The winter diet of common dolphins over the last 20 years reflects prey and predator ecological changes in the Bay of Biscay, and highlights bycatch circumstances

Bycatch (e.g., the accidental capture of non-targeted species by fisheries) is a leading cause of human-induced mortality, contributing to significant populations declines worldwide. Often stemming from the overlap between food resources and fishery target species, dietary analysis is key to understand bycatch patterns. In the Bay of Biscay, common dolphin (Delphinus delphis) bycatch has strongly increased since 2016. Addressing the potential trophic relationship between dolphins and fisheries is essential for developing effective conservation strategies and ensure the sustainability of both dolphin populations and fisheries. Using stomach content analysis, we investigated temporal changes in the occurrence, abundance and importance by mass of preys between 1999 and 2019. We found no difference in overall diet over time, still composed of pelagic energy-rich prey (pilchards Sardina pilchardus; horse mackerel Trachurus spp. and anchovy Engraulis encrasicolus). However, we observed significant decrease in the importance by mass of horse mackerel (t = 2.8365, p = 0.0052) and increase in anchovy (t = -4.2636, p < 0.005), as well as a decrease in the average size of major species, including pilchards, horse mackerel and anchovy; mainly related to environmental variations in abundance and size distribution of the small pelagic fish. We also identified a shift in minor species from upper slope habitats (e.g., blue whiting Micromesistius poutassou) to species inhabiting coastal waters (e.g., sprat Sprattus sprattus), reflecting changes in the distribution of common dolphins within the Bay of Biscay. Finally, we highlighted the consistency over time in the prevalence of fresh pilchards, anchovies and horse mackerels in the dolphin stomachs, suggesting they are more likely to feed specifically on these species when bycatch occurs. The risk of bycatch may therefore be modulated by dolphins’ target species, with a higher risk being associated with dolphins feeding on pilchards, anchovies or horse mackerel rather than other prey species Johanna Faure 1, Jasmin Niol 1, Eléonore Meheust 1, Jérôme Spitz 1,2 1 Observatoire Pelagis, UAR 3462 CNRS – La Rochelle Université, 5 allées de l’Océan, 17000 La Rochelle, France 2 Centre d’Etudes Biologiques de Chizé (CEBC), UMR 7372 CNRS – La Rochelle Université, 79360, Villiers-en-Bois, France

Bycatch ( e.g., the accidental capture of non-targeted species by fisheries) is a leading cause of human-induced mortality, contributing to significant populations declines worldwide. Often stemming from the overlap between food resources and fishery target species, dietary analysis is key to understand bycatch patterns. In the Bay of Biscay, common dolphin ( Delphinus delphis ) bycatch has strongly increased since 2016. Addressing the potential trophic relationship between dolphins and fisheries is essential for developing effective conservation strategies and ensure the sustainability of both dolphin populations and fisheries. Using stomach content analysis, we investigated temporal changes in the occurrence, abundance and importance by mass of preys between 1999 and 2019. We found no difference in overall diet over time, still composed of pelagic energy-rich prey (pilchards Sardina pilchardus ; horse mackerel Trachurus spp. and anchovy Engraulis encrasicolus) . However, we observed significant decrease in the importance by mass of horse mackerel ( t = 2.8365, p = 0.0052) and increase in anchovy ( t = -4.2636, p < 0.005), as well as a decrease in the average size of major species, including pilchards, horse mackerel and anchovy; mainly related to environmental variations in abundance and size distribution of the small pelagic fish. We also identified a shift in minor species from upper slope habitats ( e.g., blue whiting Micromesistius poutassou ) to species inhabiting coastal waters ( e.g., sprat Sprattus sprattus ), reflecting changes in the distribution of common dolphins within the Bay of Biscay. Finally, we highlighted the consistency over time in the prevalence of fresh pilchards, anchovies and horse mackerels in the dolphin stomachs, suggesting they are more likely to feed specifically on these species when bycatch occurs. The risk of bycatch may therefore be modulated by dolphins’ target species, with a higher risk being associated with dolphins feeding on pilchards, anchovies or horse mackerel rather than other prey species.

Marine mammals, trophic ecology, stomach contents, temporal variation, small schooling fish

Interactions between fisheries and marine megafauna, such as sharks, seabirds and marine mammals, have long existed in all oceans (Crowder et al., 2008; Furness, 2003; Lewison et al., 2004; Northridge, 1991). These interactions can be indirect — through fishing-induced changes in prey abundance and size structure; (Demaster et al., 2001; Goñi, 1998; Heino et al., 2015; Hsieh et al., 2006; Jennings and Kaiser, 1998) — or direct, through incidents like ship strikes, entanglements, and captures in fishing gear (Lewison et al., 2004; Read, 2008; Schoeman et al., 2020; Stelfox et al., 2016). Notably, the accidental capture of non-targeted species by fisheries, known as bycatch, is a leading cause of human-induced mortality in marine megafauna, contributing to significant population and species declines worldwide (Avila et al., 2018; Barbraud et al., 2013; Dulvy et al., 2014; Lewison et al., 2004; Read, 2008; Žydelis et al., 2013). These large marine vertebrates are particularly susceptible to fishing-related mortality due to their life-history traits, including long lifespans, low reproductive rates, and late maturity (Davidson et al., 2012; Dulvy et al., 2014; Hutchings et al., 2012). Thus, managing the environmental impacts of fishing, including bycatch, has become crucial (Gilman et al., 2022; Hazen et al., 2018; Lewison et al., 2014; Soykan et al., 2008). Among marine mammals, bycatch frequently occur in both commercial and artisanal fisheries (Jog et al., 2022). This problem often arises from the overlap between the species targeted both by these marine predators and by fisheries, leading to direct competition for resources, or at least spatial overlap between predator foraging distribution and fishing areas (Baird et al., 2021; Ferro de Godoy et al., 2020; Morissette et al., 2012). Consequently, bycatch rates can be influenced by the foraging strategies of the different predator species, and the commercial species targeted by fisheries, as well as the used fishing gears. The understand of feeding habits and trophic interactions of predators is therefore essential to shed light on their interactions with fisheries (Young et al., 2015). Stomach content analysis is commonly used in feeding ecology studies of marine to predators (Bowen and Iverson, 2013; Trites and Spitz, 2018). Despite potential biases due to variable digestion rates and rare prey, this method provides critical details on the prey species consumed including individual prey size and mass. Unlike other techniques, such as stable isotope analysis, which offer a more integrated view (Newsome et al., 2010; Trites and Spitz, 2018), stomach content analysis can focus on the most recent food intake in using only fresh remains, making it thus particularly relevant to understand some mechanisms leading to bycatch events. The Bay of Biscay in the northeastern Atlantic Ocean is a highly productive region, characterized by upwelling areas that support a rich abundance and diversity of fish species (Allain et al., 2001; Borja et al., 2019). Hence, numerous fisheries have intensively operated over time using a variety of fishing gear such as gillnets, trawls and trammel nets (Demanèche et al., 2019; Guénette and Gascuel, 2012). This productive ecosystem sustains also numerous top predators, including marine mammals, of which the short-beaked common dolphin ( Delphinus delphis ) is the most abundant (Gilles et al., 2023; Laran et al., 2017; Perrin, 2018). To meet their high energy requirements, common dolphins feed primarily on small energy-rich pelagic schooling fish, such as anchovy ( Engraulis encrasicolus ), pilchards ( Sardina pilchardus ) or horse mackerel ( Trachurus spp.), which are also important fisheries targets (Meynier et al., 2008; Spitz et al., 2014; Spitz et al., 2018). However, common dolphins are regularly caught as bycatch since decades, initially mostly in winter in pelagic trawl fishery targeting seabass ( Dicentrarchus labrax ) (Castro et al., 2024; Morizur et al., 1999; Peltier et al., 2016). Since 2016, a strong increase of bycatch level of common dolphin was observed in the Bay of Biscay (Meheust et al., 2021; Peltier et al., 2021). In addition, these high levels of bycatch were found to occur not only in pelagic trawls but also in gillnets and trammel nets targeting flatfish and European hake, Merluccius merluccius (Paillé et al., 2024). This occurred in conjunction with a change in spatial distribution of the common dolphin, more widespread and nearer to the coast than before (Laran et al., 2022). Common dolphins’ bycatch is recognized as a major issue for fisheries management and conservation in Europe. The level of this additional mortality is estimated as unsustainable for the long-term viability of this cetacean (Taylor et al., 2022) and in the Bay of Biscay, a one-month closure of the fisheries concerned has been decreed during the winter period. Understanding the mechanisms leading to their capture is essential to implement long-term effective mitigation measures allowing both to maintain dolphin population and fisheries. Stomachs of bycaught individuals generally contain fresh prey remains, providing evidence that bycatch occur when feeding and is therefore related to trophic interactions. Trophic overlap in the feeding preferences of sea bass and common dolphins has previously been identified as an underlying mechanism to explain the high bycatch rates observed in pelagic trawl fisheries in the Bay of Biscay when common dolphins feed among sea bass schools (Spitz et al., 2013). The recent increase of dolphin bycatch notably in bottom nets, associated with changes in their spatial distributions could be related to modifications in their feeding ecology. Therefore, using data from stomach content analysis conducted between 1999 and 2019, we investigated temporal changes in the feeding ecology of the common dolphin in the Bay of Biscay. The main objectives of this study were to i) explore differences in prey composition and diversity, and ii) analyze prey-specific differences in size and relative importance over time. Finally, we discussed ecological implication of our results in the context of bycatch mechanisms and changes in prey species availability.

Data collection A total of 255 stomach contents of bycaught common dolphins were collected in the Bay of Biscay during winter (December to March) from 1999 to 2019 (Figure 1). Twenty-eight individuals were directly sampled on board pelagic pair trawlers in 2005 and 2006. The remaining stomach contents were sampled from dolphins found stranded along the French coast of the Bay of Biscay whose cause of death was attributed to a bycatch event (n=227) by the French national stranding network (Wund et al., 2023). Individuals were dissected either on the field or in the laboratory, stomach were ligatured, and stored deep-frozen (-20°C) in polythene bags awaiting further analysis. Each individual has been measured and sexed and the location of capture or stranding was collected (Table 1). 17 individuals excluded from further analysis because their size or sex remained undetermined including 3 that were found stranded with their tail cut off. Data from 68 individuals, collected between 1999 and 2002 have previously been included in a scientific publication (Table 1 ; Meynier et al., 2008). Table 1. Number of samples, percentage of females, mean body size (± standard deviation) and ranges (cm) of individuals per year. | 1999 | 13 | 38,5 | 188 ± 30 | [128 - 233] | (Meynier et al., 2008) | | 2000 | 19 | 52,6 | 187 ± 18,2 | [158 - 232] | (Meynier et al., 2008) | | 2001 | 15 | 33,3 | 183 ± 29,0 | [132 - 228] | (Meynier et al., 2008) | | 2002 | 21 | 57,1 | 194 ± 20 | [147 - 223] | (Meynier et al., 2008) | | 2003 | 12 | 58,3 | 183 ± 17,9 | [144 - 202] | This study | | 2004 | 12 | 66,7 | 199 ± 20,8 | [174 - 243] | This study | | 2005 | 9 | 66,7 | 181 ± 21,9 | [150 - 221] | This study | | 2006 | 21 | 72,00 | 181 ± 20.9 | [138 - 225] | This study | | Total | 122 | 56,4 | 187 ± 22,6 | [128 - 243] | | | 2017 | 48 | 52,1 | 184 ± 22,5 | [110 - 224] | This study | | 2018 | 38 | 50,0 | 183 ± 22,9 | [109 - 228] | This study | | 2019 | 30 | 43,3 | 183 ± 19,0 | [120 - 222] | This study | | Total | 116 | 49,1 | 183 ± 21,6 | [109 - 228] | Stomach content analysis Whole stomachs were thawed and their content was washed through a sieve of 0.2 mm mesh size. From the 238 samples analyzed, hard diagnostic parts (fish bones, otoliths and cephalopod beaks) and fresh prey items were retrieved and identified by using published guides ( e.g., Clarke, 1986; Härkönen, 1986) or an internal reference collection. Fish bones and otoliths were stored dry, whereas cephalopod beaks and crustacean remains were kept in 70% ethanol. Each prey item was sorted depending on their degree of digestion. This allowed us to determine a fresh fraction, which is more representative of the composition of prey consumed close to death than a total diet composition derived from all prey items. The total number of food items was estimated as the highest number, given by either the number of paired structures ( e.g., otoliths, opercula, and hyomandibular, dentary and premaxillary bones for fishes, upper and lower beaks for cephalopods, and eyes for crustaceans) or unpaired structures ( e.g., parasphenoid bone for fishes, gladius for cephalopods, and carapace and telson for crustaceans). Diagnostic hard parts were measured (± 0.02 mm) following standards. If more than 30 remains were present for one taxon in a stomach, a sub-sample of 30 was measured. Individual prey body length (total length – TL – for fish and Dorsal Mantle Length for cephalopods, in cm) and mass (in g) were calculated using regressions from the literature or from our reference collection (Supplementary Table S1). Diversity and composition analyses Temporal variation was investigated considering two periods: “former” including individuals sampled between 1999 and 2006 and “recent” between 2017 and 2019. Diet analyses were conducted according two levels of details: Total diet, including all data regardless of the prey items’ state of digestion, and Fresh diet, only including fresh items. 15 stomach contents only contained accumulated prey items and were only considered in the Total diet analysis. Thus, for Total diet and Fresh diet analyses, a total of respectively 238 and 223 stomach contents were statistically analyzed using R (version 4.3; R Core Team, 2023)). Cumulative prey curves were constructed against the randomly pooled number of analyzed stomachs to check if a sufficient number of stomach contents had been collected to accurately describe the diet of each predator (Cortés, 1997). Curves were generated after 100 randomizations of the original data using the Vegan Community Ecology package (Oksanen et al., 2024). When curves approached an asymptote, it was considered that a sufficient number of stomachs were processed. To statistically assess the adequacy of sample size, a linear regression was performed on the final five points of the curve. The levelling off of the prey curve was considered acceptable when the slope was b < 0.05. To test temporal dietary differences, time periods were separated. Frequencies of occurrence, relative abundance and reconstructed biomass of each prey species were calculated by time period. The frequency of occurrence of a given prey taxon was calculated as the number of stomachs in which the taxon was observed. The relative abundance was assessed as the number of items found in the sample set. The reconstructed biomass was calculated as the product of the number of individuals and the average reconstituted body mass in each stomach, summed throughout the sample set. Sampling errors were assessed by generating non-parametric 95% confidence intervals around percentage number and mass using bootstrap simulations where random samples were drawn with replacement with 1000 iterations (matrices produced will be available on InDoRES data repository by the time of the publication). Total fish length distributions per period were weighed by the reconstituted mass. Fish length distributions at sample level were weighted by the number of individuals in the sample and summed to produce the overall size distribution of a prey taxon in the whole series of samples. Within and between samples diversity were investigated using alpha diversity index, including estimates of species richness, Shannon’s and Simpson‘s diversity indices based on abundance to investigate evenness and equitability of species in each sample (Peet, 1975). These analyses were performed over prey occurrence and abundance data between time periods for Total diet and Fresh diet analyses. Diet comparisons Composition Comparison of diet composition (reconstructed biomass of each prey species) between time periods was assessed using nonmetric multidimensional scaling (nMDS) ordination based on Bray-Curtis dissimilarities on log-transformed biomass data. Ordination mean plots were constructed through bootstrap average (n=100). We also tested the correlation of environmental and biological variables (longitude and latitude of sampling and size and sex of dolphins) with the ordination configuration to investigate drivers for the composition of dietary samples and test for sensibility to sampling. The significance of fitting vectors was assesses using a permutation of environmental variables (n=999). Analyses of similarities (ANOSIM) also based on Bray-Curtis dissimilarities were used to test the significance of the observed patterns in the nMDS (Somerfield et al., 2021). nMDS and ANOSIM, performed using the Vegan Community Ecology package (Oksanen et al., 2024). To assess the degree of diet overlap between the two periods, we used Pianka’s Index using the function dietOverlap() in the R package ‘FSAmisc’ (Ogle et al., 2025). At the species levels, to determine whether there was a significant difference in the contribution in mass of each prey among time periods, we performed Student t-test ( t statistic) over diet row data. When normality of data was not validated, non-parametric test Wilcoxon ( W statistics) was used. Prey size After normality was validated, comparison of fish length distributions were performed using Student t-test weighted by the number of individuals with the Survey package (Lumley, 2004).

Cumulative prey curves indicated sample size reach asymptote ( b < 0.05) at the species level for both time periods (Supplementary Figure S2). Sample sizes were thus sufficient to describe the overall and fresh diet of common dolphin. Descriptive analysis and composition of diet Former period (1999-2006) To describe the diet of the common dolphin in the former period, a total of 122 individuals were sampled from 1999 to 2006 on the French coast of the Bay of Biscay (Figure 1). These individuals included 67 females and 55 males and ranged from 128 to 243 cm (Table 1). Out of these 122 stomach contents, 28 966 prey items were retrieved, including the presence, in average, of 237 (± 336 S.D) prey items in each stomach. Among all prey items, 21 348 were considered accumulated and were not included in fresh diet analysis ( i.e ., 6 stomach contents, only containing accumulated items, were excluded). Overall, 95% of the stomachs showed the presence of fresh remains, indicating that the majority of the animals studied died during or shortly after feeding. Fish were predominant in both total and fresh diet analyses, respectively occurring in 100% and 92% of samples analyzed (Table 2). Fish were also prevalent by number and reconstructed mass in total diet (92.6% and 92.8%, respectively) and in fresh diet (78.2% and 86.4%, respectively). In comparison, cephalopods occurred in 94 of the samples (77%) (including 70% of the samples containing only fresh items) and ranked second by number in total (5.6%) and fresh diet (16.4%) but also by reconstructed mass (7.2% and 13.6%, respectively). Crustaceans were only anecdotal in both total and fresh diets. Among all prey items, 44 prey species were identified in the total diet. Out of them, Gobiidae and horse mackerel were the most occurrent and abundant preys, being present in 84% and 80% of the stomach contents analyzed and reaching 40.2% and 24.8% of preys by number, respectively. However, because of their small size, the importance of Gobiidae was only anecdotal in reconstructed mass (3.2%). On the contrary, horse mackerel were also the most important preys in terms of reconstructed mass, reaching 34.7% of relative importance. Pilchards and anchovy were also important preys in terms of occurrence (62% and 70% of samples, respectively) as well as relative importance by reconstructed mass (24.2% and 8.5%). Other common prey species were cods and hake, including the blue whiting ( Micromesistius poutassou; 37% of occurrence), the pout ( Trisopterus spp.; 48% of occurrence) and the European hake (33%), reaching respectively 6.8%, 4.0% and 4.6% of reconstructed mass. Among cephalopods, Loligo spp. and Sepiolidae were frequent preys, being present in up to 43.4% and 55.7% of stomach contents respectively, but less important species in terms of relative reconstructed biomass (reaching 5.0% for Loligo spp.). Only 28 prey species were still present in fresh diet. Amon them, anchovy and Sepiolidae were the most frequent preys (respectively occurring in 54% and 45% of stomach contents) but were of lesser importance in terms of relative abundance (respectively 19.7% and 11.0%) and reconstructed mass (respectively 11.1% and 0.8%). Few other species were less common but of major importance in terms of biomass, including pilchards (45% of occurrence), and horse mackerel (40% of occurrence) reaching respectively 38.4% and 20.7% of the reconstructed mass. It is noteworthy, that Loligo spp. occurred in 36% of the stomach contents and represented 8.2% of reconstructed mass. Recent period (2017-2019) In the recent period, a total of 116 individuals were sampled from 2017 to 2019 on the French coast of the Bay of Biscay to describe the diet of the common dolphin (Figure 1). These individuals included 57 females and 59 males and ranged from 109 to 228 cm (Table 1). Out of these 116 stomach contents, 29 051 prey items were retrieved, including the presence, in average, of 250 ± 280 prey items in each stomach. Among all prey items, 22 134 were considered accumulated and were not included in fresh diet analysis ( i.e., 9 stomach contents, only containing accumulated items, were excluded). Overall, 92% of the stomachs showed the presence of fresh remains, indicating that the majority of the animals studied died during or shortly after feeding. As in the recent period, fish prevailed in both total and fresh diet analyses, occurring at least in 99% of the samples analyzed (Table 3). Fish were also major preys by number and reconstructed mass in total diet (96.1% and 94.5%, respectively) and in fresh diet (96.9% and 97.8%, respectively). In comparison, cephalopods occurred in 102 (88%) of the samples (including 50% of the samples containing only fresh items) and ranked second by number in total (3.9%) and fresh diet (2.9%) but also by reconstructed mass (5.3% and 2.2%, respectively). Crustaceans were only anecdotal. Among all prey items, 53 prey species were identified in the total diet. Out of them, anchovy was the most occurrent and abundant preys, being present in 71% of the stomach contents analyzed and reaching 39.1% and 28.6% of relative abundance and reconstructed mass. Sepiolidae was also a common species with 76% of occurrence through the samples but represented only a small part of relative abundance and mass (respectively 2.9% and 0.2%). As in the former period, Gobiidae, including Pomatoschistus spp., and horse mackerel were frequent species, being both present in 52% of the samples analyzed but were only of secondary importance in terms of relative reconstructed mass (up to 9.9% for the horse mackerel). Pilchards were also still among the most occurrent preys (46%) reaching a contribution to the overall diet of 23.9% by reconstructed mass. Other common prey species were sandeels (Ammodytidae; 36% of occurrence), sprat ( Sprattus sprattus ; 35%) and mackerel ( Scomber spp.; 30%), reaching respectively 5.6, 3.6% and 3.6% of reconstructed mass. The presence of pout and European hake is also noteworthy with approximately 26% of occurrence and reaching 6.1% of reconstructed mass for hake. Fourty-two prey species were still present in fresh diet. Among them, anchovy was the most occurrent (62%) and abundant (48.%) prey species. Several other species were also commonly found, such as Sepiolidae, Gobiidae (mostly Pomatoschistus spp.), pilchards, horse mackerel and sprat occurring in 30% to 40% of the samples. However, considering their relative importance by number and reconstructed mass, only pilchards (respectively 15.4% and 41.3%) was also major species. In terms of reconstructed mass, sandeels and mackerels were also relatively important, reaching 5.8% and 7.8% of total mass. Diet comparison Composition No difference in diversity were found (Table 4). Analysis of reconstructed mass composition of the total diet showed a switch in major prey from the former to the recent period, with a significant decrease in horse mackerel contribution ( t = 2.8365, p = 0.0052) and an increase in anchovy contribution ( t = -4.2636, p < 0.005) to the overall diet while pilchard and European hake contributions did not vary (Figure 2A). Figure 2. Comparison of contribution in reconstituted mass of main prey species between 1999 and 2006 (purple) and 2017-2019 (green) in total diet (left panel) and fresh diet (right panel). *: p < 0.05; **: p < 0.01 and *** p < 0.005. SARD_PIL: Sardina pilchardus ; TRAC_spp: Trachurus spp.; ENGR_ENC: Engraulis encrasicolus ; MERLU_MER: Merluccius merluccius ; MICR_POU: Micromesistius poutassou ; LOLI_spp: Loligo spp.; AMMO_spp: Ammodytidae; SCOM_spp: Scomber spp.; SPRA_SPR: Sprattus sprattus In addition, few other minor preys showed differences in their contribution to the diet between the two time periods. For example, the relative importance by mass of blue whiting and squids decreased ( W = 9353, p < 0.005 and t = 2.8365, p = 0.0052, respectively). However, nMDS ordinations and statistical analyses were unable to identify any differences in dietary composition between the two time periods (ANOSIM, global R statistic 0.057, p = 0.001; Figure 3A). The diet overlap analysis also indicated a high degree of overlap in the overall diet between the two periods (Pianka’s Index, 0.77). In the fresh diet, variations in the contribution of preys were similar with a decrease in horse mackerel ( t = 2.1549, p = 0.033) and squids ( t = 3.7954, p < 0.005), and an increase in anchovy ( t = -2.7289, p =0.007; Figure 2B). In addition, the relative importance of mackerel and sprat increased in the recent period but the variation was significant for mackerel only ( t = -2.1082, p = 0.037). nMDS ordinations and statistical analyses for the fresh diet found significant difference in diet of the two time periods (ANOSIM, global R statistic 0.175, p < 0.001, Figure 3B). Environmental and biological variables were fitted to the nMDS ordination and Latitude was found to be significant to explain the ordination of samples (r 2 = 0.0756, p < 0.001) as a function of the axis MDS1 but sex and size were not significant. The diet overlap analysis however highlighted a fairly high degree of overlap in the fresh diet between the two periods (Pianka’s Index, 0.64). Figure 3. Nonmetric multidimensional scaling (nMDS) ordinations of the importance in mass of different prey species in total diet (left panel) and fresh diet (right panel) with S.E. ellipses based on the time period. Purple dots represent the samples of the former period and green dots, the samples of the recent period. Significant variable to explain the ordination of samples is represented with a dark arrow. ***: p < 0.001. Prey size In total, prey size did not vary depending on the time period (Figure 4). While a few larger preys occurred such as Boops boops, Clupea harengus, Conger conger and Solea spp., the size of some important prey increased (Table 5). Figure 4. Comparison of the global fish length distribution (cm) weighted by their contribution in mass between the former period (1999 – 2006; purple) and the recent period (2017 – 2019; green). For instance, it is the case for European hake with an increase of 2.3 cm TL of the mean size ( t = 4.58, p < 0.001) and blue whiting (+5.1 cm TL, t = 11.55, p < 0.001; Figure 5). However, between the two time periods the size of the most important preys in terms of contribution decreased, such as for pilchards (- 3.2 cm TL, t = -19.178, p < 0.001), anchovy (-2.5 cm TL, t = -31.817, p < 0.001), horse mackerel (-2.0 cm TL, t = -19.619, p < 0.001) and argentines (-5.1 cm TL, t = -13.661, p < 0.001). For some other preys such as sandeels, whiting, mackerel and sprat, no differences in size were found. Figure 5. Comparison of fish length distributions (cm) for main species weighted by their abundance between the former period (purple) and the recent period (green). * : p < 0.001.

The present study provides new information on the diet of the short-beaked common dolphin ( Delphinus delphis ) in the Bay of Biscay between 1999 and 2019. Although the overall diet of common dolphin was still mainly composed of small schooling energy-rich fish, we revealed some significant variations in the biomass contributions of different prey species over time. Additionally, a decrease in the mean size of some main prey was also found. Furthermore, the predominance of pilchard and anchovy in the fresh remains of bycatch individuals, whatever the period studied, suggests that foraging for these prey species resulted in a higher risk of bycatch. Dietary stability of the common dolphin (1999-2019) In this study we found no evidence of variations in the prey type preferences of common dolphin captured as bycatch in the Bay of Biscay in winter, which remained mainly composed of high energy density prey species (Meynier et al., 2008; Spitz et al., 2014). This aligns with existing knowledge about the common dolphin’s feeding strategy, which involves quality-based selection of prey to meet their energetically expensive life style (Spitz et al., 2012, 2010). However, we observed specific variations within energy-rich species. For instance, there was a significant decrease in the relative importance of horse mackerel over time and a corresponding increase in anchovy. This can be explained by the recent collapse of horse mackerel population whereas anchovy biomass increased following the fishery closure that occurred between 2000 and 2005 in the Bay of Biscay (Doray et al., 2018b; ICES, 2024). The diet of a predator is indeed influenced by multiple interacting factors, including prey abundance. According to optimal foraging theory, a predator ranks available prey types based on their energetic profitability ( i.e., calorie content divided by handling time), thereby maximizing energy intake to foraging time and associated energy costs. Such variations have already been observed in the common dolphin in the Bay of Biscay and Iberian waters and were related to prey abundance changes (Meynier et al., 2008; Santos et al., 2013). Specifically, pilchards increased related to increasing spawning stock biomass (SSB) in the late 20 th century and anchovy decreased between 1998 and 2002 in the Bay of Biscay. Towards a spatial shift to the coast? Regarding minor species, few changes were observed over time. Notably, the importance of blue whiting, pouts and squids decreased in favor of sandeels and sprats. Blue whiting and squids are more abundant prey in outer shelf and upper slope habitats while sandeels and sprat are more related to coastal waters (Doray et al., 2018a). These changes could be related to changes in spatial distribution of dolphins and/or shifts in spatial distribution of major prey towards more coastal waters (Laran et al., 2022), thus increasing the spatial overlap between dolphins and the main fishing effort in the Bay of Biscay which is concentrated near the coast (Demanèche et al., 2021). Although no difference was found in the overall size spectrum of prey between the two time periods, there were specific differences in size for the main prey of the diet. This was particularly significant for pilchards, anchovy and horse mackerel, which are among the most energy-rich species. Changes in size and body condition in the environment had already been identified for such species, particularly for pilchards (Doray et al., 2018b; Véron et al., 2020) and European anchovy (Taboada et al., 2024), suggesting that the decrease in size observed in the diet is related to environmental variations. Such variations in prey size for common dolphin was already observed in Portuguese waters with the disappearance of smaller size of pilchards in common dolphin diet related to poor recruitment to the stock (Marçalo et al., 2018). In the Bay of Biscay, it has also been found that the average size of small pelagic fishes varied depending of the spatial distribution. In particular, there is a decrease in mean body size when approaching the coast (Doray et al., 2018a), hence supporting the hypothesis of dolphins foraging closer to shore. However, the reduction in body size of small pelagic fish is also commonly attributed to ocean warming (Taboada et al., 2024). Dietary habits and potential link to bycatch events Stomach contents usually include fresh and accumulated remains representing a time of integration of several food intakes. By considering only the fresh fraction, we reduced the integration time to the last food intake focusing on a finer temporal scale close to death. In this study, the last food intake can inform on preys associated with the bycatch event. In addition, focusing on the fresh fraction only allowed to mitigate the potential bias associated with varied digestion time from one prey to another that can lead to a misestimation of some species (Brown et al., 2012). We found a majority (95% for the recent period) of the stomachs sampled contained fresh prey, suggesting that dolphins are being caught while feeding. The diet profile of the fresh fraction (diversity of prey and their contribution to the diet) was consistent with the overall diet mostly composed of small pelagic fish. This finding suggests there is no specific feeding strategy leading to bycatch, such as depredation. In Iberian waters, for example, common dolphins are regularly interacting with purse seine fisheries targeting small pelagic fish species (Marçalo et al., 2018). It was thus hypothesized that dolphins would take advantage of the fishery to forage, eventually leading to bycatch. However, the major prey species of common dolphins in the Bay of Biscay (small pelagic fish species) are not targeted by fisheries in which dolphins are captured (mostly fisheries targeting large demersal species), with the exception of the European hake. In the latter case, the size targeted by dolphins is from 14.5 ± 5.9 cm to 16.1 ± 6.7 cm on average (Table 5), respectively for the former and recent periods, which is almost twice as small as the minimum marketable size in Europe ( e.g ., North-East Atlantic: 27 cm, https://www.legifrance.gouv.fr/eli/arrete/2013/1/29/TRAM1240353A/jo/texte(JORF n°0045, 2013; ) suggesting that dolphins are not caught as bycatch when foraging on commercial species. Here, fresh pilchards, horse mackerels and anchovies dominated the prey species targeting during bycatch event with > 60% of the ingested biomass. At least one of these species were found in more than 80% of the animals studied, while Sepiolidae, mackerel and Loligo spp., were the more occurrent species in the remaining 20%. Trophic overlap and multiple predator species events between common dolphins and some commercial fish species, such as sea bass or hake, could rather explain bycatch, notably in pelagic trawls when they simultaneously forage on small pelagic fish (Corrales et al., 2022; Spitz et al., 2013). Furthermore, small pelagic fish species are also part of the diet of the anglerfish ( Lophius piscatorius ), in the northern Atlantic Ocean, an important species targeted by fisheries at risk for dolphin bycatch. Issac et al., (2017) suggested that pelagic species such as horse mackerel and mackerel could be ingested when they approach the bottom. One must assume that dolphin would also forage small pelagic fish close to the bottom rather in upper part of the column water. This can also explain the presence of few benthic prey in their diet such as flatfish.

This study provides a comprehensive analysis of the diet of the short-beaked common dolphin in the Bay of Biscay over two decades (1999 – 2019). The findings reveal that while the overall diet remains composed mainly of high-energy prey species, there are notable temporal variations in prey composition and size. Specifically, we observed significant shifts in the prevalence of certain species, such as a decrease in horse mackerel and an increase in anchovy, which correspond to changes in their environmental abundance. The analysis of the fresh fraction has proven to be crucial in understanding the immediate dietary patterns linked to bycatch events. The consistency between the fresh fraction and overall diet suggests that the bycatch events do not occur because of a feeding behavior that differs from the general habits of the species, but three prey species nevertheless seem to be more involved in the capture phenomenon. Among the variety of prey species included in the winter diet of common dolphin, feeding on anchovies, pilchards and horse mackerel appears to increase their risk of capture. Finally, the recent decrease in prey size for these species could also increase their predation efforts and the duration of feeding activities. This, in turn, may elevate their risk of encountering fishing gear, thereby increasing bycatch incidents.

This work was mainly supported by the Delmoges project, co-funded by the French Ministry of the Environment, in particular the Directorates DGAMPA (Fisheries and Aquaculture) and DEB (Aquatic Biodiversity), and the Fisheries Sector Association (FFP - France Filière Pêche). Pelagis and the French stranding programme (RNE) are supported by the French Ministry of the Environment and the French Biodiversity Agency (OFB). We would like to thank all the members of the RNE for their continuous efforts in collecting data on stranded cetaceans. The acquisition of certain samples was made possible by the Necessity (European Commission, SSP8-CT-2003-501605), Petracet (European Commission, FISH/2003/09) and Procet (DPMA, CNPMEM, Aglia) research projects. We are also grateful to those who contributed to the analysis of stomach contents and provided expertise on the diet of common dolphin: Anne-Kristell Jouan, Laureline Meynier, Jodie Thénard, Camille Treilhes and Vincent Ridoux. This study is part of the long-term Studies in Ecology and Evolution (SEE-Life) program of the CNRS. [1]¿p#1 Data accessibility Statement The data that support the findings of this study will be openly available through the data.InDoRES platform at https://data.indores.fr:443/privateurl.xhtml?token=05f415c2-f3cf-4e24-bf1f-db9e7a611d4e Competing Interests Statements The authors have no conflict of interest to declare. [1]¿p#1 Author contributions Johanna Faure : Conceptualization, Methodology, Formal Analysis, Visualization, Writing – Original Draft Preparation; Jasmin Niol : Investigation (data collection); Eléonore Meheust : Data curation; Jérôme Spitz : Conceptualization, Methodology, Investigation (data collection), Funding Acquisition, Writing – Review and Editing. Supplementary Material

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Authors Metrics & Citations Metrics Article Usage 447views 310downloads Citations Download citation Johanna Faure, Jasmin Niol, Eléonore Meheust, et al. The winter diet of common dolphins over the last 20 years reflects prey and predator ecological changes in the Bay of Biscay, and highlights bycatch circumstances. Authorea. 23 April 2025. DOI: https://doi.org/10.22541/au.174541573.35203069/v1 DOI: https://doi.org/10.22541/au.174541573.35203069/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.