Jean-Claude Dauvin
email: jean-claude.dauvin@unicaen.fr

Jean-Philippe Pezy

Alexandrine Baffreau

Quentin Bachelet

Noémie Baux

Yann Méar

Anne Murat

Emmanuel Poizot

Unicaen

Effects of a salmon fish farm on benthic habitats in a high-energy hydrodynamic system: The case of the Rade de Cherbourg (English Channel)

Keywords: Macrobenthic communities, Sediment characteristics, Salmon fish farm, Benthic indices, TOC enrichment

Introduction

The impact of aquaculture on benthic habitats had been highlighted since the end of the 1980s [START_REF] Brown | The effect of salmon farming on the benthos of a Scottish sea loch[END_REF]. More recently, Yokoyoma (2002), [START_REF] Buschmann | A review of the impacts of salmonid farming on marine coastal ecosystems in the southeast Pacific[END_REF][START_REF] Buschmann | Salmon aquaculture and coastal ecosystem health in Chile: Analysis of regulations, environmental impacts and bioremediation systems[END_REF]; [START_REF] Burridge | Chemical use in salmon aquaculture: a review of current practices and possible environmental effects[END_REF], [START_REF] Wilding | Salmon farm impacts on muddy-sediment megabenthic assemblages on the west coast of Scotland[END_REF] and Lu et al. (2017) have described the impact of aquaculture on sediment characteristics and macrofauna. The main impact leads to the enrichment of Organic Matter (OM) in sediments that are initially oxygenated and the accumulation of toxic compounds derived from aquaculture waste products (mainly H2S and NH3). Under salmon cages sediment contains generally high levels of organic matter due to waste products (feces, uneaten food, etc.), the driving force for most primary diagenetic redox reactions [START_REF] Kasten | Sulfate Reduction in Marine Sediments[END_REF]. The high organic matter flux creates anoxic conditions (oxygen consumption by aerobic respiration). The sulphate reduction process becomes predominant because of the availability of high concentrations of sulphate in seawater, which leads to the production of sulphides in sediment [START_REF] Jørgensen | The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark)[END_REF][START_REF] Barker | The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark)[END_REF]. This phenomenon may induce changes in the benthic communities: i.e. an increase in the abundance and biomass of species on a seabed enriched in OM [START_REF] Brown | The effect of salmon farming on the benthos of a Scottish sea loch[END_REF][START_REF] Carroll | Organic enrichment of sediments from salmon farming in Norway: environmental factors, management practices, and monitoring techniques[END_REF]Hyland et al., 2005;[START_REF] Kalantzi | Benthic impacts of fish farming: meta-analysis of community and geochemical data[END_REF]. [START_REF] Pearson | The environmental impacts of marine fish cage culture[END_REF] produced a review of the environmental impacts of marine fish cage culture showing the effects of fish farming on coastal ecosystems, concerning the influence of increased organic matter inputs on benthic habitats. Most of these previous studies indicated that the impact of fish farming is local and depends greatly on factors such as fish density, start date of activities, water depth, initial sea bottom site characteristics and hydrodynamic regime [START_REF] Frid | Environmental monitoring of caged fish farming in macrotidal environments[END_REF][START_REF] Morrisey | Predicting impacts and recovery of marine-farm sites in Stewart Island, New Zealand, from the Findlay-Watling model[END_REF][START_REF] Merceron | Environmental impact of a salmonid farm on a well flushed marine site: I. Current and water quality[END_REF][START_REF] Yokoyama | Impact of fish and pearl farming on the benthic environments in Gokasho Bay: evaluation from seasonal fluctuations of the macrobenthos[END_REF][START_REF] Tomassetti | Polychaetes as indicators of marine fish farm organic enrichment[END_REF][START_REF] Kalantzi | Benthic impacts of fish farming: meta-analysis of community and geochemical data[END_REF]Kuty et al., 2007;[START_REF] Borja | Assessing the suitability of a range of benthic indices in the evaluation of environmental impact of fin and shellfish aquaculture located in sites across Europe[END_REF][START_REF] Giles | Modelling the dispersal of biodeposits from mussel farms: The importance of simulating biodeposit erosion and decay[END_REF][START_REF] Chang | Analysis of results from the Environmental Management Program Tier 1 monitoring of salmon farms in southwestern New Brunswick, Bay of Fundy: Relationships between sediment sulfide concentration and selected parameters, 2002-2008[END_REF][START_REF] Wang | Assessing benthic ecological impacts of bottom aquaculture using macrofaunal assemblages[END_REF].

Most of these studies reveal a change in sediment biogeochemistry and benthic ecology around the cages extending to some metres distance away, in accord with the model of [START_REF] Pearson | Macrobenthic succession in relation to organic enrichment and pollution of the marine environment[END_REF] which describes the response of macrofauna to OM enrichment in the sediment. The extreme accumulation of OM tends to create an anoxic/azoic zone where both the oxygen demand and depletion are maximal [START_REF] Brooks | Interactions of Atlantic salmon in the Pacific northwest environment. II. Organic wastes[END_REF][START_REF] Brooks | Seven-year remediation study at the Carrie Bay Atlantic salmon (Salmo salar) farm in the Brougthon Archipelago, Bristihs Columbia, Candada[END_REF][START_REF] Hargrave | Empirical relationships describing benthic impacts of salmon aquaculture[END_REF][START_REF] Chang | Variables affecting sediment sulphide concentrations in regulatory monitoring at salmon farms in the Bay of Fundy[END_REF][START_REF] Backman | Operational decisions in response to a performance-based regulation to reduce organic waste impacts near Atlantic salmon farms in Bristih Columbia, Canada[END_REF][START_REF] Bannister | Changes in benthic sediment conditions under an Atlantic salmon farm at a deep, well-fushed coastal site[END_REF]Brooks et al., 2003;[START_REF] Wilding | Salmon farm impacts on muddy-sediment megabenthic assemblages on the west coast of Scotland[END_REF]. Moreover, [START_REF] Fernandez-Gonzalez | Role of fish farm fouling in recolonisation of nearby soft-bottom habitats affected by coastal aquaculture[END_REF] has highlighted the role of biofouling in fish farms appears to be an important vector for colonization of the surrounding soft-bottom communities impacted by aquaculture.

The Rade de Cherbourg (RdC) is a protected semi-open area situated on the north coast of the Cotentin Peninsula (central part of the English Channel), under a semi-diurnal high tide regime (5 m to 8 m tidal range from neap to spring tide). It is sheltered from the swell by breakwaters that enclose the outer harbour. In spite of 3.7 km of breakwaters interrupted by two mains channels in the north of the RdC and a smaller channel in the east tidal currents (up to 1 m/s) allow the mixing of water bodies and a significant concentration of suspended particulate matter in the seawater [START_REF] Merceron | Courantométrie du site d'élevage de salmonides en grande rade de Cherbourg (R.INT.DEL/94.02/BREST[END_REF][START_REF] Poizot | Self-contamination of aquaculture cages in shallow water[END_REF]. Due to these conditions, the RdC has been recognized as a suitable place for the development of aquaculture due to both the presence of highly mixed waters and presence of breakwaters suppress swell impacts. Consequently, a trout farm has been established in the RdC since the beginning of the 1990s, becoming a salmon farm (Salmo trutta fario then Salmo salar) during the early years 2000. Today, this site hosts the only marine salmon farm in France, covering an area of ~17 ha currently producing ~600 tons of salmon per year (data available in 1993, 1995[START_REF] Pearson | The environmental impacts of marine fish cage culture[END_REF][START_REF] Yokoyama | Impact of fish and pearl farming on the benthic environments in Gokasho Bay: evaluation from seasonal fluctuations of the macrobenthos[END_REF], 2006, 2011, 2013, 2016and 2017, with respective production volumes of 144, 290, 400, 800, 2300, 1200, 350, 250, 600 t) (Saumon de France, 2015).

The first study on the quality of the water column, sediments and benthic macrofauna around the fish farm was carried in the early 1990s [START_REF] Merceron | Courantométrie du site d'élevage de salmonides en grande rade de Cherbourg (R.INT.DEL/94.02/BREST[END_REF][START_REF] Bentley | Impact trophique d'un élevage intensif de salmonides sur le milieu marin (Rade de Cherbourg)[END_REF][START_REF] Kempf | Environmental impact of a salmonid farm on a well flushed marine site: II. Biosedimentology[END_REF][START_REF] Merceron | Environmental impact of a salmonid farm on a well flushed marine site: I. Current and water quality[END_REF]. Nevertheless, in these studies, the surveyed area was limited to the immediate vicinity of the aquaculture cages, and only the dominant species of the macrobenthic fauna were identified.

The development of aquaculture in a high-energy hydrodynamic regime over the past 30 years has offered the opportunity to study the impact of human pressures on the benthic habitats of the RdC. Moreover, [START_REF] Baux | An exceptional rich soft-bottom macrobenthic habitats in a semi-enclosed Bay of the English Channel: the Rade of Cherbourg[END_REF] have highlighted the great diversity of benthic communities and habitats in the RdC and have identified three habitat types corresponding to EUNIS codes giving in the classification of European benthic habitats (EUNIS, 2012) as well as two types so far not attributed to any known benthic habitat, in addition to one habitat characterized by the presence of Zostera marina beds. In this context of rich diversity, it is also interesting to study in more detail the benthic communities as well as the impact of the salmon farm.

The objective of this study is to assess the ecological quality status of benthic habitats and to determine the impact of salmon farm on these benthic habitats, by considering the sediment characteristics and macrofauna patterns in relation to distance from the salmon cages. To study the impact of salmon farming on benthic communities, univariate, multivariate analyses and benthic indices are used to assess the quality of benthic habitats.

Materials and methods

Study site

The RdC which construction ends in 1895 is the Europe's largest, extending over a total area ~1,500 ha [START_REF] Baux | An exceptional rich soft-bottom macrobenthic habitats in a semi-enclosed Bay of the English Channel: the Rade of Cherbourg[END_REF]. The 'Grande Rade' (1,300 ha) is isolated from the open sea by three breakwaters, with three channels allowing exchanges with open sea water (West Channel, East Channel and Collignon Channel; Fig. 1). The oscillating water volume is about 80,000 m 3 at spring tide, with a tidal range of 5.3 m [START_REF] Merceron | Environmental impact of a salmonid farm on a well flushed marine site: I. Current and water quality[END_REF]. Several small rivers and urbanized outfalls can give to minor fresh water inputs, into the RdC. The maximum depth is ~20 m with a mean depth ~13 m [START_REF] Gregoire | The morphosedimentology of an artificial roadstead (Cherbourg, France)[END_REF]. The mean winter water temperature is 8-9 °C and reaches 17-18 °C at the end of the summer. The predominant current is easterly during the flood tide and westerly during the ebb. The flood stream enters by the 'West Channel' and flows out mainly by the 'East Channel' and to a lesser extent by the 'Collignon Channel'. This is opposite to the situation at ebb tide. During flood and ebb tide, depending on the interactions between bathymetry, coastline morphology and tidal currents, gyres may form locally with a counterclockwise rotation [START_REF] Merceron | Courantométrie du site d'élevage de salmonides en grande rade de Cherbourg (R.INT.DEL/94.02/BREST[END_REF][START_REF] Poizot | Self-contamination of aquaculture cages in shallow water[END_REF].

The RdC sea bed is covered by four main sedimentary facies [START_REF] Baux | An exceptional rich soft-bottom macrobenthic habitats in a semi-enclosed Bay of the English Channel: the Rade of Cherbourg[END_REF]. The western sector is generally made up of organic-rich mud with decaying marine algae and coarse sand. The centre of the RdC consists of silty mud, while the east is covered by fine sand. Gravel and coarse sand are found in the channels which connect the RdC with the open sea.

The salmon farm consists of cylindrical and cubic nets aligned along an East-West axis, the concession area being located only 50 m south of the main breakwater, extending over a distance of 1.5 km with a width of 100 m at a depth of ~12 m (Saumon de France, 2015) (Fig. 1). The average height of the nets ranges from 7 to 8 m below the sea surface, and the distance above the seabed is ~ 3 to 7 m due to deformation of the fishnet cage under the influence of the variable tidal currents and range. Inside the RdC, tidal currents are mainly oriented East-West and reversely during the tide with current velocities up to 1 m/s. At the fish farm site, a gyre [START_REF] Merceron | Courantométrie du site d'élevage de salmonides en grande rade de Cherbourg (R.INT.DEL/94.02/BREST[END_REF] is responsible of an asymmetric regime of the currents velocities from flood to ebb. At neap tide, the flood currents are oriented westeast 30% of the time and ebb currents are oriented east-west 70% of the time. During spring tide, this asymmetric is reduced to respectively 40% (flood currents) and 60% (ebb currents). This asymmetry is related to the establishment of a gyre within the concession area [START_REF] Merceron | Courantométrie du site d'élevage de salmonides en grande rade de Cherbourg (R.INT.DEL/94.02/BREST[END_REF].

Sampling and laboratory treatment

Following an East-West axis parallel to the main breakwater where the cages are located and according to the main tidal currents in this area in the RdC, six benthic stations (~ 10-11 m depth) were sampled during five campaigns over an annual cycle in 2013[START_REF] Bachelet | Etude de l'influence de l'élevage de poissons en mer sur l'environnement benthique : exemple de l'élevage de saumons en rade de Cherbourg[END_REF](C1: 31 October 2013, C2: 19 February 2014, C3: 8 April 2014, C4: 13 June 2014 and C5: 18 September 2014) to identify the impact of this salmon farm. One station (CC8) was located under a cage, two stations (OR and ER) outside the fish farm (> 600 m from the central cages), and three others (OC8, EC8 and EC16) near the central cages (OC8: 50 m to the west; EC8: 50 m to the east; EC16: 500 m to the east; Fig. 1). During the five sampling campaigns, five replicates were collected using a 0.1 m² Van Veen grab covering a total sampling surface area of 0.5 m 2 at each station for the macrofauna analysis plus one supplementary sample for grain-size analysis. At station CC8, the Van Veen grab was manipulated by hand from the cage structure between the salmon nets. The sampled faunal material was sieved onboard on a 1-mm circular mesh. Fauna was preserved in 10% buffered formaldehyde prior to sorting, identified to species as far as possible. Some species are grouped to the higher classification rank when the morphological determination is doubtfully.

The taxonomic richness (TR) and abundance (A) are expressed in number of taxa and number of individuals per 0.5 m 2 , respectively. Species (or taxa) names were checked against the World Register of Marine Species (http://www.marinespecies.org) on 1 September 2018.

The fine fraction (<50 μm) was measured by wet sieving and rinsed with fresh water to remove the salt. Other sediment fractions were dried and sieved on a sieve shaker. Eightsieve column was used (8, 4, 2, 1, 0.5, 0.250, 0.125 and 0.050 mm). The sediment was classified according to Wentworth's grain-size scale (Wentworth, 1922): <50 µm: silt clay; 50-125 µm: very fine sand; 125-250 µm: fine sand; 250-500 µm: medium sand; 500-1000 µm: coarse sand; 1000-2000 µm: very coarse sand; 2000-4000 µm: gravel; and >8000 µm: pebbles). The results are expressed in percentage relative to the total dry weight of the sample. Finally, we consider four grain-size classes for the definition of sediment types: fine fraction (< 50 µm, denoted FF), fine sand (50-250 µm), medium and coarse sand (500-2000 µm) and gravels (> 2000 µm).

Sediments for Total Organic Carbon (TOC) analysis were freeze-dried and homogenized. The coarse fraction was removed with a 2-mm sieve to keep only the fine fraction. The weight of the sample was reduced to obtain 50 mg aliquots using a manual quartering method (random separation of sample aliquots). Sediment samples were acidified by H3PO4 (1 M), to remove carbonates, dried on a hot plate at 40° C and measured for TOC content by combustion in a LECO CS 744 Carbon Sulfur analyzer. Two or three replicates were analyzed per sample. The results of organic carbon measurements are expressed in percent of dry sediment. The precision determined from replicate subsampling was ± 0.05% organic carbon.

Current measurements

Two Acoustic Doppler Current Profilers (ADCPs: Teledyne RDI Sentinel S50 and V50) were installed at a distance of 20 m (East) and 50 m (West) from the eastern cage (Fig. 1). Measurements were conducted between 6 and 24 October 2016, allowing the acquisition of current velocity data during a tidal cycle (spring/neap tides). The two ADCPs measured currents at the same height in the water column, running the same firmware, and were configured for the same depth-cell size. Both ADCP's use 22 cells, with a depth-cell size of 0.5 m. Even if dates of velocities and directions measurements took place at a different moment than the bottom sampling, they were considered as characteristics of flow conditions at the studied site. The fish farm structures and shapes did not change between the two sampling surveys.

Data analysis 2.3.1. Univariate analyses

Data were used to calculate the abundance A (0.5 ind.m -2 ) and the most commonly used biodiversity indices for each station, i.e. Taxonomic Richness (TR), Shannon-Weaver diversity index (H') in log2 [START_REF] Shannon | The Mathematical theory of communication[END_REF]) and Pielou's evenness (J) [START_REF] Pielou | Shannon's formula as a measure of specific diversity: its use and measure[END_REF] for the six stations and five sampling dates. Data analysis was performed using the PRIMER® version 6 software package (Plymouth Routines in Multivariate Ecological Research) [START_REF] Clarke | PRIMER V6: User Manual/Tutorial[END_REF].

The Ecological Status indicating the quality of the station was estimated from H' values according to the thresholds defined previously by [START_REF] Vincent | Guidance on typology, reference conditions and classification systems for transitional and coastal waters[END_REF] i.e.: bad 0-1; poor 1-2; moderate 2-3: good 3-4 and high > 4. [START_REF] Dauvin | Benthic ecological status of Algerian harbours[END_REF] proposed thresholds for the taxonomic richness and J', respectively i.e: bad <10, <0.20; poor 10-20, 0.20-0.40: moderate 20-30, 0.40-0.60; good 30-40, 0.60-0.80 and high >40, > 0.80.

The biotic indices AMBI, M-AMBI and BO2A were also calculated to assess the ecological status of the macrofauna [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF][START_REF] Dauvin | Response of the Different Indices to the Diverse Human Pressures[END_REF][START_REF] Dauvin | Polychaete/ amphipod ratios: an approach to validating simple benthic indicators[END_REF]. The five ecological statuses of the European Water Framework Directive were used to assess the benthic macrofauna according to the thresholds between these statuses proposed by [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF] and [START_REF] Dauvin | Response of the Different Indices to the Diverse Human Pressures[END_REF][START_REF] Dauvin | Polychaete/ amphipod ratios: an approach to validating simple benthic indicators[END_REF]. AMBI (AZTI Marine Biotic index) [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF] is based on an a priori classification of macrofauna species in five ecological groups (EG) according to their sensitivity to OM. EG-I corresponds to taxa very sensitive to organic enrichment and disturbance which are usually only present under unpolluted conditions, EG-II to taxa indifferent to enrichment or disturbance, EG-III to taxa tolerant of excess OM enrichment, which may occur under normal conditions, but which are stimulated by organic enrichment, EG-IV, to second-order opportunistic species and EG-V, to first-order opportunistic species, able to resist strong disturbance. The relative abundances of each EG serve to calculate benthic indices following the formula given in [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF]. AMBI = 0 EGI + 1.5 EGII + 3 EGIII + 4.5 EGIV + 6 EGV The index is computed using the AZTI program (version 5.0) freely available online at http://www.aziti.es. AMBI values lying between 0 and 6 can be converted into EcoQ using the fixed scale provided by [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF].

M-AMBI is a multimetric index for assessing the ecological quality status of marine and transitional waters [START_REF] Bald | Assessing reference conditions and physico-chemical status according to the European Water Framework Directive: a casestudy from the Basque Country (Northern Spain)[END_REF][START_REF] Muxika | Using historical data, expert judgement and multivariate analysis in assessing reference conditions and benthic ecological status, according to the European Water Framework Directive[END_REF]. It is based on benthic macroinvertebrates and combines AMBI, which is a biotic index based on species sensitivity/tolerance, with diversity and taxonomic richness. The M-AMBI algorithm incorporates different metrics by means of factor analysis (FA). Reference values are considered to represent an unperturbed environment, in which the Taxonomic Richness (TR) and H' values are highest for the same type of benthic communities (respectively 50 for TR and 3.5 for H'). We also assume that, in an unperturbed environment, first-order opportunistic species (EG V of the AZTI list) would be absent or poorly represented with a AMBI value of 2.5.

The BO2A (Benthic Opportunistic Annelids Amphipods Ratio) (Dauvin and Ruellet, 2009;[START_REF] Dauvin | Twenty years of application of Polychaete/Amphipod ratios to assess diverse human pressures in estuarine and coastal marine environments: a review[END_REF] is calculated following the formula:

BO2A= log10 [foa/(fsa +1) +1]
where foa is the frequency of opportunistic annelids (Clitellata and Polychaeta) (i.e., number of opportunistic polychaetes corresponding to Ecological Groups IV and V of Grall and Glémarec (1997) and used for calculating AMBI [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF], plus the Oligochaeta and Hirudinea, divided by the total number of individuals in a sample), and fsa is the sensitive amphipod frequency (i.e., number of amphipods, excluding the opportunistic Jassa amphipods, divided by the total number of individuals in a sample), and foa + fsa ≤ 1.

A two-way ANOVA is used to test spatio-temporal changes (stations and campaigns factors) for TS, A, H', J, AMBI, M-AMBI and BO2A. A Shapiro-Wilk normality test and a Bartlett test for homogeneity of variances are performed prior to each ANOVA. The Tukey Honestly Significant Difference test is applied when ANOVA shows significant differences.

Multivariate analysis

The spatial structure of the sediment characteristics and the benthic community are analysed using a multivariate analysis following [START_REF] Clarke | PRIMER V6: User Manual/Tutorial[END_REF] with the PRIMER software package (Plymouth Routines In Multivariate Ecological Research). Data analysis is performed using non-metric multidimensional scaling ordination (MDS), and a Hierarchical Ascendant Classification (HAC) is created by group average linking using the Bray-Curtis similarity measure [START_REF] Beals | Bray-Curtis Ordination: An Effective Strategy for Analysis of Multivariate Ecological Data[END_REF]. In comparison with other similarity measures, the Bray-Curtis is known to be one of the more efficient indexes to highlight the similarity between samples [START_REF] Bloom | Similarity indices in community studies: potential pitfalls[END_REF]).

Sorensen's coefficient for Presence/Absence of taxa, and Log10+1-transformed abundances (0.5 ind.m²) are used to down-weight the effect of very abundant species. To identify the species within different groups which principally account for the observed assemblage differences, SIMPER (SIMilarity PERcentage) routines are performed using a decomposition of Bray-Curtis similarity on log-transformed abundance data (Clarke and Warwick, 1994).

Results

Hydrological conditions

Table 1 summarizes the main characteristics of the water column during the year 2013-2014. While the temperatures show considerable seasonal variations, with a maximum at the end of summer in September-October (17.9 °C) and a minimum in winter in February (8.2 °C), the salinity remains stable throughout the year with values included between 34.4 and 34.7. The diurnal oxygen saturation reaches a maximum in September 2014 (123%), probably in relation with a high primary production at that time; the turbidity is low, with very low values (1.2 NTU) at both of the early sampling dates and low values during the rest of the year (< 14 NTU).

Currents near the cages

Ebb currents flow westward at almost 270°, while flood currents flow eastward at almost 90°. Ebb currents start to flow a few hours before high tide, and flood currents start after low tide during spring tides and before low tide during neap tides. During neap tides, the ebb current runs for 60% of the time, and the flood current for 40% (Table 2). During spring tides, flood currents run for 30% of the time, and ebb currents make up the remaining 70%.

At West ADCP location, mean velocities and directions highlight a more erratic behaviour during ebb currents (270°) than at East ADCP location. This is the reverse for flow currents (90°) in a lesser extent. At both ADCPs, the maximum velocity increases twofold between neap tide and spring tide. The maximum velocity is three times higher during spring tide compared with neap tide.

Sediment characteristics

Using HAC and MDS to analyse the grain-size characteristics of the sediments from the five stations at the five sampling dates, we can distinguish three main groups (Fig. 2).

The first group is made up of five observations, four from CC8 and one from OC8.

For this group, the mean percentage of Fine fraction (FF)= 11.4, fine sand= 51.3, coarse sand and gravel= 37.3. The sediment type corresponds to sand and gravel with a low percentage of FF, mainly restricted to station CC8 located under the cage.

The second group comprises 10 observations, 4 from EC8, 4 from EC16 and those from OR in October 2013 and OC8 in June 2014. For this group, the mean % FF= 16.2, fine sand= 76.2, coarse sand and gravel= 7.6, corresponding to fine sand sediment mainly represented at EC8 and EC16 (two stations in the eastern part of the salmon cages).

The third group comprises 14 observations, including the five sampling dates at ER, four at OR (except at OR in October), the observation from EC16 in June 2014, three observations from OC8 (February, April and September 2014), one from CC8 in September, and one from EC8 in June 2014. For this group, the mean % FF= 14.9, fine sand= 60.1, coarse sand and gravel= 25.0, corresponding to heterogeneous muddy sediment, according to the Wentworth classification (Wentworth, 1922). This group comprises both reference stations OR and ER plus station OC8 in the western sector of the salmon cages.

The TOC levels remain relatively stable throughout the year (Table 3) with a global mean of 0.55 +/-0.04%. While seasonal changes are very slight, there are nevertheless some differences in TOC between the stations, with a minimum at station EC8 in the eastern part of the salmon cages (0.35+/-0.06%) ranging up to a maximum at station CC8 under the cages (0.82 +/-0.05%).

Figure 3 shows the mean annual values of TOC for the five stations as a function of FP (fine particles < 50 µm). The mean values of TOC at four stations (OR, EC8, EC16 and ER) follow a linear trend from 0.3% TOC and 5% FP to 0.6% TOC and 16% FP. The reference stations OR and ER also yield the highest TOC, with the latter being the richest in FP content. The stations (EC8 and EC16) located in the eastern sector of the salmon cages are characterized by lower fine fraction percentage (6-8%) and TOC (0.3-0.4%). For these four stations, there is no organic matter enrichment. Station CC8 under the cages and OC8 in the western sector of the cages show different patterns, with a low percentage of FP and, on the contrary, relative high percentages of TOC (>0.6%) in comparison with other stations.

The similarity between the stations is high (80 %), but three groups can be identified with granulometric data combined with those of FP and TOC (Figs. 2 and3; Table 3). Firstly, stations CC8 and OC8 under and near the western sector of the cages are characterized by mixed sediment with high TOC%. Secondly, stations EC8 and EC16 in the eastern sector of the cages are located on fine sand with low TOC%. Lastly, the reference stations OR and ER are distant from the cages, showing high percentages of fine particles correlated with high percentages of TOC on heterogeneous mixed sediment.

General characteristics of the fauna

A total of 182 taxa are identified during the study, the polychaetes dominate in terms of diversity with 71 taxa representing 38% of the TR (Taxonomic Richness), the molluscs with 36 taxa (20% of TR) and the amphipods with 32 taxa (17% of TR). These three dominant zoological groups comprise a total of 139 taxa (75% of the fauna recorded). A total of 119,296 individuals are sampled, with 98 taxa being represented only by a few number of individuals (<10), making up only 0.27% of the collected fauna. The tanaids account for 61% of the recorded individuals, followed by the polychaetes (25%), amphipods (6%) and molluscs (1.5%). The TR varies from 22 to 78 and the abundances from 455 to 11,355 individuals. 0.5m -2 without any clear seasonal pattern or differences between the six stations (Table 3).

Moreover, the ten most abundant species account for 106,921 individuals making up 89.6% of the macrofauna. The tanaid Apseudopsis latreillii represents 60.3% of the total number of collected individuals, then the polychaetes Notomastus latericeus (9.4%) and Dipolydora giardi (9.0%), the Maldanidae, mainly represented by Euclymene oerstedii (4.3%), the amphipod Ampelisca tenuicornis 1.7%, the polychaetes Phyllodoce mucosa (1.3%) and Capitella minima (1.3%), the amphipod Phtisica marina (1%), the polychaete Chaetozone gibber (0.7%) and the bivalve Thyasira flexuosa (0.7%).

To summarize, although the fauna is diversified, it is dominated numerically by a small number of species, especially by the tanaid Apseudopsis latreillii (species identified in December 2017 by specialist P. Pesquete, Aveiro University, personal communication).

Spatio-temporal patterns of the macrofauna

Using the abundance data and applying a Bray-Curtis similarity level of 50%, the HAC and MDS analyses allow us to identify three groups (Fig. 4). Group 1 includes the 10 observations from stations OC8 and CC8. Group 2 corresponds to the observations from reference sites OR and ER, plus two sampling dates for EC16 (June and September) and June for EC8. Group 3 includes seven observations from EC8 (four dates) and EC16 (three dates).

Table 4 show significant differences in abundances, H' and J' between stations, while only differences of TR are observed between ER and EC8.

Finally, analyses of sediment characteristics and macrofauna abundance lead to the identification of a spatial pattern with three groups of stations: two stations (CC8 and OC8) located under the cages and in the western sector of the cages, two stations (EC8 and EC16) in the eastern sector of the cages, and, finally, the reference stations OR and ER. A weak pattern is observed at the scale of the observations during the 2013-2014 sampling year (Tables 3 and4). Table 4 shows significant differences of Taxonomic Richness between C1 and C5, while significant differences in abundances are observed between C4 and C5, and for both diversity indices H' and J' between C2 and C4.

The tanaid Apseudopsis latreillii dominates the macrofauna in the three groups of stations, with an abundance ≥ 2,000 ind.0.5m²; similarly, the polychaete Notomastus latericeus shows noteworthy abundance in all three groups, but decreases in abundance from stations under and in the western sector of the cages towards the reference stations (Table 5).

Opportunistic species (GIV and GV) are only abundant in the first group of stations (CC8 and OC8); conversely, sensitive species are present and abundant at both references stations (OR and ER) (Table 5).

Ecological quality status

The values of the three benthic indices (AMBI, M-AMBI and BO2A) and their corresponding Ecological Quality Status (EcoQS) allow us to identify three groups of stations without seasonal changes: references stations ER and OR plus EC16 and OC8 show a good to high EcoQS, station EC8 shows moderate to high EcoQS depending on the sampling campaign or the biotic index, and station CC8 shows a moderate EcoQS. The diagnosis based on H' and J' yields a poor to moderate EcoQS, but both of these indices are clearly influenced by high Apseudopsis latreillii abundances which lower their values.

Discussion

The Rade de Cherbourg has hosted a salmon farm since the beginning of the 1990s [START_REF] Kempf | Environmental impact of a salmonid farm on a well flushed marine site: II. Biosedimentology[END_REF][START_REF] Merceron | Environmental impact of a salmonid farm on a well flushed marine site: I. Current and water quality[END_REF]. Our recent study (2013)(2014) offered the opportunity to assess the ecological quality status of benthic habitats in the RdC three decades after the initial establishment of aquaculture in this zone (firstly a trout farm and then a salmon farm). [START_REF] Bachelet | Etude de l'influence de l'élevage de poissons en mer sur l'environnement benthique : exemple de l'élevage de saumons en rade de Cherbourg[END_REF] highlighted the effects of anaerobic degradation due to sulphate-reducing processes at the water-sediment interface in areas east of the salmon cages, as well as high ammonium concentrations resulting from faeces and non-consumption of feed for farmed fishes at the station located under the cages [START_REF] Bachelet | Etude de l'influence de l'élevage de poissons en mer sur l'environnement benthique : exemple de l'élevage de saumons en rade de Cherbourg[END_REF]. Both the highenergy hydrodynamic regime in the RdC and the acceleration of currents under the cages [START_REF] Poizot | Self-contamination of aquaculture cages in shallow water[END_REF], has led to the dispersal of excess organic waste. The increase of current velocity under the fishnets is also responsible of erosion, which has resulted in modifications of the bottom morphology. As measured during this study, the maximum current velocity reaches 0.7 m.s -1 , which is two times higher during Spring tide compared with Neap tide.

Nevertheless, this maximum velocity is very slow in comparison with values recorded off the

North Cotentin coast especially in the Raz Blanchard, a zone where the maximum velocity can reach 5.0 m.s -1 during Spring tide (Salomon andBreton, 1991, 1993). Velocity measurements near the cages show that the fish farm is always under the influence of tidal currents, including during neap tides. The water within the farm is regularly replenished and homogenized [START_REF] Poizot | Self-contamination of aquaculture cages in shallow water[END_REF]. The strong asymmetry of the tidal currents (as regards both duration and intensity of flow) leads us to consider that ebb current is dominant on the farm [START_REF] Poizot | Self-contamination of aquaculture cages in shallow water[END_REF]. These conditions correspond to the coastal farming classification of [START_REF] Holmer | Review: environmental issues of fish farming in offshore waters: perspectives, concerns and research needs[END_REF], preventing the accumulation of OM under the cages due to dispersal of the residual organic matter derived from the waste-feed and faeces, in contrast to enclosed salmon farms such as in Norwegian fjords [START_REF] Kutti | Effects of organic effluents from the salmon farm on a fjord system. II. Temporal and spatial patterns in infauna community composition[END_REF][START_REF] Holmer | Review: environmental issues of fish farming in offshore waters: perspectives, concerns and research needs[END_REF]. Moreover, the moderate fish production over the past 25 years (mean ~ 700 tonnes of salmon per year) has caused a negligible supplementary input of organic matter in the RdC [START_REF] Murat | SALMOCOT : Enrichissement organique à proximité d'un élevage de salmonidés en Manche[END_REF].

The six stations chosen in this study are located in two main EUNIS benthic habitats [START_REF] Baux | An exceptional rich soft-bottom macrobenthic habitats in a semi-enclosed Bay of the English Channel: the Rade of Cherbourg[END_REF]: A5.334 Melinna palmata in mixed muddy fine sand (OC8, CC8, and EC8) and a new habitat with Spio decorata and Apseudopsis latreilli in very fine sand and fine sand (OR, ER and EC16). Analyses of the six stations sampled during the five campaigns show that the macrofauna can be separated at a similarity of 50 % in three benthic assemblages (Fig. 4; Table 5).

The first group corresponds to reference stations OR and ER at some distance from the cages, with high percentages of fine particles (≅ 14%) higher than at other studied stations (≅ 7%) and relatively high mean TOC (≅ 0.56%) on heterogeneous mixed sediment. The relationship between TOC and fine particles suggests that both stations are in dynamic equilibrium outside the influence of the cages (Fig. 3).

The second group of stations EC8 and EC16 is located in the eastern sector of the cages on fine sand (≅ 7.2% of FP) with low TOC content (0.37%). These lower values (FP and TOC) are consistent with higher energy hydrodynamic conditions (stronger currents and enhanced turbulence) linked to an acceleration of currents under the cages [START_REF] Poizot | Self-contamination of aquaculture cages in shallow water[END_REF]. These hydrodynamic conditions result in a winnowing of fine particles and organic matter.

The third group corresponds to stations CC8 and OC8 (under and near the cages in their western sector), which are characterized by mixed sediment with the highest TOC% (≅ 0.73%) and the lowest FP% (≅ 6.5%). Low FP percentage of the sediment is linked to the impact of cages on water flow (higher turbulence and current intensity) [START_REF] Poizot | Self-contamination of aquaculture cages in shallow water[END_REF].

In contrast to the stations in the second group, stations CC8 and OC8 do not appear to have a faunal community consistent with fish farm (high OM content) due to the presence of faeces and feed-waste. Despite the high-energy hydrodynamic conditions, the input of organic matter is sufficient to enrich the underlying sediment.

During the present study, very high densities of the tanaid Apseudopsis latreillii are observed under salmon cages, as well as in other muddy sand habitats of the RdC. This species is known to have an affinity with clean fine sands (Bakalem et al., 2009). The three benthic assemblages for each of the three communities identified are dominated by A. latreillii with very high abundances both in terms of time (year scale) and space. High abundances of this species are observed in most of the benthic habitats of the RdC except the mud habitat [START_REF] Baux | An exceptional rich soft-bottom macrobenthic habitats in a semi-enclosed Bay of the English Channel: the Rade of Cherbourg[END_REF]. This species is widely distributed all along the North Cotentin coastline, and a very high abundance is found outside the fish farm and the RdC in its eastern part [START_REF] Martinez | Caractérisation des habitats benthiques de la baie du Becquet (ZNIEFF-mer 25M00009)[END_REF] as well as in some sand and gravel habitats in the western sector of the RdC (personal observations). Such high abundances have been observed in other marine areas (Table 6), but the present study represents the first report of such high abundances of this species associated with a fish farm. The abundances observed in the RdC and in its eastern part (20,000-23,000 ind.m -2 ) are among the highest recorded values. Only the abundance observed in the Persian Gulf (Dubai creek) reaches higher values, with 43,000 ind.m -2 , but this site is subject to the impact of organic pollution [START_REF] Saunders | The impact of orgnanic pollution on the macrobenthic fauna of Dubai Creek (UAE)[END_REF]. The relationship between fish farm and its increasing of Organic Matter and abundance of A. latreillii need to be clarified in the future.

Except for the Aber Wrach in the western part of the English Channel along the Brittany coast where the abundance exceeds 3,000 ind.m -2 [START_REF] Dauvin | Conditions of the peracarids populations of subtidal communities in northern Brittany ten years after the Amoco Cadiz oil spill[END_REF][START_REF] Hily | Modifications of the specific diversity and feeding guilds in an intertidal sediment colonized by an eelgrass meadow (Zostera marina)[END_REF], A. latreillii is a rare species in the benthic communities of the English Channel. It is mostly associated with the Melinna palmata muddy sand community as in the Rivière de Morlaix station in the Bay of Morlaix, in an area naturally enriched in OM [START_REF] Dauvin | The muddy fine sand Abra alba-Melinna palmata community of the Bay of Morlaix twenty years after the Amoco Cadiz oil spill[END_REF], and is very rarely recorded in the eastern part of the English Channel. Moreover, A. latreillii never occurs as dense populations or as a dominant species such as observed in the North Cotentin. Abundant populations occur elsewhere principally on fine sand and muddy fine, in Zostera marina meadows [START_REF] Hily | Modifications of the specific diversity and feeding guilds in an intertidal sediment colonized by an eelgrass meadow (Zostera marina)[END_REF][START_REF] Esquete | Peracarid assemblages of Zostera meadows in an estuarime ecosystem (O Grove inlet, NW Iberian Peninsula): spatial distribution and seasonal variation[END_REF]Gamito et al., 2011), and rarely on coarse sand [START_REF] Lourido | Assemblages of peracarid crustaceans in subtidal sediments from the Ria de Aldan (Galicia, NW, Spain)[END_REF].

The response of A. latreillii to pollution continues to be debated, and the sensitivity of this species to pollution is unclear since it is reported as being both tolerant and sensitive (dela- [START_REF] De-La-Ossa-Carretero | Sensitivity of tanaid Apseudes latreilli (Milne-Edwards) populations to sewage pollution[END_REF]. It is classified in the list of tolerant species (GIII in AMBI, [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF]Table 5). Along the Castellon coast (W. Mediterranean) de-la-Ossa-Carretero et al. ( 2010) has shown that the species responds negatively to sewage pollution, displaying reduced abundances near outfalls with a higher input of suspended solids, nutrients and a possible trend to hypoxia at stations near the discharge channels. As A. latreillii is a GIII species largely dominant in the three benthic assemblages recorded under and near the cages, the AMBI and M-AMBI benthic indices lead mainly to the attribution of a good EcoQS at the stations except for station CC8 under the cages where a moderate EcoQS is obtained (Table 3). The use of BO2A yields the same EcoQS for the six stations (Table 3). Nevertheless, both diversity indices (H' and J; Table 3) assign degraded poor and moderate EcoQS for these stations in relation to high abundances of the dominant species A. latreillii and the poorly structured distribution of abundances between species.

Apart from the top dominant species, the other dominant taxa show different patterns of abundances in the three assemblages (Table 5). Opportunistic species such as the polychaetes Dipolydora giardi, Capitella minima, Chaetozone gibber, Caulleriella alata and Cirriformia tentaculata and the amphipod Jassa herdmani (EG IV and V) are abundant at stations under and near the cages (OC8 and CC8) or in the eastern sector of the cages, and rare at the reference stations. EG III species such as the polychaetes Phyllodoce mucosa and Neanthes acuminata, the amphipod Gammarella fucicola and the tanaid Zeuxo holdichi show similar patterns with decreasing abundance from the stations under the cages to the reference stations. Conversely, the polychaetes Maldanidae (mainly Euclymene oerstedii), Melinna palmata, Hilbigneris gracilis and Acromegalomma vesiculosum, the bivalve Thyasira flexuosa and the tanaid Apseudes talpa are abundant only at the reference stations (Table 5).

The amphipods sensitive to organic pollution (Ampelisca tenuicornis and Phtisica marina) and the polychaete Lanice conchilega show similar abundances at both eastern stations and at the reference stations (Table 5).

In summary, these is a decrease in abundance of opportunistic species away from stations OC8 and CC8 near the cages and, conversely, an increase in abundance of sensitive species from stations under and near the cages towards the reference stations (OR and OR).

The abundances of both opportunistic and sensitive species show intermediate abundances at stations in the influence zone (EC16 and EC8).

All the results (sediment type, fine particles, TOC percentages, macrofauna abundances and biotic indices), show that the fish farm impact remained relatively low in this tidal regime: but three benthic habitats can be distinguished: i)

an impacted zone (under the cages and in the western sector of the cages; stations CC8 and OC8), exhibits moderate accumulation of OM and an enhanced hydrodynamic regime;

ii) an influenced zone (stations EC8 and EC16 in the eastern sector of the cages) with a single impact (enhanced hydrodynamics);

iii) A non-impacted zone corresponding to both reference stations (ER and OR) (Tables 3,5 and Figs. 2,4).

Our results highlight a local impact of the salmon fish farm, with a relative lower number of species under cages and the reduction in the rate of degradation of benthic OM inputs since the establishment of fish farming in the 1990s. In addition, these observations appear to support the Intermediate Disturbance Hypothesis, which proposes that the diversity and abundance of organisms increases when the disturbance is intermediate in frequency and intensity [START_REF] Huston | A General Hypothesis of Species Diversity[END_REF][START_REF] Wilson | The "intermediate disturbance hypothesis" of species coexistence in based on patch dynamics[END_REF][START_REF] Dial | Theory of marine communities: the intermediate disturbance hypothesis[END_REF][START_REF] De Backer | Similar diversity-disturbance responses to different physical impacts: three cases of small-scale biodiversity increase in the Belgian part of the North Sea[END_REF][START_REF] Pezy | What are the most suitable indices to detect the structural and functional changes of benthic community after a local and short-term disturbance?[END_REF].

Although tidal currents prevent the accumulation of OM in the RdC, [START_REF] Bachelet | Etude de l'influence de l'élevage de poissons en mer sur l'environnement benthique : exemple de l'élevage de saumons en rade de Cherbourg[END_REF] observed the presence of local anaerobic degradation by sulphate-reducing processes at the water-sediment interface to the east of the salmon cages, as well as high ammonium flux under the cages, resulting from faeces and waste-feed derived from the salmon farm at the station under the cages. Again, the impact in the RdC appears very limited.

Moreover, many studies have shown that certain parameters play a major role in determining the impact of farms, such as high fish density, age of the concession, water depth, previous sediment types located at the site and hydrodynamic regime [START_REF] Frid | Environmental monitoring of caged fish farming in macrotidal environments[END_REF][START_REF] Morrisey | Predicting impacts and recovery of marine-farm sites in Stewart Island, New Zealand, from the Findlay-Watling model[END_REF][START_REF] Merceron | Environmental impact of a salmonid farm on a well flushed marine site: I. Current and water quality[END_REF][START_REF] Borja | Assessing the suitability of a range of benthic indices in the evaluation of environmental impact of fin and shellfish aquaculture located in sites across Europe[END_REF][START_REF] Giles | Modelling the dispersal of biodeposits from mussel farms: The importance of simulating biodeposit erosion and decay[END_REF][START_REF] Chang | Analysis of results from the Environmental Management Program Tier 1 monitoring of salmon farms in southwestern New Brunswick, Bay of Fundy: Relationships between sediment sulfide concentration and selected parameters, 2002-2008[END_REF]. The current velocity appears to be one of the most important parameters because it favours the dispersal of waste and ensures the absence of anoxic conditions under the fish cages [START_REF] Black | The relationships between hydrodynamics, the concentration of hydrogen sulphide produced by polluted sediments and fish health at several marine cage farms in Scotland and Ireland[END_REF][START_REF] Keeley | Novel observations of benthic enrichment in contrasting flow regimes with implications for marine farm monitoring and management[END_REF]. Facing with the large increase of OM in sediments under the cages and in surrounding habitats in coastal ecosystem with weak currents, fish farmers have chosen to develop aquaculture in deeper waters, mainly in Norwegian fjords and in higher energy hydrodynamic regimes [START_REF] Holmer | Review: environmental issues of fish farming in offshore waters: perspectives, concerns and research needs[END_REF][START_REF] Falconer | Using physical environmental parameters and cage enginnering desing within GIS-based seite suitability models for marine aquaculture[END_REF][START_REF] Bannister | Changes in benthic sediment conditions under an Atlantic salmon farm at a deep, well-fushed coastal site[END_REF].

In the case of the RdC, the abundances of opportunistic and sensitive species are clearly linked to the impact of salmon cages; opportunistic species are more abundant in the assemblages impacted by the farm and, in the contrary, sensitive species are more abundant in the reference non-impacted assemblages (Table 5). Moreover, in this particular megatidal environment, the benthic habitats are impacted by two main factors: the input of OM in relation to the fish farm and the increase in currents near and under the fish cages. The RdC is particularly suitable for aquaculture, with an increase of current velocities under the cages in shallow waters due to the reduction of the open water column under the cages. This phenomenon prevents a large accumulation of OM under the cages and favours the dispersal of OM outside the fish farm, which is reinforced by tidal currents.

In their prospect, the 'GMG Saumon de France' company envisaged the transfer of cages to the neighbouring eastern part of the RdC, which exhibits a higher energy hydrodynamic regime than in the RdC. Our results do not justify this repositioning of the fish farm outside the RdC, the existing salmon cages showing a relative low and local impact. Some studies had shown that after the cessation of fish production or after fallowing (i.e. period during which there is a pause in production), the opportunistic taxa undergo a decline in only two months [START_REF] Zhulay | Effects of aquaculture fallowing on the recovery of macrofauna communities[END_REF], or six months (Brooks et al., 2013a), whereas the impact could have a duration of several years [START_REF] Brooks | Seven-year remediation study at the Carrie Bay Atlantic salmon (Salmo salar) farm in the Brougthon Archipelago, Bristihs Columbia, Candada[END_REF][START_REF] Pereira | Recovery of sediments after cessation of marine fish farm production[END_REF]Keeley et al., 2012aKeeley et al., ,b, 2015)). In the case of the cessation of activity in the RdC salmon farm, it will be interesting to survey the resilience of benthic habitats in this area.

Figure 1 .

 1 Figure 1. Location of the six sampling stations near the salmon farm in the Rade deCherbourg and benthic habitats identified by[START_REF] Baux | An exceptional rich soft-bottom macrobenthic habitats in a semi-enclosed Bay of the English Channel: the Rade of Cherbourg[END_REF]. * Location of the two sites used for ADCP current measurements.

Fig. 3 .

 3 Fig. 3. Relationship between percentage of TOC and fine particles (< 50 µm), showing evidence of the enrichment of TOC at stations CC8 and OC8 (see Fig. 1 for location of stations).

Fig. 4 .

 4 Fig. 4. A: Dendrogram showing hierarchical clustering of data from 30 sites in the Rade de Cherbourg, using group average linking of Bray-Curtis similarities on Log(x+1) abundance of the taxa. The three groups of samples are separated at 50% similarity. B: MDS ordination of the 30 sites sampled in the Rade de Cherbourg, based on standardized and Log (x+1) transformation of abundances and Bray-Curtis similarities. (see Fig. 1 for location of the stations; oct13: October 2013; feb: February 2014; apr14: April 2014; jun14: June 2014; sept14: September 2014).

Figure 2 Figure 4

 24 Figure 2

Table 1 .

 1 Mean seawater characteristics during the five sampling campaigns. C1: 31 October 2013, C2: 19 February 2014, C3: 8 April 2014, C4: 13 June 2014 and C5: 18 September 2014.

		C1 C2 C3 C4 C5
	Salinity (psu)	34.5 34.4 34.6 34.7 34.6
	Temperature (°C)	17.7 8.2 11.1 14.7 17.9
	Oxygen saturation (%) 102 101 103 107 123
	Turbidity (NTU)	1.2 2.1 14.0 11.0 8.0

Table 2 .

 2 Main characteristics of tidal currents measured by the eastern ADCP.

	Main characteristics	Neap tide	Spring tide
	Tidal range (m)	2.0	6.5
	Maximum flood current velocity (m.s -1 )	0.10	0.20
	Maximum ebb current velocity (m.s -1 )	0.30	0.70
	Flood current (% of time)	40%	33%
	Ebb current (% of time)	60%	66%

Table 3 .

 3 Characteristics of sediments and macrofauna collected from sampling stations. TOC (%): Total Organic Carbon; FP (%): Fine Particles < 50 µm in dry sediment. TR: Total taxonomic richness per 0.5 m²; A: Total abundance (ind.0.5m²); H': Shannon-Weaver diversity index in log2; J': Pielou's evenness index. Mean values of the Benthic Indices (AMBI, M-AMBI, and BO2A) calculated with the five replicates for each station. C1: 31 October 2013, C2: 19

	February 2014, C3: 8 April 2014, C4: 13 June 2014 and C5: 18 September 2014. Ecological Quality Status (EcoQS), red: bad; poor: orange; moderate:
	yellow; good: green; high: blue.
	Stations/campaigns TOC (%)	% FF	TR	A	H'	J'	AMBI	M-AMBI	BO2A
	OR	C1	0.59	10.0	55	3,920	1.53	0.26	2.72	0.69	0.002
		C2	0.53	11.5	59	3,381	1.70	0.29	2.64	0.73	0.005
		C3	0.35	11.3	51	3,340	1.64	0.29	2.63	0.69	0.005
		C4	0.54	11.8	44	2,935	2.30	0.42	2.32	0.76	0.008
		C5	0.43	17.4	78	7,022	211	0.34	2.37	0.88	0.005
	ER	C1	0.53	17.5	70	3,250	3.14	0.51	2.42	0.95	0.024
		C2	0.70	14.0	58	3,089	2.63	0.45	2.28	0.86	0.012
		C3	0.62	18.4	55	3,190	2.51	0.43	2.27	0.83	0.010
		C4	0.61	17.6	45	3,065	2.56	0.47	2.51	0.77	0.031
		C5	0.67	12.7	57	7,792	2.21	0.38	2.39	0.80	0.010
	EC16	C1	0.41	6.5	38	1,248	1.74	0.33	3.03	0.61	0.046
		C2	0.43	5.6	49	3,007	2.59	0.46	3.01	0.75	0.072
		C3	0.33	5.5	33	2,164	2.08	0.41	2.21	0.70	0.008
		C4	0.37	15.1	47	8,451	2.06	0.37	2.71	0.71	0.024
		C5	0.39	7.8	53	3,889	2.28	0.40	3.02	0.73	0.065
	EC8	C1	0.32	1.6	22	445	2.65	0.59	3.13	0.63	0.123
		C2	0.41	2.7	32	1,973	2.91	0.58	3.66	0.65	0.142
		C3	0.34	6.5	34	3,119	1.97	0.39	2.81	0.64	0.024
		C4	0.29	19.5	39	7,324	1.70	0.32	3.21	0.60	0.077
		C5	0.37	1.8	45	2,054	2.23	0.41	3.50	0.65	0.147
	OC8	C1	0.59	4.7	37	1,502	2.17	0.42	2.75	0.68	0.022
		C2	0.61	5.8	57	3,648	2.36	0.40	3.04	0.76	0.052
		C3	0.61	14.6	55	6,913	1.78	0.31	2.99	0.69	0.015
		C4	0.73	4.4	59	11,355	1.76	0.30	3.19	0.69	0.039
		C5	0.67	16.1	65	7,964	2.36	0.39	3.25	0.77	0.082
	CC8	C1	0.79	2.1	48	3,177	3.07	0.55	3.79	0.72	0.162
		C2	0.85	1.2	35	2,113	2.80	0.55	3.88	0.63	0.157
		C3	0.68	6.5	43	1,693	2.34	0.43	2.97	0.70	0.031
		C4	0.94	5.7	34	3,284	2.04	0.40	3.52	0.58	0.065
		C5	0.83	6.0	48	2,989	2.35	0.42	3.60	0.66	0.164

Table 4 .

 4 Results of two-way ANOVA statistical tests. C1: 31 October 2013, C2: 19 February 2014, C3: 8 April 2014, C4: 13 June 2014 and C5: 18 September 2014.

				F	p-value Tukey test
	Taxonomic richness Dates	4	2.82 <0.05	C1≠C5
		Stations 6	2.31 <0.05	ER≠EC8
	Abundance	Dates	4	14.43 <0.001 C2≠C4, C5
						C1≠ C4, C5
						C3≠ C4, C5
		Stations 6	6.57 <0.001 OC8≠EC16, CC8, EC8, ER, OR
	H'	Dates	4	3.87 <0.01	C2≠C3, C4
		Stations 6	4.41 <0.001 OR≠ER, CC8
						OC8≠ER, CC8
						CC8≠EC16
	J'	Dates	4	4.42 <0.01	C4≠C1, C2
		Stations 6	11.67 <0.001 OR≠EC16, ER, EC8, CC8
						CC8≠EC16
						OC8≠ER, EC8, CC8
	AMBI	Dates	4	2.45 <0.05	C5≠C3
		Stations 6	24.75 <0.001 ER≠EC16, OC8, EC8, CC8
						OR≠OC8, EC8, CC8
						EC16≠OC8, CC8
						CC8≠OC8
	M-AMBI	Dates	4	1.14 0.34	-
		Stations 6	4.73 <0.001 ER≠EC16, EC8, OC8, CC8
	BO2A	Dates	4	6.37 <0.001 C3≠C1, C2, C5
						C5≠C2
		Stations 6	23.3 <0.001 OR≠OC8, CC8, EC8
						ER≠EC8, CC8
						EC16≠EC8, CC8
						OC8≠EC8, CC8
						CC8≠EC16
	∑		143

Table 5 .

 5 Mean abundances per 0.5 m² for dominant taxa in the three zones identified around the salmon cages in the Rade de Cherbourg. EG: Ecological Group from AZTI consulted on 15 th February 2018 (see Fig.1for location of the six stations).

	Taxa	EG	CC8/OC8 EC16/EC8	OR/ER
	Apseudopsis latreillii	III	2538.9	1980.8	2669.5
	Notomastus latericeus	III	574.3	375.0	169.4
	Dipolydora giardi	IV	554.9	452.1	68.0
	Capitella minima	V	115.2	34.0	1.8
	Phyllodoce mucosa	III	66.3	70.7	15.5
	Gammarella fucicola	III	65.9	-	1.1
	Chaetozone gibber	IV	60.8	12.7	11.8
	Jassa herdmani	V	60.2	0.3	0.2
	Caulleriella alata	IV	50.0	10.3	7.6
	Cirriformia tentaculata	IV	44.6	16.7	2.6
	Cheirocratus intermedius	I	41.8	3.9	8.2
	Zeuxo holdichi	III	39.1	6.5	5.5
	Abludomelita obtusata	II	38.5	1.4	9.7
	Steromphala umbilicalis	I	19.6	-	0.5
	Neanthes acuminata	III	15.8	7.7	0.1
	Ampelisca tenuicornis	I	4.0	99.9	99.5
	Phtisica marina	I	0.3	60.0	62.0
	Maldanidae	I	1.1	59.4	450.1
	Lanice conchilega	II	1.1	34.7	31.9
	Tritia reticulata	II	15.1	12.7	9.0
	Hilbigneris gracilis	III	5.0	9.3	34.6
	Melinna palmata	III	-	0.9	80.7
	Thyasira flexuosa	III	0.1	6.4	77.6
	Acromegalomma vesiculosum	I	2.9	1.0	38.7
	Apseudes talpa	II	-	0.1	25.8
	Abra alba	III	3.9	7.4	12.0

Table 6 .

 6 Sites and habitats showing high abundance (mean abundance and maximum abundance ind.m²) of the tanaid Apseudopsis latreilli in the World Ocean. OM: % Organic Matter in dry sediment.

		Sites	Benthic habitat	Abundance mean max	OM (%)	Reference
	English Channel	Rade de Cherbourg, Normandy, France Mixed, coarse sand and gravel	5,078 20,700	3.5	This study
			Muddy fine sand	4,339 10,320	3.0	This study
			Fine sand	3,962 14,640	2.5	This study
			A. squamata and A. latreilli mixed sediment 3,151	4,777	3.7	Baux, 2015
			M. palmata muddy sand	2	3	6.5	Baux, 2015
			M. palmata mixed muddy fine sand	2,690	4,720	3.2	Baux, 2015
			S. decorata fine sand	453	2,130	1.2	Baux, 2015
			A. latreilli and S. decorata in very fine sand	2,250	6,280	3.5	Baux, 2015
		Baie du Becquet, Normandy, France	A. squamata and A. latreilli mixed sediment 8,900 22,787	1.6	Martinez, 2017
			C. coarse mixed sediment	2,563	5,150	2.0	Martinez, 2017
			Amphipods and polychaetes in muddy sand	6,822 13,770	1.7	Martinez, 2017
		Aber Wrach, Brittany, France	Intertidal S. decorata fine sand	740	-	-	Hily and Bouteille, 1999
			Intertidal Z. marina meadows	3,086	-	-	Hily and Bouteille, 1999
			Fine sand A.alba-H. bilineata community	2,535	3,608	-	Dauvin and Gentil, 1990
	Atlantic	Galicia, Ria de Grove, Spain	Subtidal Z. marina meadows	5,940	9,640	-	Esquete et al., 2011
			Fine sand community	398	2,161	1.8	Lourido et al., 2008
			Coarse sand community	348	2,289	1.5	Lourido et al., 2008
			Muddy fine sand community	1,778	5,321	3.1	Lourido et al., 2008
		Ria de Formosa, Portugal	Subtidal Z. marina meadows	1,528	-	2.8	Gamito et al., 2012
	Mediterranean Sea South-eastern Spain	Sandy soft-bottom in Portman Bay	1,432	-	-	Marin-Guirao et al., 2005
			Marine caves of Granada's coast	2,106	-	-	Navarro-Barranco et al., 2013
		Castellon coast Spain	Well-sorted fine sand community	-	1,033	-	De-la-Ossa-Carretero et al., 2010
		western Italian coast	Well-sorted fine sand community, Pisa	4,335	-	-	Tataranni and Lardicci, 2010
	Marmara Sea, Turkey	Well-sorted fine sand community	80	166	2.85	Ates et al., 2014
	Persian Gulf, Dubai creek	Muddy sediment under organic pollution	17,650 43,000		Sauders et al., 2007

Acknowledgements

We are grateful to the staff of "GMG Saumon de France" and the crew of the company's vessel for allowing us to sample close to the cage in the Rade de Cherbourg. We thank the GEDANOR project for support funded by the Conseil Régional de Basse-Normandie (CRBN) and Ports Normands Associés (PNA). We also thank the crew of the N.O. Côtes de la Manche (CNRS-INSU) for sampling during one campaign, the staff of M2C laboratory for their help in sampling and technical operations, particularly C. Brunaud and C. Thouroude.

Finally, we thank M. Carpenter for the post-editing the English style and grammar, and the reviewer of the first version of this paper for his very useful comments and suggestions.


Raluca I Băncilă
email: raluca.bancila@ieu.uzh.ch

Roger Pradel

Rémi Choquet

Rodica Plăiaşu

Olivier Gimenez

Using temporary emigration to inform movement behaviour of cave-dwelling invertebrates: a case study of a cave harvestman species

Keywords: animal movement and behavior, cave-dwelling species, individual heterogeneity, season, temporal emigration

Cave systems are less transparent than surface habitats and significantly less is known about movement patterns and intrinsic behavioural processes of cave-dwelling animals. Here, we introduce temporary emigration (TE), the probability that an individual is absent from a cave at a given sampling occasion, to inform about movement and behavior of cave-dwelling animals. Because these animals are difficult to study with traditional tracking techniques, we resort to the capture-recapture (CR) modeling framework to assess TE to account for detectability less than one and unobservable states. We investigated the influence of cave, surface weather variables, season, individual heterogeneity and sex on the TE of the subtroglophile harvestman species, Paranemastoma silli. We used multistate CR models on 999 individually marked adults of two P. silli populations inhabiting two caves, Closani and Lazului Caves, located in the Mehedinti Mountains in southwestern Romania. Adult harvestmen demonstrated clear seasonality and heterogeneity in TE. We found that TE of harvestmen exhibited between caves variation and there was little support for surface weather variables in controlling TE and for sex specific TE patterns. Our results highlight that the study of TE patterns is useful to gain valuable insights into movement and intrinsic behavioural processes of cave-dwelling species.

Introduction

Understanding animal movements is important in animal ecology because of the profound implications for population dynamics [START_REF] Piper | Making habitat selection more ''familiar'': a review[END_REF]. Animal movements are inextricably bound to behavioural processes [START_REF] Miriam | The behavioral ecology of animal movement: reflections upon potential synergies[END_REF]), e.g. animals move to feed, find mates and avoid predators or unfavorable environmental conditions. Hence understanding why and how animals move is not complete without an understanding of the animal behaviour. Furthermore, several studies have documented individual variation (individuality) in movement patterns [START_REF] Vardanis | Individuality in bird migration: routes and timing[END_REF][START_REF] Brodersen | Variable individual consistency in timing and destination of winter migrating fish[END_REF].

The recognition of individuality can provide fresh insights in the causes and consequences of individual differences in movement that behavioural ecology has already taken.

The literature exploring animal movement and behaviour is primarily focused on terrestrial and oceanic systems. Cave systems are much less transparent and significantly less is known about movement of cave-dwelling animals. Understanding where and how cave species travel through space is particularly important. This will allow us to gain valuable insights into broader ecological effects and consequences of general processes such as subsurface dispersal or migration [START_REF] Culver | The biology of caves and other subterranean habitats[END_REF] or behavioural processes such as foraging and breeding movements [START_REF] Woiwod | Insect movement: mechanisms and consequences[END_REF].

However, cave-dwellers, especially invertebrates, are difficult to study. The most commonly used techniques (e.g. radio transmitters or satellite technology) to track animal movement patterns and behavioural processes [START_REF] Rubenstein | From birds to butterflies: animal movement patterns and stable isotopes[END_REF]) are of no use in caves. Individual tags provided good data for larger animals but not for many smaller ones. Advances in the use of stable isotope markers offer new promise for insects [START_REF] Hobson | Flying fingerprints: making connections with stable isotopes and trace elements[END_REF] but the cost are still prohibitive.

Multistate capture-recapture (CR) methods allow estimation of temporary emigration (TE) [START_REF] Pradel | Multievent: An extension of multistate capture-recapture models to uncertain states[END_REF]. TE is the transition from an "observable" state to an "unobservable" state, and back to an "observable" state (Chapple el al. 2016). TE is widely recognized in animals and plants and was attributed to a physiological condition, reproductive status, behavioural state or location [START_REF] Schaub | Estimating survival and temporary emigration in the multistate capture-recapture framework[END_REF]). Here, we argue that an important application of TE, especially for cave-dwellers, is to inform about animal movements over space, e.g. emigration from and immigration to cave. Frequently sampling is conducted only in cave. Considering a multistate CR model with states "observable" and "unobservable" between which individuals are allowed to move, the transition from an "observable" to an "unobservable" state is the emigration, the probability that an individual has moved out (emigrated) from cave between capture occasion i and i+1 while the transition back to an "observable" state is the immigration, the probability that an individual has returned (immigrated) to cave during capture occasion. Mutistate CR methods have been also developed to model individual heterogeneity in capture and demographic rates [START_REF] Pledger | Open capture-recapture models with heterogeneity: I. Cormack-Jolly-Seber model[END_REF][START_REF] Pradel | The stakes of capture-recapture models with state uncertainty[END_REF][START_REF] Gimenez | Individual heterogeneity in studies on marked animals using numerical integration: capture-recapture mixed models[END_REF] and there is increased evidence that unmodeled individual heterogeneity causes bias in capture and survival estimates [START_REF] Pledger | Correction of bias due to heterogeneous capture probability in capturerecapture studies of open populations[END_REF][START_REF] Fletcher | Bias in estimation of adult survival and asymptotic population growth rate caused by undetected capture heterogeneity[END_REF]. While individual heterogeneity is expected in movement patterns, no CR study investigated the effect of individual heterogeneity on TE.

Subtroglophiles are a category of non-obligate cave-dwellers defined as species intimately associated to cave environments, but requiring the epigean habitats during some period, or to complete some biological function such as reproduction or feeding (Lunghi et al. 2013). Those species likely have strong seasonality in the emigration/immigration rates. One can hypostatize high emigration and low immigration in spring, summer and fall and low emigration and high immigration in winter. Due to reliance on surface habitat of the subtroglophiles the emigration/immigration rates may depend also on cave microclimate or surface environmental factors, e.g. surface weather variables. Moreover, it is commonly assumed that intra-specific competition avoidance plays an important role in promoting emigration [START_REF] Dieckmann | The evolutionary ecology of dispersal[END_REF]. In caves the food resources are very scarce and cave-dwellers might experience strong inter-and intra-specific competition. The variation in competitive abilities may generate individual heterogeneity in emigration/immigration rates. Individuals that decide to remain inside cave are expected to be more competitive and fight for these scarce resources while individuals that decide to leave and feed outside cave are expected to be less competitive.

In this study we investigated the influence of cave, surface weather variables, season, individual heterogeneity and sex on the TE of the subtroglophile harvestman species, Paranemastoma silli. We used a multistate CR model on 999 individually marked adults of two P. silli populations inhabiting two caves, Closani and Lazului Caves, located in the Mehedinti Mountains in southwestern Romania. To quantify individual heterogeneity, we used two discrete classes of individuals that refer to two possible paths: competitive individuals and uncompetitive individuals. We predict that season and individual heterogeneity have significant effect on TE. We expect that less competitive individuals to show high seasonal TE while more competitive individuals will experience lower TE regardless of season. We predict that TE is different between the two caves because the caves are characterized by different microclimates. We expect that surface weather variables have an effect on TE because weather is known to affect the demography of organisms. We predict sex-specific patterns in TE because of possible disparate demography or behaviour between the sexes.

Material and methods

Study area and capture details

Paranemastoma silli (Hermann 1871; Dyspnoi: Nemastomatidae) is an subtroglophilic endemic harvestman species in the Carpathian Mountains [START_REF] Avram | Contributii la cunoasterea raspindirii geografice si a ecologiei opilionidelor cavernicole, endogee si epigee, din Romania[END_REF]) that inhabits caves but also habitats outside caves, especially beech forests.

We conducted a CR study of P. silli in two caves: Closani and Lazului. The caves are located in Mehedinti Mountains, southwestern Romania, at 433 and 370 m a.s.l., respectively. The caves are ca. 3 km apart.

The caves differ in the hydrological regime, surface vegetation and food resources. Closani Cave is a hydrologically inactive cave and the outside habitat consists mainly of dry scrubs. Lazului Cave is a hydrologically active cave and is located in a beech forest. Closani Cave has an old guano deposits whereas in Lazului Cave little amounts of fresh guano are present.

We conducted visual encounter surveys to search for harvestmen in teams of two researchers once a month between December 2008 and November 2009, separately in each cave. We individually marked all caught adult harvestmen using non-toxic acrylic ink (Mestre amd Pinto-da-Rocha 2004). A combination of different colour marks that were still recognizable after several recaptures was used to individually mark each harvestman. The marked individuals were released at the place where they were collected. The immature stages were not marked because they moult six times in ~2.5 months [START_REF] Avram | Contribution à la connaissance du développement embryonnaire et postembryonnaire chez Nemastoma cf. sillii Herman (Opiliones, Nemastomatidae)[END_REF]. The adult harvestmen were sexed based on the cheliceral morphology.

The daily temperature and rainfall values were recorded at Closani village, with an iROX PRO-X portable weather station. Temperature and rainfall were included in the study as mean air surface temperature and mean precipitation amount during the day of capture occasion and during the week prior to the capture occasion (tempm, precipm, tempw and precipw, respectively). All environmental covariates were standardized for the analysis.

Data analysis

We analysed the CR data of 999 adult harvestmen for a period of 11 months using multistate CR models [START_REF] Lebreton | Modeling individual animal histories with multistate capture-recapture models[END_REF]) expressed as multievent models [START_REF] Pradel | Multievent: An extension of multistate capture-recapture models to uncertain states[END_REF]. We recorded the 'events' as "seen" and "not seen" and considered the states "alive and observable" (OU) and "alive and unobservable" (UO) and "dead" (D). Individuals that are in the state "unobservable" and dead during a sampling occasion are unavailable for recapture, therefore the corresponding recapture probability is fixed to 0.

To include individual heterogeneity in TE, we built two discrete classes of individuals, g 1 and g 2 , which refer to two possible paths at first observation. The classes are states of the multievent model: "alive and observable in g 1 ", "alive and unobservable in g 1 ", "alive and observable in g 2 ", "alive and unobservable in g 2 ". The classes are associated with distinct values of the initial states.

For model formulation we considered: i) , the vector of probabilities of presence in the various initial states ( ), ii)

, the matrix of survival probabilities (S); iii) , the matrix of transition probabilities ( ), corresponding to TE ; and iv) , the matrix of detection probabilities (p). The full description of the vectors and matrixes can be found in the Appendix S1.

Because the transition probabilities depend only on the states in which an individual was before a transition we modeled TE as a Markovian or state-dependent transition [START_REF] Schaub | Estimating survival and temporary emigration in the multistate capture-recapture framework[END_REF]).

Model selection

We were interested in TE, but different model structures can be used to model the sampling process and the survival on which the TE is conditional. In total we designed 119 models. We used E-SURGE 1.9.0 (Choquet et al. 2009a) to perform model selection based on Akaike's Information Criterion corrected for small sample size (QAICc) (Burnham and Anderson 1998) and to obtain maximum likelihood estimates of the parameters. In the results, for brevity we choose to present only the models with ΔAICc < 20.

Goodness-of-fit

Goodness-of-fit tests were performed on CR histories. We used Test3.SR for transience (lower chance of recapture of first encountered individuals than individuals already encountered [START_REF] Pradel | Capture-recapture survival models taking account of transients[END_REF]) and Test2.CT for trap-dependence and for trap-happiness (higher probability to encounter at time t+1 the individuals encountered at time t than to individuals not encountered at time t but known to be alive because of previous and future recaptures). We computed the overall and separately on cave and sex GOF-statistics for transience and trap-dependence, from Test3 and Test2 respectively. The GOF tests were performed using U-CARE (Choquet et al. 2009b).

Results

The directional tests for transience and trap-happiness were statistically not significant, thus there was no need to correct for transience or trap-happiness or to account for overdispersion (Table 1).

From the set of the candidate models, the model with heterogeneity and season effect on TE, along with cave and sex on survival and cave, sex and tempm on recapture was best supported by the data (Table 2). The model with no heterogeneity, season and cave effect on TE, along with cave and sex on survival and time on recapture received some support and was the second-ranked at 3.860 ∆AICc after the best model (Table 2).

We estimated parameters from the best model (Fig 123). Recapture decreased with the increase of the temperature during the day of capture occasion in both caves and sexes (Fig. 1). Recapture was the higher in males from Closani than from Lazului cave and lower in females from Closani than from Lazului cave (Fig. 1). The emigration probability was high in autumn, spring and summer for g 1 individuals and low for g 2 in all seasons except winter (Fig. 2). The immigration was high in autumn for g 1 individuals and high in spring and summer for g 2 individuals (Fig. 3).

Discussion

In this study the application of a multistate CR model was able to estimate relevant demographic parameters to inform movement patterns and behavioural processes, when data cannot be collected for other established frameworks, e.g. animal tracking techniques. Our approach revealed clear seasonality and captured individual heterogeneity in the TE of a subtroglophile harvestman species.

Seasonal variation in TE is likely due to foraging behavior. Foraging movements from cave were reported for many bat [START_REF] Kunz | Ecology of Bats[END_REF]) and several cricket species [START_REF] Fagan | Population and community consequences of spatial subsidies derived from central-place foraging[END_REF]). Strong seasonality is also in agreement with studies on activity rhythm of non-obligate cave harvestmen species. Several studies showed that harvestmen use caves as shelters and reproductive sites but leave the cave periodically to forage [START_REF] Hoenen | Activity rhythms and behavioral characterization of two epigean and one cavernicolous harvestmen (Arachnida, Opiliones, Gonyleptidae)[END_REF][START_REF] Machado | Aspects of the population ecology of Goniosoma sp (Arachnida Opiliones Gonyleptidae) in limestone caves in southeastern Brazil[END_REF].

Using assumptions of two heterogeneity classes during model construction, we showed significant different patterns of emigration/immigration among seasons between the two classes.

Although we can only speculate on this the heterogeneity in TE is likely linked to differences in the competitive abilities for food. The scarcity of the food resources in caves is common and it has been shown that non-obligate cave-dwellers forage primarily external rather than within cave [START_REF] Kunz | Ecology of Bats[END_REF][START_REF] Fagan | Population and community consequences of spatial subsidies derived from central-place foraging[END_REF]). However, some individuals reside inside the cave and rely upon the resources available in cave, maybe due to their better competitive abilities. This question requires experimental studies to test whether individuals showing good competitive abilities are more likely to reside in cave and if these inter-individual differences are consistent across time.

The results further indicated that once the heterogeneity is not accounted for, season and cave drive the observed emigration/immigration rates. The two studied caves have different microclimate [START_REF] Decu | Recherches sur les Grottes du Banat et d'Olténie[END_REF]. They are characterized by different hydrological regime, airflow, temperature, humidity, guano deposit and outside surface habitat. This indicates that TE may be promoted by variability in cave microclimate.

We found little support for surface weather variables in controlling TE. Weather is known to affect directly and indirectly the demography of organisms [START_REF] Moreno | Extreme climatic events in relation to global change and their impact on life histories[END_REF]. The lack of an effect of surface weather variables on TE suggests that caves buffer well the fluctuations of weather factors in the habitats above and reinforce the hypothesis of caves as refuges from unfavorable surface environmental conditions (e.g., too dry or too hot) [START_REF] Manenti | Habitat features and distribution of Salamandra salamandra in underground springs[END_REF][START_REF] Manenti | Caves as breeding sites for Salamandra salamandra: habitat selection, larval development and conservation issues[END_REF][START_REF] Manenti | Odonata occurence in caves: active or accidentals? A new case study[END_REF][START_REF] Ficetola | Do cave salamanders occur randomly in cavities? An analysis with Hydromantes strinatii[END_REF].

Results also showed no strong support for sex differences in TE. Sexual disparity in TE was linked to disparate demography or behaviour between sexes [START_REF] Chapple | A novel application of multi-event modeling to estimate class segregation in a highly migratory oceanic vertebrate[END_REF]. Movement is the result of interactions between morphology, physiological performances (movement capacity), and behavioural decisions (movement decision) (Ducatez et al., 2013). Additional studies are necessary to test TE sexdependency in correlation with various movement traits.

TE was until recently viewed as causing problems in CR studies and several approaches have been developed for dealing with it [START_REF] Fujiwara | Estimating population projection matrices from multi-stage markrecapture data[END_REF][START_REF] Kendall | Estimating state-transition probabilities for unobservable states using capture-recapture/resighting data[END_REF]. In this study we presented TE [START_REF] Schaub | Estimating survival and temporary emigration in the multistate capture-recapture framework[END_REF]) as the parameter of biological interest and we provided a pertinent framework within which to consider TE as transition between an observable and unobservable state. The TE approach of a multistate CR model can be applied to other studies to gain information on movements pattern and behavioural processes of animals difficult to track with the available intrinsic and extrinsic tracking techniques. Moreover, it can be used when a robust design [START_REF] Pollock | A capture-recapture design robust to unequal probability of capture[END_REF]), e.g. sampling replication (secondary occasions) within each season (primary occasion) cannot be implemented.

However, it is still preferable to sample all potential locations of marked individuals and we recommend muti-state CR approach where unobservable states are inevitable but in general we recommend setting up a study under [START_REF] Pollock | A capture-recapture design robust to unequal probability of capture[END_REF] robust design. 2).

Figure 2. Estimated seasonal emigration rates (SE) of harvestmen form the two heterogeneity classes, g 1 (grey) and g 2 (black). Estimates are based on the most parsimonious CR model (Model 1 in Table 2). 2).

Figure captions Figure 1 .

 captions1 Figure captions

Figure 3 .

 3 Figure 3. Estimated seasonal immigration rates (SE) of harvestmen form the two heterogeneity classes,

  For, example one may hypothesize that the recapture differs among observers due to different levels of experience. Both capture and survival probabilities may vary with time, (e.g. occasion, season) or time specific covariate (e.g., weather) or harvestmen can exhibit cave and sex-specific capture and survival probabilities. Consequently, we

considered models representing different combinations of observer, time, cave, sex, season, tempm and precipm on capture probability and cave, sex, season, tempw and precipw on survival probability. To assess the hypotheses of TE we built models representing combinations of same covariates as for survival probability but we distinguished between models with and without individual heterogeneity by considering no classes or two classes, respectively, in matrix of transition probabilities, associated with distinct values of the initial states (for details see Appendix S1).

Table 1 .

 1 Goodness-of-fit tests showing no transience, trap-happiness or overdispersion.

	Closani
	Cave

Table 2 .

 2 Model selection with deviance (DEV), Akaike's Information Criterion corrected for small sample size (AICc), differences in AICc (ΔAICc) and number of estimable parameters (K) for the candidate models with ΔAICc < 20 (for brevity models with ΔAICc >20 are not shown).The candidate models vary in the presence (h+) and absence (h-) of heterogeneity in temporary emigration (TE) and effect of cave (a group variable with two categories: Closani Cave and Lazului Cave), sex (a group variable with two categories: female and male), time (capture occasion time effect), tempm (mean air surface temperature during the day of capture occasion), tempw (mean air surface temperature during the week prior to the capture occasion).

	Model		Survival	TE	Recapture	K	DEV	QAICc	∆AICc
	1	h+	cave+sex+cave:sex season	cave+sex+tempm	30	5.931.324 5.991.946 0.000
	2	h-	cave+sex	cave+season	time	23	5.949.438 5.995.806 3.860
	3	h+	cave+sex	cave+season	cave+sex+season	28	5.942.382 5.998.924 6.979
	4	h+	cave+sex	cave+season	sex+season	28	5.943.232 5.999.774 7.828
	5	h+	cave+sex	cave+sex+tempw cave+sex+tempm	46	5.907.283 6.000.735 8.789
	6	h+	cave+sex	season	cave+season	26	5.947.876 6.000.344 8.398
	7	h+	cave+sex	sex+season	cave+sex+tempm	38	5.924.895 6.001.888 9.942
	8	h-	cave+sex+season	cave+sex	cave+sex+tempw+precipw	28	5945.477 6002.020 10.074
	9	h-	cave+sex+season	cave+sex	cave+season	20	5966.626 6006.905 14.959
	10	h-	cave+sex+season	cave+sex	cave+sex+season	24	5959.481 6007.881 15.935

Acknowledgements

This work was supported by a short-term postdoctoral research fellowship offered by French Embassy and French Institute of Romania and a grant of Romanian National Authority for Scientific Research, CNCS -UEFISCDI, projects number: PN-II-RU-TE-2014-4-1536 to RB.