generally more patent in their masculinizing effects than natural androgens [START_REF] Hunter | Hormonal sex control and lts application to flsh culture[END_REF].

The potency of 17amethyltestosterone ( 1 7a-MT) is attributed to the presence of the 1 7 a.-methyl group which makes its elimination slower than in natural androgen like testosterone (Fagerlund and McBride 1978;[START_REF] Donaldson | Hormonal enhancement of growth[END_REF].

A study of the masculinizlng potential of 11 B-OH~4 and 17a.-MT was conducted in O. niloticus to compare the respective performances of both artificial and natural hormones; to test the hypothesis of a possible role of t l B-0Hà4 in the process of testicular differentiation; and also to study possible alternatives to traditional sex reversai treatments.

Materlals and Methods

Animais Tested

Two types of male broodfish of O. nllot/cus of the "Bouake" strain (Baroiller 1988b) were used for the production of fry families: normal males (XV) and sex reversed males (XX). The latter came from two related families and yielded, for most of them, significant proportions of males in their progenies produced by single pair matings (Baroiller. this vol.).

Breedlna

Single male broodfish were placed in 400-1 aquaria with normal females at a sex ratio of 4: 1. The water in the_breeding aquaria was filtered and maintained at a constant temperature of 27°C. Each animal was identified by a mark inserted in the dorsal muscles. Reproduction was detected at the onset of maternai mouthbrooding behavior which is characterized by a dilation of the mouth. On the first day of incubation, ail other individuals were removed to leave the mouthbrooding female on her own in the breeding aquaria. Five days after hatching. i.e., nine days post-fertilization (dPF). the fry were removed from the mouth. Each brood, identlfled by the fertilization date and the marks on the respective parents, was divided into two to five batches of at least 1 OO fry reared separately in 2.00-1 aquaria.

Hormone Treatment

Steroids were administered through the feed. A first feeding salmonid feed (Aqualim) was impregnated with an alcohol solution contalning steroids. Concentrations of 1 to 45 ug of steroids per gram of feed were tested. For the contrai batches, the same preparation was applied except that the feed did not contain any steroid.

Fry were fed ad libitum six times dally using an automatic feeder during the 12-hour photoperiod, seven days a week.

Fry at 10-15 dPF were treated th us for 45 and 21 days. To avoid a poten• tial masculinizing effect from the tem• perature (Baroiller et al., in press), the water, which was filtered and aerated, was maintained at 28± t .5°C.

Nurslng and Sexlng the Fry

At the end of the treatment, fry aider than 31 dPF were transferred to 1.5-m3 outdoor tanks where they were fed the same diet until sexing was possible. At 60-90 dPF, when the histological differentiation of the male and fe• male gonads had already taken place (Baroiller 1988a and b). ail fry were sexed by microscopie examination of the gonad squash (x 125). The presence of previtellogenic (auxocytosis) or vitellogenlc oocytes, and the lobular configuration showed the female and male sexes, respectively.

A x 2 test was used ta compare the sex ratios of the batches that had received treatment and the control batches (a .. Q.05).

Results

After treatment and at the moment of sexing, there was no significant difference in the survival rate of the fry between the contrais and the batches that underwent hormonal sex reversai, regardless of the treatment duration and the concentrations used (Table 1 ).

The microscopie examination of the gonad squashes of fry treated with l 1 B-OH.6.4 ( 1 ,63 t sexed animais) or t 7a-MT (4 t 6 fry) did not show any hermaphrodite characteristics, sterilîty, or structural abnormality. The gonads of the individuals treated with 11 B-OH.6.4 were functional, irrespective of theîr genotype: functional sex reversed male XX and normal males were obtalned and identified by progeny testing at the end of these treatments (Baroiller, this vol.).

Of the 1 7 batches treated with l l B-OHA4, only one batch showed no deviation of sex ratio compared to the controls (Table 2). The progeny of the XY4 male underwent hormonal sex reversai only after treatment 15 dPF whereas the t 3 other families received treatment after 10-14 dPF. Since sex reversai was produced in the four other batches also treated with 5 µg•g• 1 , this concentration did not seem to be the cause of absence of a deviation in sex ratio. This result could instead indicate a critical period of hormonal sensitivîty.

Ali treatments using l t 6-0HA4 on Fry of Iess than 1 5 dPF signîficantly affected the sex ratio towards the male sex compared to the controls, regardless of the concentration used (Table 2). ln contrast, 17u-MT had masculînizing effects only in a range of concentrations between 5 and 45 µg• g-1 •

The 21-day treatment yielded ail-male populations using hormonal concentrations between 10 and 35 µg•g-1 (1 tB-241 OH.6.4) and at 5 µg•g• 1 ( l 7a-MT). With concentrations lower or equal to 5 µg of 118-0H.6.4 per gram of feed administered over a period of 21 days, the percentage of males was proportlonal to the concentration used (Fig. t ).

There was no significant difference in masculinizing potency between the two hormones for concentrations of t 0-45

µg•g• 1 • With concentrations of 5 µg•g• 1
and higher, significant differences were observed in the results of both treatments: using 5 µg•g-1 , only 17u-MT yielded 100% male populations against a maximum of 88% wîth 1 t B•OH.6.4. lnversely, t 7o.-MT was not effective if used at l µg•g-1 , whereas l ts-OH.6.4 used at the same concentration signifîcantly altered the percentage of males compared to the contrais.

Replicates (2-4) were conducted for four different concentrations of 11 B-OH.:\4 (5, 20, 30 and 35 µg•g-1 ) (Table 2). There was no signiflcant difference in sex ratios between repllcates of the same treatment.

Discussion

The hormones used for masculinization of flsh are generally artificial molecules derived from testosterone: 170.-MT, 17u-ethynyltestosterone, dihydrotestosterone acetate and testosterone propionate. These synthetk androgens are considered more patent than natural androgens for the hormonal sex reversai of gonochoristic teleost fish species [START_REF] Hunter | Hormonal sex control and lts application to flsh culture[END_REF].

Many authors have also produced l 00% male tilapia popùlations using these steroids, despite a great heterogenelty ln the experimental conditions: especially concentrations varying from l 0 to 240 µg• g-1 of feed [START_REF] Baroiller | Contribution of research ln reproductive physiology to the culture of tllaplas[END_REF]. The optimum concentrations usually suggested are 30 µg•g-1 for 17a-MT and 60 µg•g-1 for ethynyltestosterone (Pandian and Varadaraj 1987;McGeachin et al. 1987[START_REF] Rothbard | Changes in steroid concentrations during sexual ontogenesis in tilapia[END_REF]Barolller and Jalabert 1989). For O. mossambicus, the minimum concentration for the production of ail-male batches using 17a-MT is 5 µg•g-1 (Pandian and Varadaraj 1987). Other artificial androgens such as 17a-methyl-5androsten-3B-17B-diol [START_REF] Varadaraj | Masculinization of Oreochromls mossamb/cus by administration of 17a.-methyl-5androsten-3&-17&-dlol through rearing water[END_REF] and rnibolerone (Guerrero and Guerrero, this vol.) have also been used.

The present study indicates that in O. niloticus, the minimum concentration of 17a-MT yielding a 100% male population ls also 5 µg• g-1 for a 21-day treatment period; there was no deviation in sex ratio with concentration of

1 µg•g-1.
As the steroidogenesis in the period of gonadal differentiation has not been extensively studied in teleost fish, particularly in tilapia (Baroiller 1988a and b;[START_REF] Baroiller | Precocious steroidogenesis in the gonads of Oreochromis niloticus du ring and after sexual differentiation[END_REF], only a limited number of na.tura.I androgens have been 243 tested as masculinizing agents. Among these, two t t -oxygenated derivatlves, administered through the rearing water and through the feed, respectively, can modify the differentiation process in tilapia: adrenosterone causes a destruction of the ovarian structures in O.

niloticus at a concentration of 5 mg-1-1 [START_REF] Katz | Effects of adrenosterone on gonadal and body growth ln T!lapla n//otlCil (Teleostei, Cichlidae)[END_REF]) and 11 ketotestosterone, at 200 µg•g-1 , inhibits the formation of the ovarian cavity in O. mossambicus but does not prevent the formation of young oocytes that will subsequently degenerate [START_REF] Nakamura | Effects of 11-ketotestosterone 245 on gonadal sex dlfferentlation in Tilapia mossambica[END_REF]). However, considering the high levels used, the hypothesis of toxlc or paradoxical effects cannot be discounted [START_REF] Hunter | Hormonal sex control and lts application to flsh culture[END_REF]. ln O.

niloticus, the rnasculinizing effects of adrenosterone have also been demonstrated after treatrnents through the feed at 45 µg•g• 1 (Baroiller 1988b).

Until then, 11 B-OHLi4 had not been used in hormonal sex reversai treatment in tilapia. This natural androgen shows a masculinlzing potency comparable to that of 1 7 a-MT at concentrations of 10-35 µg• g• 1 • ln addition, deviations of sex ratio are observed at concentrations lower than 5 µg•g• 1 , which is not the case for 17a-MT.

The microscopie examination of the gonads of fry treated for 60-90 days dld not show any abnormality in the course of gonadal ontogenesis. Moreover, the functional reversai of the gonads was demonstrated by the identification of the sex reversed males after progeny testing of the individuals treated in the present study (Baroiller, this vol.).

The gariepinus [START_REF] Van Den Hurk | Effects of 17a-methyltestosterone and 11 &-hydroxyand rostenedione on gonad differentlatlon in the Afrlcan catfish, Clarias gariepinus[END_REF]), 17a-MT significantly bîases the sex ratio towards males (65%) at concentrations of 30 µg•t-1 and towards the female sex at 1 OO µg• 1-1 • ln Oncorhynchus myklss, testosterone derivatives may not be essential for testicular differentiation: testosterone and its 11-oxygenated derivative can be synthesized by the testls only stages beyond those at which 11 B-0Htl4 has been identified (van den Hurk et al. 1982). Furthermore, a treatment using cyproterone ac:etate did not affect the sex ratio of the batches of trout (van den Hurk and van Oordt 1985) and tilapia fry [START_REF] Hopkins | Estrogen sex reversai of T!lapla aurea[END_REF])_ The androstenedione 11-oxygenated derivatives may be involved in some stages of testicular differentiation ln these three species.

ln O. niloticus, response to the hormone treatment is observed during a determined critic:al period_ To be efficient, treatment must begin between nlne and 13 dPF. Beyond this perlod, dlfferentiation seems definitively according to the genotype; from then on, ît could only be affected by ex:ogenous steroid factors.

The 2 1-day treatment is therefore applled between 9-30 dPF. From a histological perspective. at 27°C, oogonia proliferatlon occurs in females between 20-28 dPF and is followed by their flrst meiotic prophase at 28-35 dPF (Barolller 1988a and b). In males, a highly progressive multiplication of somatlc cells and spermatogonia is observed during the same period [START_REF] Baroiller | Precocious steroidogenesis in the gonads of Oreochromis niloticus du ring and after sexual differentiation[END_REF]. Exogenous hormones are therefore administered to the fry before the establishment of the se histological processes.

  Fig. t. E.fflclency of hormonal sex reversai using dlfferent sterolds and different concentrations ln tllapla feed.

Table 1 .

 1 Survival of Oreochromi5 nllotlcus fry according to hormone treatment applled for sex reversai.

	Treatment	Survival	Number	Survival	Number
		after treatment	of batches	at sexing	of batches
		(%)	tested	(%)	te:;ted
	11 B-0Hii4	83.8	15	55.2	17
	17a-MT	85.6	4	56.7	4
	Control	79.6	12	50.2	14

Table 2 .

 2 Sex reversai treatments admlnistered to Oreochromis niloticus fry, the progeny of normal male (XYJ or

	sex rever$ed male	broodfîsh pairs.
	Broodfish
	male type
	and no.
	XYl


Slgnlficant Proportions of Unexpected Males in Progenies from Single Pair Matings with Sibling Sex Reversed Males of Oreocbromls nllotlcus

The profltablllty of tilapia culture ls dlrectly influenced by the efficient control of lts reproduction through artlfklal me.ans. Thus, many studies have focused on gonadal differentlatlon and sex determinatlon. ln Oreochromis niloticus, XX sex reversed males (produced by hormone treatment and analysîs of the sex ratio of their progenles from single pair matlngs) can be used as a reliable tool to analyze both the efficiency of masculinizing treatments and sex determination. Twentytwo broodfish from two sets of sibling sex reversed males were tested through 35 single pair matlngs by microscopie sexing of all 4.546 fry. The 35 crosses ylelded progenies composed of a majority of females, but all-female progenîes were observed in only five reproductions. The other progenles yielded sex ratios slgnificantly different from 1: 1, ranging from 65 to 99% female with an average of 85%. Three unexpected males from these crosses were further tested; their progenîes also lncluded unexpected males. These results cannot be explained by a simple monofactorîal sex determlnlng system. Environmental factors may be involved in the sex determlnlng mechanisms or o. nllot/cus.

Introduction

Oreochromis niloticus is one of the two major cultured specles of tilapias in the world [START_REF] Pull In | Choice of tilapia species for aquaculture[END_REF]. lts ability to reproduce early {Baroîller and jalabert t 989) leads, in c.ontrolled environments, to rapid overpopulation with a tendency to stuntlng. The efficient contrai of tilapia reproduction through artificial means directly influences the profitability of its culture. Sînce growth potential is higher in male tilapias than in females [START_REF] Hanson | Growth comparlsons of monosex tilapia produced by separation of sexes, hybridization and sex reversai[END_REF], three methods are -Current address: CIRAD-EMVT, Laboratoire de physiologie des poissons, INRA, Campus de Beaulieu, 35042 Rennes Cedex, France. usually proposed to produce ail-male populations. These are manual sexing. hybridization, and hormonal sex reversai (see reviews of Guerrero 1982: Lovshin t 982; [START_REF] Wohlfarth | Applled genetlcs of tllapla, 2nd edltlon[END_REF][START_REF] Pandian | Techniques to regulate sex ratio and breeding in tllapia[END_REF]. On the average, manual sexing involves 2. 7-10% error, and also requires the elimination of 50% of the population after two to three months of rearing [START_REF] Baroiller | Contribution of research in reproductive physlology to the culture of tilapias[END_REF]. Because of its efficiency and reliability, hormonal sex reversai is today the most widely used method among the three. However, since the metabollsm and the effects on the envîronment of the degradation products of synthetic androgen are not fully understood in fish, this technique is not authorized in some countries. Hybridization between two parental species of aquaculture interest such as Oreochromis ni/oticus and O. aureus does not generally produce ail-male populations [START_REF] Majumdar | Sex ratios from interspecific crosses within the tilapias[END_REF][START_REF] Wohlfarth | The heredi.ty of sex determination in tilapias[END_REF].

The viability of the YY genotype in O. niloticus, as demonstrated by Baroiller ( 1988b), [START_REF] Baroiller | Contribution of research in reproductive physlology to the culture of tilapias[END_REF] and [START_REF] Scott | The YY supermale ln O. niloticus {l.) and its potential in aquaculture[END_REF]. suggests a fourth, intraspecific approach. ln O. niloticus

where the male is heterogametic, a feminization treatment produces XY sex reversed females which, when crossed with an XY male, produce a new YY genotype. Each progeny from crosses between this homogametic YY male and any homogametic XX female is theoretically an all-male population. However, these inter-and intraspecific crosses sometimes yield sex ratios which cannot be explained by a simple XX/ XY monofactorial sex determining mechanism in O. niloticus. To explain these unexpected sex ratios, three other models have been proposed (see reviews of [START_REF] Avtalion | Sex determining genes ln tllapla: a mode! of genetic recombination emerging from sex ratio results of three generations of diploid gynogenetic Oreochromis aureus[END_REF][START_REF] Mair | Sex reversai. gynogenesis and triploidy in O. niloticus {L.)[END_REF][START_REF] Wohlfarth | The heredi.ty of sex determination in tilapias[END_REF]: a theory combining two alleles from an autosomal locus and two of the three sex chromosomes (Avtalîon and Hammerman 1978;Hammerman and Avtalion 1979): a polygenic mode! [START_REF] Majumdar | Sex ratios from interspecific crosses within the tilapias[END_REF]and finally, a monofactorial sex determination with the possible presence of a rare autosomal gene, epistatic to major genes. However, these three theories do not explain ail the results Found ln the literature.

ln order to analyze sex determination in O. niloticus, two sets of sibling sex reversed males were pr.oduced and tested in single pair matings.

Materlals and Methods

Animais

PRODUCTION OF TWO STOCKS OF SEX REVERS ED MALES (FIG. 1)

A progeny of fry from O. nllotlcus of the "Bouake" strain (Baroiller et al, in press) was produced by crossîng a male with one of the four females placed in the same 400-1 aquarium. The progeny, removed after 10 days post-fertilization (PF), was exposed to a hormonal sex reversai treatment using 1 7a methyltestosterone added to the feed at 30 µg•g-1 (Baroiller and Toguyeni, this vol.). Thirty males from the resulting ail-male population were kept to analyze the sex ratio of their progenies in 400-1 aquaria. Twenty of these males were bred with normal females. Thirty juveniles of each of these progenles were killed to determine the sex ratio of each population. Sexing by gonadal squash was done after 90 days PF when the histological characteristics of differentlation were already established for both sexes (Baroiller 1988a and b). The 13 broodfish with progenies showing no significant difference in sex ratio compared to a 1: 1 population were eliminated (sex ratios ranging from 33.3 to 60% male). The seven other males with all-female progenies (30 females in the sample of 30 juveniles sexed in each progeny) were kept as sex reversed males. One of them was crossed again and its entire progeny was exposed to a hormonal sex reversai treatment using 11 B-OHi.\4 added to the feed at 30 µg•g-1 (Baroiller and Toguyeni, this vol.). The resulting ail-male population was theoretically an exclusively XX sex reversed male population; 60 of them constituted Stock 1 and were identified by a letter. The progeny of one of them, sex reversed male J, was exposed to a hormonal sex reversai treatment using 118-0HL14 at 20 µg•g• 1 (Baroiller and Toguyeni, this vol.). The resulting sex reversed ail-male population constituted Stock 2, each broodfish being identified by a number. Thlrteen "sex reversed male" broodfish from Stock 1 and nine sex reversed males from Stock 2 were crossed in single pair matings with one or several females in 400-1 aquaria. The water in the spawning aquaria was filtered permanently and maintained at 27°C using a thermoregulation device. Each animal was tagged in the dorsal musculature. Reproduction was detected by the onset of maternai mouthbrooding characterized by dilation of the female's mouth. As soon as this particular characteristic was observed, all other individuals were removed to leave the female mouthbrooding in the aquarium. Five days after hatching, i.e., nine days PF, the fry were removed from the mother's mouth. The entire progeny (or a sample of at least 100 fry from the batch), identified by its fertilization date and the respective tags on the parents, were transferred to 200-1 aquaria. The Fry were fed ad libitum six times a day, seven days a week, with a first-feeding salmonid feed (Aqualim) distributed by automatic feeder du ring a 12-hour photoperiod. The water, aerated and filtered, was kept at 28± 1.5°C. After three weeks of rearing in aquaria, when the gonadal differentiation was already well established (Baroiller t 988a and b), fry were transferred to external 1. S-m3 tanks until sexing and were given the same feed. After 60-90 days PF, when the histological characteristics of the male and female differentiation were already established (Baroiller t 988a and b). all fry were sexed by examination of the gonad squash at x125 magnification. The presence of previtellogenic or vitellogenic oocytes and the lobular configuration showed the female and male sexes, respectively. Four sex reversed males from Stock 2 (XX4, XX9, XXt 7 and XX18) were each successively crossed with three or four different females.

Results

The sex ratios of ZZ progenies from sex reversed males were determîned by sexing ail 2,429 fry produced, i.e., approxirnately 110 fish per batch. Four of them, i.e., 18% of the progenies, gave sex ratios of 0: 1 (male:female). ln the 18 other progenies, unexpected males based on a monofactorial sex determînation were produced in proportions rangîng from 1 to 40.5% (Table 1 and Fig. t). Only one of these sex ratios (40.5% male) was not signiflcantly different from a 1: 1 ratio. Compared to the ratio of an "average" synthetic population of 5,299 fry from crosses of 42 normal broodfish of the same spedes ( t .39: t ), the 40.5% male showed a hîghly signlficant difference. The male frequency distribution in these progenies was unimodal, around 0-10% male.

Since the progeny of the sex reversed male is theoretic::ally composed of 100% XX individuals, four of the naturally unexpected males from the XXI sex reversed male (Stock 1) were kept until their sexual maturity to analyz.e the sex ratio of their progenies. Three unex~ pected males (Dl, 02 and 03) naturally originating from XXI sex reversed males were bred with different females (Table Z). Their progenies, female for most of them included, however, 2.5-9.2% unexpected males. The sex ratios of the progenies of the Dt, 02 and 03 males ail differed significantly from the sex ratios of the progeny of the XXI sex reversed male. There was no signiflcant dlfference between two of the three unexpected male progenies (02 and 03).

The XX4 sex reversed male was suc.çessively bred three times with two females. There was no sîgniflcant difference between the three sex ratios whether the progenies came from the same pair or only had the same sire (Table 3).

Four progenies were produced by crossing an XX9 sex reversed male with four different females. Three progenies (XX4 male x XX11, XX12 and XX16 females) out of four yielded sex ratios which did not differ significantly among themselves. ln contrast, the cross with the XX 10 female gave a progeny with a sex ratio differing significantly from that of the progenies from the XX 11 and XX 12 females.

The same applied to the sex ratios of progenies from XX 18 sex reversed males as three of them did not show any significant difference (progenies from XX16, XXl 7 and XX20 females).

ln the XX 17 male, there was no significant difference between the proportions of unexpected males in two out of the three crosses (XX 17 x XX t 5 and XX11 females). However, only the XX.17 x XX 11 cross produced an allfemale population. The progeny from the same male crossed with the XX t 6 female gave 39.6% males. Therefore, the same XX 1 7 male successively produced 0 and 39.6% unexpected males with two different females.

Finally, a diallel cross experiment was made involving two sex reversed males and two females (Table 4). A maternai Influence was observed in the two progenies from XX9 sex reversed males. ln contrast, a paternal influence was observed between the two progenies from the XX 16 female.

Discussion

ln species such as O. niloticus where the female ls considered homogametic, the progeny from a sex reversed male ls theoretlcally all-female, based on a •When x! is low.er than or close to the level of significance vis-à-vis a 1: 1 population, sex ratios are tested against a 1.39: 1 sex ratio which is the mean sex ratio (min.: 15.8% males; max.: 77. t %)

from ail 42 single pair matlngs of XY males, i.e., 5,299 sexed fry. Levels of significance were: '=p<0.05; "=p<0.01; and •••=p<0.001. ND = No data on survival: the initial number of fry was not recorded.

Table 2. Crosses of a sex reversed male Oreochromis niloticus with three of the unexpected males found in its progeny: progenies from the same sex reversed male characterized with the same lower case letter (a, b or c) show significant dffferences (p•0.05) in their sex ratios; O. ni/oticus, three other studies have analyzed the sex ratios of progenies from sex reversed males. ln the first study of eight male progenies produced by masculinlzing treatment, four progenies were all-female, one showed a sex ratio not significantly different from t: 1 and three ylelded unexpected high proportions of females in the range 65.2-81.8% (Jalabert et al. t 974). However, it is impossible to attribute the paternity of these unexpected sex ratios to sex reversed males rather than to XY males, considering the large range of sex ratios that these XY males show in O. ni/ot/cus [START_REF] Shelton | Progeny sex ratios from lnterspecific pair spawnings of Tilaplà àUrt!à and T. niloticà[END_REF]Wedekind 1987 cited by Wohlfarth andWedekind 1991 : Lester et al. 1989). ln the second study [START_REF] Calhoun | Sex ratios of progeny from mass spawning of sex reversed broodstock of Tilapia nilotlcà[END_REF] Finally, in the third study, four sex reversed males were identifled by the sex ratio of their progenies in 11 single pair matings. Two progenies out of 1 1 yielded unexpected sex ratios, with 6. 7 and t 7 .4% males. Nine other progenies were consîdered all-femal e. However. the number of sexed fish was still very low, with an average of 20 fry per batch (min.: 6; max.: 43), and low percentages of unexpected males may therefore have escaped notice. These four different studies show sex ratios that cannot be explained by a monofactorial sex determination al~ though they can constltute 50 [START_REF] Calhoun | Sex ratios of progeny from mass spawning of sex reversed broodstock of Tilapia nilotlcà[END_REF] to 88% (thls study) of the progenies. However, as the number of these unexpected proportions of males was often fairly low, they may have been masked, at least partially, in other studies based on a very limited sample size.

235

The percentage of unexpected males in successive progenles from the same sex reversed male differs from progenies to progenies; a paternal and maternai effect may be involved. A maternai influence is also suggested by the differences in the proportions of unexpected males observed in the mass breeding of sex reversed males depending on the batch of females used [START_REF] Calhoun | Sex ratios of progeny from mass spawning of sex reversed broodstock of Tilapia nilotlcà[END_REF]. ln diallel crosses involving five pairs of broodfish of O. nilot/cus, there is no such maternai or paternal influence [START_REF] Mair | Sex reversai. gynogenesis and triploidy in O. niloticus {L.)[END_REF].

Therefore, deviations from the theoretical mode! may be found in high proportions and seem to be incompatible with rare autosomal factors. The conditions under which these reversais occur in relation to the expected phenotype have not yet been dearly established. The hypothesis of genetic contamination seems unlikely since [START_REF] Majumdar | Sex ratios from interspecific crosses within the tilapias[END_REF] demonstrated the presence of unexpected sex ratios in isolated natural populations. More complex, polygenic genetic models could be lnvolved. However, the absence of sex-specific genetic markers in tîlapias renders difficult genetic analyses such as those conducted on platyfish (Xiphophorus maculatus) [START_REF] Kallman | A new look at sex determination in poeclliid fishes[END_REF].

The deviations observed could also result from the potential effects of external factors on differentiatîon. Depending on the genotype, one or more external factors could influence the phenotypic sex of the fry during the gonadal differentiation of sex. This hypothesis, far easier to test than a complex sex determining mechanism, could explain some unexpected results such as the presence of males in progenies from sex reversed males as well as the presence of a natural XY sex reversed female in O. niloticus [START_REF] Scott | The YY supermale ln O. niloticus {l.) and its potential in aquaculture[END_REF]Mairet al. 1991 ). Consequently, a study on the effects of temperature on the sex ratio

FryFig. 1 .

 1 Fig. 1. Production of two stocks of sex reversed males of Oreochromis niloticus.

  Sex ratio (d :9 and % of males) in a diallel cross expîrement involving two sex reversed males and two females: progenies from the same sex reversed male characterized wlth the same lower case (a, b or c) show significant differences (p=0.05) in thelr sex ratios. of 35 crosses of sex reversed males conducted in the present study, only five progenies, i.e., 14.3%, correspond to the monofactorial theoretical model. ln the 31 other progenies, 1-40.5% of unexpected males were observed. These males dld not result from sexing errors during microscopie examination of gonads and showed normal fertility: three of them bred.

Table 1 .

 1 Single pair matings of sex reversed males from Stocks and Z of Oreochromls nlloticus

	with XX females.
	Crosses	No. of	Survival	No. of	No. of	Males	x 2 (level of
	(Mx F)	progeny at	(%)	males	females	(%)	sîgniflcance)•
		sexing					1 :1	t .39: t
	XXA x XXA	t 16	55	47	69	40.5	2. t (NSJ 6.9(**)
	XX7 x XX7	179	89.5	63	116	35.2	8 ( .. )
	XX9 x XXtO	146	65.2	50	96	34.2	7.4 ( .. )
	XXD x XXD	149	ND	38	1 1 t	25.5	t 8.9( .. *)
	XXE x XXE	141	47	36	105	25.5	t 7 .9( ... )
	XXIO x XX20	89	ND	21	68	23.6	13.3( ... )
	XXF x XXF	199	ND	45	154	2Z.6	3Z.Z (* .. )
	XXG x XXG	152	ND	34	118	22.4	25. t ( .. ")
	XXH x XXH	27	ND	6	21	ZZ.3	4.44 (*)	7. 7(**)
	XXI x XXI	175	54.2	35	140	20	34.5 ('**)
	XX8 X XX8	85	85	15	70	17.6	19.8 (* .. )
	XX) x XX)	70	ND	1 1	59	15. 7	18.6('*')
	XXK x XXK	119	95.2	14	105	11.8	40.6 ('*')
	XXL x XXL	76	ND	6	70	7.9	32. 7 (' **)
	XX18 x XX16	101	95.3	4	97	4	54.2 ( .. ')
	XX17 X XX15	54	51.9	2	52	3.7	29.5 ('")
	XX4 X XX7	103	94.5		102	1	65.1 ( ... )
	XXI x XX1	96	90.6		95	1	60.5 (*")
	XXC X XXC	74	66.1	0	74	0	74 (***)
	XXB x XXB	86	43	0	86	0	86 (***)
	XXM x XXM	32	ND	0	32	0	32 ( ... ,
	XXI 1 x XXl 1	160	93.6	0	160	0	160 (**•)

Table 3. Successive matings of four sex reversed Oreochoromls niloticus males: progenles from the same sex reversed male charaderlzed wlth the same lower case Jetter (a, b or c) show signlficant dlfferences (p~0.05) ln thelr sex ratios; NS was conducted on progenies from these sex reversed males (Baroiller et al., this vol.).