Hernvann Pierre-Yves
email: pierre.yves.hernvann@gmail.com

Gascuel Didier

Exploring the impacts of fishing and environment on the Celtic Sea ecosystem since 1950

Keywords:

Analyzing the long-term changes in ecosystems and disentangling the influence of overfishing and the environment require historical data integration. Fisheries-independent data are available only since the mid-1980s in the Celtic Sea and thus provide a short-term and truncated vision of fishing impacts. We conducted a catch-based study over the 1950-2015 period. We successively (i) reconstituted catch time-series, including unreported catch estimates, and studied them through a trophic-spectrum approach and ecosystem indicators based on trophic level, size and species diversity; (ii) estimated biomass trends of the main Celtic Sea target species from catch and effort time-series, using production models that detect potential shifts in productivity; and (iii) explored linkages between species productivity potential and both fishing and environment using long-term time-series of large-scale climatic indices, locally observed hydroclimatic variables and plankton data from the Continuous Plankton Recorder. Our results highlight that fishing has caused a drastic depletion of the main exploited species in the Celtic Sea since World War II. The biomass reduction

Introduction

The second half of the twentieth century was characterized by a dramatic increase in fishing pressure worldwide (Rousseau et al., 2019, Bell et al., 2017), and European seas were affected too.

Although World War II gave a respite to European fish stocks [START_REF] Letaconnoux | Effets de la guerre sur la constitution des stocks de poissons[END_REF], this respite was short [START_REF] Holm | World War II and the 'Great Acceleration' of North Atlantic Fisheries[END_REF]. Postwar fishing capacity rebuilding plans, intra-European competition and, later, fleet modernization plans drove large increases in the fishing effort until the late 1980s [START_REF] Meuriot | Fishing fleet replacement[END_REF][START_REF] Mesnil | Public-aided crises in the French fishing sector[END_REF]Guénette and Gascuel, 2009). Despite the establishment of the Common Fisheries Policy (CFP) in 1983 and its consecutive reforms, overfishing was not reduced before the late 2000s (European Commission, 2001, 2009;Daw and Gray, 2005;[START_REF] Gascuel | Fishing impact and environmental status in European seas: a diagnosis from stock assessments and ecosystem indicators[END_REF]. In addition to fishing, European seas have experienced major environmental changes since the mid-twentieth century. Long-term variability in hydroclimatic conditions induced critical changes in planktonic production [START_REF] Planque | Long-term changes in zooplankton and the climate of the North Atlantic[END_REF][START_REF] Edwards | Long-term and regional variability of phytoplankton biomass in the Northeast Atlantic (1960-1995)[END_REF][START_REF] Beaugrand | Monitoring pelagic ecosystems using plankton indicators[END_REF], and these changes propagated through the food web to finally impact the overall dynamics and productivity of Northeast Atlantic ecosystems. In particular, such changes could push some ecosystems from one stable state to another through ecosystem regime shifts [START_REF] Scheffer | Catastrophic shifts in ecosystems[END_REF][START_REF] Collie | Regime shifts: Can ecological theory illuminate the mechanisms?[END_REF].

Elucidating the long-term impacts of fishing and environment on ecosystems and disentangling their respective effects is a key challenge to better understanding ecosystem functioning [START_REF] Kirby | Synergistic Effects of Climate and Fishing in a Marine Ecosystem[END_REF][START_REF] Gaichas | What drives dynamics in the Gulf of Alaska? Integrating hypotheses of species, fishing, and climate relationships using ecosystem modeling[END_REF][START_REF] Lotze | Historical Changes in Marine Resources, Food-web Structure and Ecosystem Functioning in the Adriatic Sea, Mediterranean[END_REF] and providing valuable insight for fisheries management [START_REF] Pitcher | Fisheries Managed to Rebuild Ecosystems? Reconstructing the Past to Salvage the Future[END_REF][START_REF] Borja | Medium-and Long-term Recovery of Estuarine and Coastal Ecosystems: Patterns, Rates and Restoration Effectiveness[END_REF][START_REF] Thurstan | Filling historical data gaps to foster solutions in marine conservation[END_REF]. In this context, the analysis of historical abundance trends of all ecosystem compartments is a cornerstone. By combining both fisheries-dependent and -fisheries-independent information, usual stock assessments provide valuable information. However, stock assessments refer to a limited part of marine resources and often begin once stocks have already been intensely exploited. One way to address this issue is to extend biomass estimates beyond the period covered by a stock assessment based on partial timeseries of catch per unit efforts (CPUEs) [START_REF] Rosenberg | The history of ocean resources: modeling cod biomass using historical records[END_REF][START_REF] Eero | Extending time series of fish biomasses using a -simple surplus production-based approach[END_REF]. When information is scarce, another option is to work with catch data while noting that it is influenced by fishing strategies (Pauly et al., 2013).

As one of the most exploited seas in Europe and its neighboring ecosystems that has experienced major environmental changes in recent decades (Reid et al., 2001;[START_REF] Southward | The Western English Channel-an inconstant ecosystem[END_REF], the Celtic Sea likely underwent deep changes in its structure and functioning since 1950. However, these changes are poorly known, and few stock assessments started before the 1980s in this area.

Stock assessments show that the fishing pressure has continuously increased since the mid-1980s and culminated in the late 1990s when the abundance of exploited stocks reached its historical minimum level [START_REF] Bentorcha | Using trophic models to assess the impact of fishing in the Bay of Biscay and the Celtic Sea[END_REF][START_REF] Gascuel | Fishing impact and environmental status in European seas: a diagnosis from stock assessments and ecosystem indicators[END_REF]. Additionally, fisheries-independent survey data show that fishing altered the trophic and size structures of the ecosystem (Blanchard et al., 2005;Shephard et al., 2012;[START_REF] Pinnegar | Long-term changes in the trophic level of the Celtic Sea fish community and fish market price distribution[END_REF][START_REF] Gascuel | Fishing impact and environmental status in European seas: a diagnosis from stock assessments and ecosystem indicators[END_REF]. A meta-analysis performed by [START_REF] Rochet | Combining indicator trends to assess ongoing changes in exploited fish communities: diagnostic of communities off the coasts of France[END_REF], as well as more recently published ecosystem models [START_REF] Bentorcha | Using trophic models to assess the impact of fishing in the Bay of Biscay and the Celtic Sea[END_REF][START_REF] Moullec | Trophic models: What do we learn about Celtic Sea and Bay of Biscay ecosystems[END_REF], confirm that the Celtic Sea ecosystem has been highly impacted by fishing and was still deteriorating in the mid-2000s. The latter results suggest that a decrease in fishing pressure recently promoted the recovery of the ecosystem, but only to a level comparable to that of the 1980s. Additionally, [START_REF] Guénette | Shifting baselines in European fisheries: The case of the Celtic Sea and Bay of Biscay[END_REF] suggested that most changes in the area may have occurred between 1950 and 1970; however, these analyses have failed to identify the impact of environment on ecosystem functioning. Therefore, our aim is to look deeper into the Celtic Sea history, from post-WWII fisheries development to the most recent years characterized by the implementation of the FMSY approach in Europe and to better quantify changes induced by both fishing and environment in the ecosystem after 1950. Thus, we successively (i) reconstituted catch time-series, including unreported catch estimates, for all species and studied them through a trophic-spectrum approach and ecosystem indicators; (ii) reconstituted effort time-series for 14 key species of the Celtic Sea ecosystem and fisheries; (iii) integrated reconstructed catch and fishing effort time-series in ad hoc production models to estimate biomass trends of the 14 species while exploring changes in their productivity; and (iv) explored linkages between species productivity potential and both fishing and environment.

Material and Methods

Study area

The Celtic Sea is a large region of the northwestern European continental shelf that spreads from the western English Channel to the start of the Porcupine Bank, Southwest Ireland. The Celtic Sea continental shelf displays a great variety of oceanographic conditions and habitats that result in wide biological diversity [START_REF] Ellis | Epibenthic assemblages in the Celtic Sea and associated with the Jones Bank[END_REF][START_REF] Martinez | The fish and fisheries of Jones Bank and the wider Celtic Sea[END_REF]. The Celtic Sea, here matching with ICES Divisions 7e-j, has been a crucial area for European fishing fleets for decades and is still so.

With more than 420,000 tons landed yearly during the last decade, it is essentially exploited by French (39% of catches for the 2010-2015 period, according to ICES statistics), Irish (21%) and UK fisheries (18%). Vessels operating in the Celtic Sea use several demersal gears, including bottom trawlers, beam trawlers, gillnetters, longliners and dredgers, and pelagic gears, including mainly pelagic trawls [START_REF] Laurec | Modelling technical interactions[END_REF][START_REF] Pelletier | A multivariate approach for defining fishing tactics from commercial catch and effort data[END_REF][START_REF] Marchal | Impact of technological creep on fishing effort and fishing mortality, for a selection of European fleets[END_REF]. Based on the targeted species, the fishing strategies result in heterogeneous exploitation of the continental shelf [START_REF] Sharples | Fishing and the oceanography of a stratified shelf sea[END_REF][START_REF] Mateo | Highly mixed fisheries: fine-scale spatial patterns in retained catches of French fisheries in the Celtic Sea[END_REF]. (iccat.int/en/accesingdb.html). Information on the landings of primary benthic producers was issued in the literature [START_REF] Arzel | Aménagement des pêcheries côtières de Nord-Bretagne, algues-crustacés[END_REF][START_REF] Arzel | L'exploitation des algues en Bretagne[END_REF]. The Statlant database includes only official landings; thus, it underestimates the total removals. To address this issue, complementary information from the Sea Around Us project (SAUP, [START_REF] Pauly | Sea Around Us Concepts, Design and Data[END_REF] was used. Catch reconstruction by the SAUP for the Celtic-Biscay Shelf Large Marine Ecosystem [START_REF] Miller | Reconstructing Ireland's marine fisheries catches: 1950-2010[END_REF][START_REF] Bultel | Catch reconstruction for the French Atlantic coast, 1950-2010[END_REF][START_REF] Gibson | Preliminary reconstruction of total marine fisheries catches for the United Kingdom and the Channel Islands in EEZ equivalent waters[END_REF] was used to estimate the relative quantity of unreported landings and discards regarding official landings. These rates were then applied to the reconstituted landings to convert them into catches. More details about Celtic Sea catch reconstitution can be found in Appendix S1.

Catch study

For the sake of clarity in the description of the results, low catch species were pooled using the trophic groups of the Celtic Sea Ecopath model developed by [START_REF] Moullec | Trophic models: What do we learn about Celtic Sea and Bay of Biscay ecosystems[END_REF]. The following study relies on reconstituted catches, and the approach described in paragraph 2.4 will also be explored using Statlant landings only.

• Catch-based indicators

The overall changes in catch composition were investigated by calculating Shannon's diversity index (H'y) [START_REF] Shannon | Communication Theory of Secrecy Systems*[END_REF], the mean maximum length (MMLy) the mean trophic level (MTLy) [START_REF] Jiming | A tentative analysis of the trophic levels of North Sea fish[END_REF][START_REF] Pauly | Fishing Down Marine Food Webs[END_REF] and the high trophic level Indicator (HTIy) [START_REF] Bourdaud | New trophic indicators and target values for an ecosystem-based management of fisheries[END_REF] for each year y. H' represents the species diversity and can identify whether fisheries rely on few species or exploit a wide range of species. The MML quantifies the relative importance of small and large species and is expected to decrease due to fishing, which drives down the proportion of species with large asymptotic size and slow growth and maturation. The last two indicators are related to the food-web structure. The MTL and the HTI, which correspond to the fraction of top predators in the total catch, also decline with increasing fishing impact.

The indices are calculated as follows:

′ = ∑ , * ( , ) (eq.1)
where Ps,y is the proportion of the species s in the total landings of year y.

= ∑ * , ∑ , (eq.2)
with L∞s the asymptotic length of species s, as reported in FishBase [START_REF] Froese | Fishbase[END_REF], and

Ys,y the catches of species s in year y. This index excludes invertebrate species.

= ∑ , * ! (eq.3)

where TLs were estimated from the Celtic Sea Ecopath model for monospecific groups and from FishBase for other species; Y tot y is the total catch amount in the Celtic Sea.

" = #$%& #$%' * 100 (eq.4)
where Y TL≥2 y and Y TL≥4 y are the total catches in year y of species with a TL higher than or equal to 2 and 4, respectively, in the Celtic Sea.

Finally, to obtain a more synthetic overview of changes in the trophic composition of catches, the trophic spectra of mean yearly catches [START_REF] Gascuel | The trophic-level based model: A theoretical approach of fishing effects on marine ecosystems[END_REF][START_REF] Gascuel | The trophic spectrum: theory and application as an ecosystem indicator[END_REF] were built by decades using the ET-Transpose function of the EcoTroph R package, which distributes each group/species on a range of TLs according to its own trophic variability [START_REF] Gascuel | The trophic-level-based ecosystem modelling approach: theoretical overview and practical uses[END_REF][START_REF] Colleter | An introduction to the EcoTroph R Package: analyzing aquatic ecosystem trophic networks[END_REF].

Trends in fishing pressure for the main targeted species

A standardized index of species-specific fishing pressure was estimated over the period of 1950-2015 for 14 species among the most exploited species in the Celtic Sea and those currently assessed by ICES. These species were selected because enough information can be found in the literature to reconstruct past fishing pressures; the species include cod (Gadus morhua), hake For the recent period, the F estimates available from ICES stock assessments, denoted * +,-, are the most reliable index of fishing pressure. To fill the gap between 1950 and the starting year of assessments, a search for data on species-specific fishing effort was first conducted. The ancillary documents and statistics that were gathered enabled the production of the nominal fishing effort time-series, . /01 (see Appendix S1 for methodological description), and these time-series were converted into effective fishing effort time-series using the mean annual rate of increase in fishing efficiency, α [START_REF] Gascuel | Analyse de l'évolution des puissances de pêche par l'analyse des corhortes : application aux senneurs exploitant l'albacore (Thunnus albacares) dans l'Atlantique est[END_REF]. α allows us to consider changes in fishing efficiency related to innovation in fishing technologies [START_REF] Fitzpatrick | Technology and fisheries legislation[END_REF][START_REF] Marchal | Impact of technological creep on fishing effort and fishing mortality, for a selection of European fleets[END_REF], and its value is defined according to the empirical equation proposed by [START_REF] Pauly | An empirical equation to predict annual increases in fishing efficiency[END_REF]. Effective efforts were expressed as a fishing mortality, * , by standardizing . /01 and * +,-over the first three years of the stock assessment, as follows:

(
* = 2
134/ 567:79' (: 5 ;<= ) 134/ 567:79' (-5 7!> ) * . /01 * (1-∝) /A , B < D * +,-, B ≥ D (eq.5)

where n is the first year of the assessment. Sensitivity analyses of the final results to the poorly known parameter α were conducted using α +/-50%. More details on fishing effort reconstruction are provided in Appendix S1.

Biomass trends for the main targeted species

For the 14 selected species, the time-series of catch and fishing pressure were used to fit the surplus biomass production model over the whole 1950-2015 period. Additionally, to estimate biomass trends since 1950, the approach we developed enables the detection of long-term changes in the productivity potential of each species.

• Formulation of the surplus production model Surplus production models assume that stock productivity depends on its abundance. Regardless of the initial conditions, the biomass of a stock fished with a constant effort tends to an equilibrium state determined by the fishing pressure level. According to the Fox model [START_REF] Fox | An Exponential Surplus-Yield Model for Optimizing Exploited Fish Populations[END_REF], biomass at equilibrium, B eq , can be expressed as a function of the fishing mortality, F, as:

F 3G = 3.I : J K . L A M M J K (eq.6)
where MSY is the maximum sustainable yield, which can be considered as a proxy of the productivity potential of the stock, and FMSY is the fishing mortality consistent with achieving the

MSY.

Though this equilibrium situation never occurs in reality [START_REF] Hilborn | Quantitative Fisheries Stock Assessment: Choice, Dynamics and Uncertainty[END_REF], [START_REF] Beckett | Fitting the generalized stock production model by least-squares and equilibrium approximation[END_REF] proposed to fit the model to observed abundance and effort data using a "pseudo-equilibrium" method. This empirical approach, considered as more robust than dynamic production models [START_REF] Fonteneau | Vingt-cinq ans d'évaluation des ressources thonières de l'Atlantique : quelques réflexions méthodologiques[END_REF][START_REF] Meissa | Assessing stocks in data-poor African fisheries: a case study on the white grouper Epinephelus aeneus of Mauritania[END_REF], assumes that the abundance at year y (By) is in equilibrium with the cumulative fishing mortality (FFox) over the k previous years in such a way that:

F = 3.I : J K . L NM MOP M J K (eq.7)
where MSY is the parameter to estimate, FMSY is taken from the ICES stock assessments (current estimates or proxy, or older estimates when not available), and FFox y is a linear combination of fishing mortality (Fy) over the k previous years:

* :QR = : NS * TU⋯U: NW * (XAT)U: * X TU⋯U(XAT)UX (eq.8)
k is related to the number of main exploited age classes and is conventionally fixed to 2 for small pelagic species and 3 for other species.

• Integration of changes in species productivity potential Usually, surplus production models assume that fishing pressure drives changes in species abundance, in turn modifying the products according to a specific relationship. Hereafter, we considered that the environment could affect production by inducing a shift from one biomassproductivity relationship to another. This effect was integrated in our approach following Fréon (1988), who assumed that the environment impacts both the carrying capacity and the rate of population increase (K and r, respectively, while maintaining ln(K)/r constant), resulting in a timevarying MSY within the Fox model (while one FMSY value per productivity regime would also be required within Schaeffer 's model;Fréon, 1988). Temporal changes in the MSY value were postulated to follow regime-like dynamics. The timing of shifts in the productivity potential was investigated and integrated into the surplus production model to estimate one different MSY for each putative regime.

MSY shifts identification was based on rearranged eq.9:

D( YZ) = D F + D(* I ) + : MOP : J K + 1
(eq.9)

Eq.9 implies that a shift in the MSY is consistent with a shift in the right-hand side of the equation.

To identify productivity regimes prior to any fit and because By was available only once the model was fitted, the timing of potential MSY shifts was explored using observed data and investigating shifts in the D F 0\ + D(* I ) + : MOP : J K time-series, where B obs y is calculated as the ratio between the reconstructed Yy and Fy.

Breakpoints in these time-series were detected using the STARS method [START_REF] Rodionov | A sequential algorithm for testing climate regime shifts[END_REF] parameterized for a cut-off length of 10 years and a probability level set to 0.05. A "pre-whitening" procedure was applied prior to running STARS to avoid potential misinterpretation of stochastic fluctuations as regime shifts [START_REF] Hsieh | Distinguishing random environmental fluctuations from ecological catastrophes for the North Pacific Ocean[END_REF][START_REF] Rodionov | Use of prewhitening in climate regime shift detection[END_REF]. STARS was executed under the R software using the function developed by [START_REF] Seddon | A quantitative framework for analysis of regime shifts in a Galápagos coastal lagoon[END_REF].

• Fit of the model

In view of a parsimonious approach, shifts identified through data analysis were integrated in our approach only if they statistically improved the surplus biomass production model. Thus, by including or excluding breakpoints among those identified, we listed all potential partitions of the MSY regime (hence ranging between 2 and the number of regimes identified through data analysis)

and fitted one model for each of them.

For all considered species and possible partitioning of MSY regimes, the surplus biomass production model defined in Eq. 7 was fitted to the observed 1950-2015 time-series of biomass B obs y (calculated as the ratio between the reconstructed Yy and Fy) and Fox's fishing mortality (FFoxy) using the maximum likelihood method and assuming a lognormal distribution of the residuals. The fit was performed using weighted nonlinear least squares regression. Note that in our study, the Yy/Fy ratio was preferred to the ICES estimates of biomass because the Celtic Sea area did not systematically match the spatial extent of the assessed stocks.

Finally, the Akaike criterion [START_REF] Akaike | A new look at the statistical model identification[END_REF] was used to select the regime shift combination leading to the best surplus production model. Once the model was fitted, the By values were predicted over 1950-2015 from Eq. 9 using the MSY estimates, FFoxy, and FMSY.

A sensitivity analysis to the hypothesis of regime shift existence in productivity potential was conducted by comparing the results to those that could be obtained under the hypothesis of (1) a constant productivity potential defined by a single MSY and (2) a productivity potential with continuous and monotonous change. The latter was modeled by expressing the MSY at year y as the product of an initial value MSY0 and of an annual multiplier ] = (1 -^) AT_`a , where β is a yearly change rate to be estimated.

Linking changes in species productivity potential to environmental variables

To explore common patterns in the variability of productivity potential among the 14 selected species and to identify the drivers of changes, we performed principal component analysis (PCA).

Years were considered as statistical individuals, species productivity potential values were variables, and environmental data were supplementary variables. Environmental data included local biotic data from the Continuous Plankton Recorder survey and abiotic variables reflecting hydroclimate conditions at the local or regional scale. We also used egg and larvae counts from the CPR survey as potential indicators of overall fish productivity in the Celtic Sea (all variables are summed in Tab. 1). Based on preliminary tests, correlations with the principal component of PCA on productivity potentials were analyzed with a 3-year lag. This choice is consistent with the fact that the environment mainly affects early life stages (through recruitment, growth or modification of the spawning feature of parents), thus delaying the observation of any effect in catches and estimated productivity potential. The environmental variables, their running means and their paircorrelations are plotted in Appendix B Fig. B1 andB2.

Results

Catch reconstruction

The relative importance of unreported catches declined over the study period, from 50% of the reported landings in 1950 to 10% in 2015 (Fig. 1-A). This trend was due to the reduction in the proportion of unreported landings, while the global discard rate remained quite stable (near 10%, in spite of a late increase in demersal fish). Despite such variations, trends in the Statlant landings and SAUP catches were similar. Due to their large and highly variable landings, pelagic species shape the temporal profile of catches (Fig. 1-A, B). Mackerel, horse mackerel, and boarfish have

Trophic and sized-based analysis of catches

• Ecosystem indicators Both trophic and size-based indicators (Fig. 2) displayed similar declines between 1950 and 2010.

The mean trophic level (MTL) was reduced by 0.38 and the mean maximum length (MML) was reduced by 25 cm, while the proportion of predators in the catch (HTI) declined by more than half.

Thus, fisheries targeted species with a smaller size and lower trophic level over time. This trend is explained by both the greater amount of pelagic fish and invertebrate removals and the changes in the composition of demersal fish catches; additionally, when these values were calculated excluding horse mackerel and mackerel species, the MML increased up to 88 cm in 1975 but then declined by more than 40 cm over the next 25 years. The Shannon diversity index confirms the catch diversification. Setting aside its temporary drops due to peaks in horse mackerel and mackerel catches, the value showed a continual increase over the study period, highlighting a dramatic transition from low to high diversity in targeted species.

All trophic and size-based indicators increased after 2010, thus exhibiting values close to those observed in the late 1980s. Such changes were driven by the increasing catches of large and/or high-TL demersal species (e.g., haddock, anglerfish, and hake).

• Trophic spectra

The trophic spectra point to the main changes in catch patterns (Fig. 3). Spectra were split into two main areas: around a TL of 2 for primary consumers (e.g., shellfish such as great scallop or blue mussel) and from a TL of 3.0 to a TL of 4.7 (Fig. 3-A showed a two-phase trend marked by an initial increase until the 1990s, followed by a return to a situation equivalent to the 1970s. In summary, the exploitation of the Celtic Sea developed on all components of the ecosystem between 1950 and 2015 and was particularly enhanced for low TL resources.

Fishing pressure

The mean time-series of reconstructed fishing mortality for the 14 selected stocks (Fig. 4) highlighted a dramatic increase in the fishing effort since 1950 marked by the overrun of the mean FMSY in 1978 and a peak in 1991. Over this period, the mean fishing mortality increased approximately 12-fold (between 8 and 18 for the extreme values assumed for the fishing efficiency increasing rate, α) to reach a value 1.7 times the mean FMSY. Despite the major European decommissioning plans that followed (e.g., the French Mellick plan in 1991), fishing mortality was quite stable until 2005; then, the value declined by more than 30% in just 5 years before stabilizing at approximately 1.1 times the mean FMSY over the last 4 years. The smoothed trend of the mean fishing mortality hides a large variety of fishing mortality trajectories (e.g., early exploitation for cod, later for anglerfish and sea bass) and mean values (the lowest for pilchard and the highest for hake).

Biomass reconstruction

•

Fox model fit and regime shifts detection

The Fox model fitted correctly to all species, highlighting that abundances were mainly regulated by fishing (models fits in Appendix C). The best fits of the final model were obtained for hake, cod, whiting, sole, plaice and Nephrops (R 2 values higher than 0.8, Tab. 1), while a poor fit was observed only for haddock (R 2 = 0.37). All the studied species displayed shifts in their productivity potentials (Fig. 5) that explained abundance variability better than the constant productivity scenario (AICs in Tab. 1). These shifts suggest that fishing was not the only driver of changes in abundance. Some species presented one shift, pulling apart two long periods of different productivity potential (e.g., herring and haddock). Others exhibited several shifts in productivity potential during the study period. For some species (e.g., megrim and sea bass), successive regimes followed one other in a monotonous trend. Regime shifts can also be "punctual" (e.g., mackerel and Nephrops), suggesting a temporary perturbation (favorable or not) prior to an apparent return to standard conditions. To finish, some regime shift successions form smooth fluctuations (e.g., hake, cod, and whiting).

According to these cases, changes in species productivity potential were either synchronic or not, with periods of high fishing pressure, potentially favoring or preventing a decline in stock biomasses.

• 1950-2015 biomass trends for the studied species

The fitted production models highlight a global decline in fish biomass since 1950 (Fig. 5). This reduction in the abundance of Celtic Sea stocks is a direct consequence of the post-World War II rise of fishing pressure in the area. The rate of decline and its timing differ according to the exploitation history of each stock. Demersal species that were already commonly targeted in the 1950s (i.e., cod, hake, whiting, sole, and plaice) showed the steepest decrease in biomass. Over 30 years, these values have fallen to between 15% and 25% of their 1950 levels, while some species, such as sole or hake, continued to decline and bottomed out during the 1990s. Demersal species that have been targeted more recently (i.e., anglerfish, megrim or haddock and, one decade later, sea bass) also displayed a similar decline in biomass, but this decline was delayed and of lower amplitude. For 75% of the studied species, the lowest biomass levels were reached between the early 1990s and the mid-2000s.

Since the late 2000s, all demersal fish species have shown increasing biomasses, suggesting a stock recovery. The intensity of this recovery is highly variable according to species and is influenced by their productivity potential regimes. Anglerfish, megrim, hake, and haddock appear to benefit from both declining fishing pressure and favorable environmental conditions and consequently display the most impressive recovery among the studied species. Biomass trajectories of pelagic species exhibit more complex trends because of their higher sensitivity to environmental conditions and to their highly variable fishing intensity throughout the studied period. Nonetheless, they all present decreases in biomass over the whole period and no particular improvement in abundance, except for herring.

The mean trend of the biomass for the 14 studied species, expressed in relative values (Fig. 5),

shows an especially fast and strong decrease over the 1950s and 1960s. Compared to 1950, the mean stock biomass was reduced more than four-fold in the 1990s but has increased by approximately 40% over the last ten years.

Productivity regime shifts and link to environmental variations

The overall productivity potential of the 14 studied species fluctuated over time, peaking in the 1970s and the 1990s, and there was no clear trend over the entire period (Fig. 6A). In contrast, the mean value of the MSY/MSY1950 ratio increased by 50% over the study period. This trend was due to the increase in productivity potential observed for numerous low productivity and low abundance species, while a decrease occurred for a few abundant and more productive species, therefore leading to the overall stability observed since 1950 at the ecosystem level.

The first two principal components of the PCA performed on the productivity potential explained 64% of all variability of the data and highlighted the similarities and divergences among species.

PC1 compares species with increasing productivity potential over the study period (i.e., gadoids, anglerfish, sea bass, and megrim) to species with decreasing productivity potential (i.e., herring, pilchard, and sole) (Fig. 7). Plotting the PC1 scores against time (Fig. 6-B) highlights a transition period that initiated in the late 1970s and ended in the late 1980s, when major shifts occurred toward higher (hake, cod, whiting, haddock, megrim, and plaice) or lower (pilchard and mackerel) productivities (Tab. 1). PC2 splits each period before and after the 1980s, in two subperiods. Two additional transition phases characterized by declining productivities are thus identified: one in approximately 1960 (for hake, whiting, sole, plaice, pilchard, and lobster) and a second in approximately 2000 (for hake, cod, whiting, horse mackerel, and plaice). The two-dimensional space defined by the first two PCA dimensions (Fig. 7) draws groups of similar years regarding productivity potential. These groups broadly correspond to decades, and the diametrical opposition of the 1950s and the 2010s highlights that these two decades were particularly dissimilar in terms of productivity potential.

Numerous biotic and abiotic parameters computed as supplementary variables were highly correlated with PC1 (corr > 0.4, Fig. 7) and presented a marked increase or decrease over the study period. Thus, the first PC, linked to the transition period of the 1980s, was positively correlated with parameters related to the wind regime (corr=0.93 and 0.65 with the wind speed and the northward westward wind component, respectively) and the Gulf stream index (0.59) and negatively correlated with the sea surface salinity (SSS, -0.62) and several biotic variables (0.47, -0.58 and -0.59 with phytoplankton color index, small copepods abundance, and larvae abundance,

Discussion

Capacities and limits of the data reconstruction and biomass production model

• Biomass production model.

In a first attempt to estimate the biomass trends for the main exploited species since 1950, the catch-based dynamic production models developed by [START_REF] Martell | A simple method for estimating MSY from catch and resilience[END_REF] and Froese et al.

(2017) were considered. These methods provided reliable estimates of reference points but did not appear to handle stock dynamics unrelated to fishing well. They resulted in biomass trends that were particularly smooth and failed to reproduce observed abundance patterns when catch timeseries displayed few exceptionally low or high values (see applications to the Celtic Sea stocks in [START_REF] Froese | Estimating fisheries reference points from catch and resilience[END_REF]. By incorporating potential changes in productivity potential, the method we developed overcame this issue but also allowed integration of local knowledge on fishing effort.

• Catch data reconstruction.

Statlant file provided by the ICES provided catches for the overall ecosystem compartments. The main drawbacks of this database lie in the spatial allocation of catches, the lack of information for some underreported species, and the lack of discard estimates. Catch data are treated by carefully working at the spatial reallocation and searching complementary information on particular species and crosschecking catch time-series with national databases; this method allows us to assume that our catch reconstruction is the best that can be currently done with existing data. Additionally, the integration of SAUP data into such modeling approaches is quite rare and provides an added value to our study (Zeller et al., 2016).

Sensitivity analysis showed that the estimated biomass trajectories were very similar after 1970 for models using either SAUP or Statlant data. However, SAUP-based models consistently produced steeper biomass declines with the decrease in unreported catches, hence providing a supplementary argument for the usefulness of the integration of unreported catches in such approaches. In a complementary analysis, however, catch data could also be compared to other worldwide catch databases (Watson et al., 2017).

•

Fishing pressure.

The model appeared sensitive to the assumed value for α, defining the yearly rate of increase in fishing efficiency. Assuming lower or higher values for α (±50%) led to lower and higher mean biomass estimates in 1950 (-30% and + 60%, respectively). Nonetheless, the overall decrease in the abundance of exploited stocks between the 1950 and the late 1990s remained in the same order of magnitude regardless of the value of α considered (from a 3-fold to a 7-fold reduction). In addition, the values of α calculated according to [START_REF] Pauly | An empirical equation to predict annual increases in fishing efficiency[END_REF] seemed to be plausible and even conservative [START_REF] Villasante | Estimating the effects of technological efficiency on the European fishing fleet[END_REF]. Furthermore, [START_REF] Millischer | Estimation of the overall fishing power: A study of the dynamics and fishing strategies of Brittany's industrial fleets[END_REF] estimated an α value of 12% for the French fleet from Concarneau targeting cod in the Celtic Sea over 1983-1994.

The process of rebuilding fishing effort time-series in a data-poor context produces large uncertainties. In our case, this uncertainty was higher for pelagic species, whose activity is highly dependent on seasonal and environmental conditions. In the case where several independent time-series of fishing effort are available from the literature for a given species, uncertainty might be assessed through bootstrapping procedures. However, the various time-series usually have different levels of reliability; thus, we preferred to select the most reliable one for each species and time period based on expert judgment. Analyzing the sensitivity of the final results to a large range of α coefficient values is assumed to be sufficient to explore the uncertainty of the trends in fishing efforts.

• Productivity changes.

The "reference", "continuous" and "constant" scenarios on changes in productivity potential provided similar estimates of biomass for 1950 (Appendix, Fig. D1). The main difference, however, was in the 1950-2015 biomass trajectories, with the reference scenario producing more realistic trends and statistically better fits (see Tab. 1). Regime-like responses in fish productivity could notably reflect shifts in recruitment [START_REF] Ottersen | Temporal shifts in recruitment dynamics of North Atlantic fish stocks: effects of spawning stock and temperature[END_REF][START_REF] Kell | Is spawning stock biomass a robust proxy for reproductive potential?[END_REF], growth [START_REF] Clausen | Shifts in North Sea forage fish productivity and potential fisheries yield[END_REF] and survival [START_REF] Mueter | Recruitment and survival of Northeast Pacific Ocean fish stocks: temporal trends, covariation, and regime shifts[END_REF]. Many studies confront two hypotheses about fish stock productivity: productivity is determined by the spawning stock size (i.e., standard surplus production model) vs productivity is determined by the environment, notably following regime-like dynamics [START_REF] Gilbert | Towards a new recruitment paradigm for fish stocks[END_REF][START_REF] Szuwalski | Examining common assumptions about recruitment: A meta-analysis of recruitment dynamics for worldwide marine fisheries[END_REF]. Here, we proposed a more conservative option corresponding to the mixed hypothesis tested by [START_REF] Vert-Pre | Frequency and intensity of productivity regime shifts in marine fish stocks[END_REF] in their meta-analysis, which proved to be relevant for a substantial proportion of analyzed stocks. The amplitudes of the shifts identified in our study were high, but they were globally in the same range of values as those of MSY values determined by [START_REF] Clausen | Shifts in North Sea forage fish productivity and potential fisheries yield[END_REF] for North Sea species (up to 68% variation in 30 years).

The early alteration of the ecosystem: a fishing-induced trophic cascade?

As in many ecosystems worldwide, the dramatic expansion of fishing during the second half of the twentieth century has deeply impacted the structure and functioning of the ecosystem [START_REF] Myers | Rapid worldwide depletion of predatory fish communities[END_REF][START_REF] Tremblay-Boyer | Modelling the effects of fishing on the biomass of the world's oceans from 1950 to 2006[END_REF][START_REF] Christensen | A century of fish biomass decline in the ocean[END_REF]. Fishing pressure has constantly increased in the Celtic Sea since 1950 and culminated in the early 2000s, subsequently driving down the biomass of exploited species, with declines up to eight-fold for some of them. A substantial part of the biomass decline occurred between 1950 and 1980, highlighting an early alteration of the ecosystem and providing new evidence of shifting baselines in European seas [START_REF] Pauly | Anecdotes and the shifting baseline syndrome of fisheries[END_REF][START_REF] Guénette | Shifting baselines in European fisheries: The case of the Celtic Sea and Bay of Biscay[END_REF].

Such an impact of fishing may have cascaded through the food web. Among the main exploited species in 1950, hake, cod, whiting and megrim are important predators in the Celtic Sea, and they structure the food web through top-down control. The drastic reduction in their abundance must have benefitted their prey, in particular, small pelagic species, which are known to be sensitive to fishing-induced predation release [START_REF] Daskalov | Overfishing drives a trophic cascade in the Black Sea[END_REF][START_REF] Frank | Trophic Cascades in a Formerly Cod-Dominated Ecosystem[END_REF][START_REF] Savenkoff | Changes in the northern Gulf of St. Lawrence ecosystem estimated by inverse modelling: Evidence of a fishery-induced regime shift?[END_REF]. This result could explain the upward shifts in mackerel productivity potential and biomass in the late 1960s, which indicated a shift toward a more pelagic-dominated ecosystem. The outburst in pelagic fish was concomitant with a decrease in the abundance of copepods (especially those smaller than 2 mm, SmCop in Appendix B Fig. B.1), the primary food source of adults and larvae, and an increase in phytoplankton abundance (two-fold increase in the phytoplankton color index PCI, Appendix B Fig. B.1). These opposite abundance trends between successive trophic levels suggest a fishinginduced trophic cascade [START_REF] Frank | Trophic Cascades in a Formerly Cod-Dominated Ecosystem[END_REF].

This potential trophic cascade may have impacted fishing. Because the outburst was contemporaneous with main demersal stock depletion, pelagic fish may have been considered as an alternative fishing target, resulting in a transfer of fishing effort [START_REF] Lockwood | Mackerel research in the South-West[END_REF].

The depletion of historically targeted species also affected the diversification in fish production, shifting toward few large fish species of lower commercial value at that time (i.e., anglerfish) and mainly toward low TL and small demersal fish and crustaceans [START_REF] Pinnegar | Long-term changes in the trophic level of the Celtic Sea fish community and fish market price distribution[END_REF], Gascuel et al. 2016). These patterns would suggest a significant alteration of the Celtic Sea trophic structure typical of the "fishing down the marine food-web" [START_REF] Pauly | Fishing Down Marine Food Webs[END_REF]; however, they patterns may have resulted from independent changes in fishing strategies, hence indicating the co-occurrence of "fishing through the marine food-web" [START_REF] Essington | Fishing through marine food webs[END_REF].

Although pelagic fish have dominated the biomass in the Celtic Sea since the early 1970s, several species have succeeded in a "pelagic waltz". Quickly overexploited, mackerel biomass severely declined in the early 1980s [START_REF] Lockwood | An assessment of the Western mackerel stock[END_REF][START_REF] Hawkins | Detection of environmental change in a marine ecosystem-evidence from the western English Channel[END_REF]. Horse mackerel, which was mainly a bycatch at the mackerel fishery apogee, likely benefitted from the vacant niche left by mackerel and turned into a new target species of pelagic fleets [START_REF] Eaton | Scad in the North-East Atlantic[END_REF]. Once horse mackerel were depleted, especially after 2000, boarfish became the most fished pelagic species.

The influence of multi-decadal variability of Atlantic hydroclimate

Confirming insights from a previous trophic modeling work, the productivity of the Celtic Sea ecosystem appears to be under the influence of North Atlantic conditions. Positive and negative phases of the NAO match favorable and unfavorable periods for most gadiformes (i.e., cod, hake, and whiting), flatfish species (i.e., sole and plaice), and horse mackerel. The NAO has been shown

to affect fish population dynamics for a wide diversity of species, mainly through recruitment (Reid et al., 2001b;[START_REF] Stige | Cod and climate:: effect of the North Atlantic Oscillation on recruitment in the North Atlantic[END_REF][START_REF] Goikoetxea | Links between the recruitment success of northern European hake (Merluccius merluccius L.) and a regime shift on the NE Atlantic continental shelf[END_REF] or post-recruitment growth [START_REF] Ottersen | Ecological effects of the North Atlantic Oscillation[END_REF] processes.

Several mechanisms can explain the connection between this index and the productivity potential of fish species in the Celtic Sea. The NAO is closely linked to water circulation [START_REF] Hurrell | North Atlantic climate variability: The role of the North Atlantic Oscillation[END_REF] and notably influences the distance of the North Atlantic Subpolar Gyre to the Celtic shelf [START_REF] Alheit | What happened in the mid-1990s? The coupled ocean-atmosphere processes behind climateinduced ecosystem changes in the Northeast Atlantic and the Mediterranean[END_REF]. The inflow of Atlantic waters from the subpolar gyre to the Celtic Sea favored during positive phases of the NAO is likely to influence zooplankton composition. Such inflows are associated with C. finmarchicus advection up to the Celtic Sea [START_REF] Sundby | Recruitment of Atlantic cod stocks in relation to temperature and advectlon of copepod populations[END_REF]. Compared to the dominant species in the Celtic Sea (C. helgolandicus), C. finmarchicus develops earlier in the year [START_REF] Planque | Calanus and environment in the eastern North Atlantic. I. Spatial and temporal patterns of C. finmarchicus and C. helgolandicus[END_REF] and has an upper position within the water column [START_REF] Williams | Vertical distribution of Calanus finmarchicus and C. helgolandicus in relation to the development of the seasonal thermocline in the Celtic Sea[END_REF]. Its higher abundance would possibly increase the food availability of the early spring larvae of several species and improve their survival [START_REF] Beaugrand | Plankton effect on cod recruitment in the North Sea[END_REF], thus explaining the positive correlation observed between NAO and C. finmarchicus.

Another likely mechanism of the NAO action on productivity is the modification of local wind patterns. In the Celtic Sea, winds mainly directed toward the northeast would be further directed to the north when the NAO is high (similar trends in NAO and northward wind component, Appendix B Fig. B.1). The resulting increase in the Ekman transport of superficial water masses to the east would favor eggs and larvae retention on the shelf and larvae transport to nurseries [START_REF] Goikoetxea | Links between the recruitment success of northern European hake (Merluccius merluccius L.) and a regime shift on the NE Atlantic continental shelf[END_REF]. The lower amplitudes of changes in sole and plaice productivity potential would thus result from their higher dependence on coastal areas for coastal and nursery sites [START_REF] Ellis | Spawning and nursery grounds of selected fish species in UK waters[END_REF].

The unfavorable conditions in the early 1960s due to the NAO shift into a strongly negative phase may have accelerated the biomass decline of some species, such as hake and whiting. Conversely, the favorable conditions in the 1990s caused by the strong NAO positive phase may have mitigated fishing impact, preventing species from further collapse while fishing pressure culminated at that time. The benefits from these conditions were likely amplified because the environment is known to affect recruitment deeper for spawning biomass levels [START_REF] Brander | Cod recruitment is strongly affected by climate when stock biomass is low[END_REF].

The Celtic Sea ecosystem also appeared to be influenced by global thermal conditions of the North Atlantic. This effect was mainly perceived through the dynamic of pilchard productivity. In 1961, contemporary to the downward shift in the AMO index and in SST, pilchard productivity potential declined by 40%, leading to an abrupt decline in its biomass. After another decline in 1981, it shifted upward in the early 1990s, when the AMO and SST increased. This result corroborates previous studies based on long-term variability in pilchard egg numbers [START_REF] Southward | The Western English Channel-an inconstant ecosystem[END_REF][START_REF] Southward | Fluctuations in the herring and pilchard fisheries of Devon and Cornwall linked to change in climate since the 16th Century[END_REF][START_REF] Edwards | Marine Ecosystem Response to the Atlantic Multidecadal Oscillation[END_REF], highlighting that its productivity changes according to the transition between warm and cold phases of the North Atlantic. Such dependence on thermal conditions is responsible for the alternation between pilchard and herring as the dominant pelagic species over time [START_REF] Edwards | Marine Ecosystem Response to the Atlantic Multidecadal Oscillation[END_REF]. However, despite the cold period from the 1960s to the 1980s, which should have been favorable, herring did not show any improvement in productivity potential. This result is probably linked to its overexploitation at that time (biomass was divided by 6 over [1950][1951][1952][1953][1954][1955][1956][1957][1958][1959][1960] and trophic limitation [START_REF] Lauria | Influence of Climate Change and Trophic Coupling across Four Trophic Levels in the Celtic Sea[END_REF].

A major environmental shift in the late 1980s

The Celtic Sea is part of the North Atlantic Ocean and is impacted by the great hydroclimatic changes that occurred at the global scale [START_REF] Reid | Global impacts of the 1980s regime shift[END_REF]. Hence, as observed in numerous neighboring Northeast Atlantic ecosystems, the Celtic Sea experienced an ecosystem shift during the 1980s [START_REF] Southward | The Western English Channel-an inconstant ecosystem[END_REF]Reid et al., 2001a;[START_REF] Mollmann | Effects of climate and overfishing on zooplankton dynamics and ecosystem structure: regime shifts, trophic cascade, and feedback loops in a simple ecosystem[END_REF][START_REF] Conversi | The Mediterranean Sea Regime Shift at the End of the 1980s, and Intriguing Parallelisms with Other European Basins[END_REF] . Such a major change in the ecosystem was reflected in the species productivity potential values.

The 1980s step increase observed in the main pattern of productivity variability matched a period of major changes in the environmental conditions in the Celtic Sea. The increase in wind speed, the variable that was best correlated with this pattern, may have improved water mixing, hence leading to an increase in primary production [START_REF] Reid | Is observed variability in the long-term results of the Continuous Plankton Recorder survey a response to climate change?[END_REF][START_REF] Reid | Changing states of the Large Marine Ecosystemsof the North Atlantic[END_REF], which was likely boosted by the dramatic increase in SST in the Celtic Sea in 1989. Thus, primary producer biomass almost doubled between 1980 and 2000.

However, this increase in primary producer biomass did not result in a more productive ecosystem.

The year 1988 marked a severe decline in the abundance of small copepods, while larger copepod abundance has increased since that period (Appendix B). These changes are identified as a consequence of synergetic effects of multiple hydroclimatic factors [START_REF] Beaugrand | Reorganization of North Atlantic Marine Copepod Biodiversity and Climate[END_REF][START_REF] Beaugrand | Long-term changes in copepod abundance and diversity in the north-east Atlantic in relation to fluctuations in the hydroclimatic environment[END_REF].

These changes are probably what triggered opposite trajectories in the mean productivity potential between pelagic and demersal fish species. Indeed, small copepods constitute the primary food source of small pelagic larvae [START_REF] Checkley | Selective Feeding by Atlantic Herring (Clupea harengus) Larvae on Zooplankton in Natural Assemblages[END_REF][START_REF] Peterson | Diets and selective feeding by larvae of Atlantic mackerel Scomber scombrus on zooplankton[END_REF][START_REF] Last | The food of herring, Clupea harengus, in the North Sea, 1983-1986[END_REF][START_REF] Fernandez | Analysis of feeding of Sardina pilchardus (Walbaum, 1792) larval stages in the central Cantabrian Sea[END_REF]. In contrast, larvae and young stages of large demersal fish are more dependent on larger species of zooplankton [START_REF] Bromley | Diel feeding patterns and the development of food webs in pelagic 0-group cod (Gadus morhuaL.), haddock (Melanogrammus aeglefinusL.), whiting (MerlangiusmerlangusL.), saithe (Pollachius virensL.), and Norway pout (Trisopterus esmarkiiNilsson) in the northern North Sea[END_REF][START_REF] Heath | A synthesis of large-scale patterns in the planktonic prey of larval and juvenile cod (Gadus morhua)[END_REF][START_REF] Morote | Feeding selectivity in larvae of the European hake (Merluccius merluccius) in relation to ontogeny and visual capabilities[END_REF]. Furthermore, zooplankton abundance was probably not limiting for large demersal fish production compared to pelagic species.

Last, independent of any trophic control, the productivity of various species could have been directly driven by environmental changes, especially those related to temperature [START_REF] Rijnsdorp | Resolving the effect of climate change on fish populations[END_REF]. In particular, species with the highest thermal preferenda, such as sea bass, megrim and anglerfish, would be positively impacted, as established by [START_REF] Auber | Regime Shift in an Exploited Fish Community Related to Natural Climate Oscillations[END_REF] in the adjacent eastern English Channel. This change would also partly explain the growth in boarfish abundance during the last decade (Coad, 2014). As suggested by trends in the catch-based Shannon index, warming would also favor the settlement of Lusitanian species in the Celtic Sea, such as gurnards, mullet and pouts [START_REF] Ter Hofstede | Regional warming changes fish species richness in the eastern North Atlantic Ocean[END_REF].

First signs of recovery

Over the last decade, fishing pressure in the Celtic Sea has been sharply reduced, but the mean F/FMSY for our studied stocks was still equal to 1.15 in 2015. In addition, this average value masks the great heterogeneity among species. The exploitation rate was considerably reduced for some species (e.g., F=0.72*FMSY for herring) but was still high for others (e.g., F=1.3*FMSY for sea bass), and 6 species remained overexploited. The concomitance between fishing pressure reduction and favorable or unfavorable environmental conditions determines the recovery dynamic: few species have benefitted from higher productivity over the last 15 years (i.e., megrim, sea bass, and anglerfish), but most have lower productivity than their values in the 1990s, thus recovering slowly relative to the reduction in fishing effort.

The greatly degraded status of several historically exploited stocks and the consequent reduction in their resilience capacity would also explain this slow improvement [START_REF] Neubauer | Resilience and Recovery of Overexploited Marine Populations[END_REF]. Froese and Proelß (2010) were correct when predicting that the target of "phas[ing] out overfishing by 2015" (European Commission, 2011) would not be met. Nonetheless, slight though they may be, the post-2000 trends in biomass are encouraging in terms of fisheries management [START_REF] Cardinale | Rebuilding EU fish stocks and fisheries, a process under way?[END_REF][START_REF] Fernandes | Reversal of Fish Stock Decline in the Northeast Atlantic[END_REF]. Additionally, all the size-and trophic-based indicators calculated on catches indicated a recent improvement of the ecosystem status and structure characterized by a return of larger and higher TL species. The drastic reduction in mackerel and horse mackerel abundance may also lead to a progressive rebalancing between demersal and pelagic compartments, highlighting similar changes as those observed in the eastern English Channel [START_REF] Auber | Regime Shift in an Exploited Fish Community Related to Natural Climate Oscillations[END_REF][START_REF] Mclean | Functional reorganization of marine fish nurseries under climate warming[END_REF].

Nonetheless, the current recovery in the last decade must be analyzed considering the historical biomass levels. Species abundance and ecosystem indicators in the Celtic Sea are only reaching values equivalent to those in the late 1980s, when stock biomasses were, on average, almost 3 times smaller than those in the year 1950, i.e., this was certainly not the beginning of the fishing impacts on marine resources. However, it must be emphasized that environmental changes that occurred over recent decades actually prevent any return to the state of marine ecosystems in 1950. Furthermore, future changes, especially those linked with ongoing climate change, must be considered in fisheries management in upcoming decades.

Figures

  followed each other as dominant species in catches since the 1970s. Mackerel and horse mackerel catches peaked in 1976 and 1995, respectively, while boarfish appeared in statistics in 2003 and was the most fished species in the Celtic Sea in the late 2000s. Catches of demersal species (Fig.1-C) progressively increased after 1950, peaked the first time in 1972-1976, culminated in the 1990s, and then declined by 30% between 1997 and 2008. Since then, demersal species have returned their 1980s level. The proportion of emblematic species such as hake, cod, whiting and sole (i.e., the primary exploited demersal species after WWII) progressively declined while fisheries started targeting new species such as anglerfish and pout in the 1970s or smaller demersal species in the 1980s. Invertebrates made important contributions to catch diversification (Fig.1-D). Their removal continuously increased until 2006, and they were driven by spectacular bivalve fisheries development after 1971 and by a gradual increase in cephalopod and crustacean exploitation.

  ) for other species. Since 1950, catches have always been dominated by TLs of approximately 3.6 (i.e., mainly small pelagic species), but several changes occurred over the study period. Catches in primary producer (TL=2) were almost negligible until the late 1970s, and then they strongly increased. At low TLs (2.8-3.2, i.e., small crustaceans and gastropods), catches continuously increased until the 2000s, when they were 3.5 times larger than the value in 1950. Catches of intermediate TLs of approximately 3.6 reached large values in the 1970s (mackerel) and 1990s (horse mackerel), and the values more than doubled at the end of the period compared to the value of the 1950s. Higher TLs between 4.0 and 4.5 (i.e., demersal fish)

  respectively). This result suggests a major change, arising around the 1980s, in the environment of the Celtic Sea ecosystem, toward more turbulent conditions, less saline conditions, and, in spite of the lower correlation with SST, warmer water associated with great changes in small zooplankton abundance (Appendix B Fig.B.1). On PC2, which was mainly related to the late 1960s, 1980s and early 2000s changes in fish stock productivity, the NAO was by far the most correlated environmental variable (corr=0.80), and it likely had a relationship with its strong and long-term contrasts over the study period and Appendix B Fig. B.1). Despite the shorter time-series available (n=55 once lagged), PC2 also seemed to be linked to mesozooplankton abundance and composition because it was negatively correlated with the total large copepod and Calanus helgolandicus species abundances (-0.65 and -0.58, respectively) and positively correlated with Calanus finmarchicus abundance (0.34).

Figure 1 -

 1 Figure 1 -ICES landings and SAUP reconstructed catches in the Celtic Sea between 1950 and

Figure 2 -

 2 Figure 2 -Trends over the 1950-2015 period for ecosystem indicators calculated upon catch

Figure 3 -

 3 Figure 3 -Decadal trophic spectra of catches (mean annual catch on the decade by trophic

Figure 4

 4 Figure4-F/FMSY ratio for the 14 modeled species (colors) and mean ratio (black). Lines

Figure 5 -Figure 6 -

 56 Figure 5 -Temporal trends in observed and estimated biomass from the surplus production

Figure 7 -

 7 Figure 7 -Correlations circle (left) graph of individuals (right) of the PCA for PC1 and PC2. Black

•

  Reported and reconstructed catchesLandings of all fish species between 1950 and 2015 were taken from the Statlant database (available on ICES website, ices.dk/marine-data/dataset-collections/Pages/Fish-catch-and-stock-

	assessment.aspx),	and	the	data	for	tuna-like	species	came	from	ICCAT

Acknowledgments

We thank the Système d'Information Halieutique and the Sir Alister Hardy Foundation for Ocean Science for providing French fishing effort data and CPR data respectively. This work has been partly realized in the frame of a Ph.D. co-funded by Ifremer and the Region Bretagne.

Tables Table 1 -Parameters of the biomass production models and the final combinations of productivity regime shifts they include after selection according to Akaike's scores. The selection is used Global fit quality of the model is given by the usual R²; AIC is the Akaike's coefficient of the model; AIC cst. prod. is the Akaike's coefficient of the model based on the hypothesis of a constant productivity; corr ICES is the correlation coefficient between the predicted biomass time-series and the biomass estimates from the ICES working groups; Nb shifts is the number of shifts considered in the reference model; BPn and MSYn refer to years of shifts and values of the productivity potential MSY before that shift.