# Stable isotopes reveal intrapopulation heterogeneity of porbeagle shark (Lamna nasus)

Sandrine Serre, Armelle Jung, Yves Cherel, Caroline Gamblin, Cédric Hennache, François Le Loc'h, Anne Lorrain, Anne Priac, Gauthier Schaal, Éric Stephan

<figure><img src="image_1.png" /></figure>

## To cite this version:

Sandrine Serre, Armelle Jung, Yves Cherel, Caroline Gamblin, Cédric Hennache, et al. Stable isotopes reveal intrapopulation heterogeneity of porbeagle shark (Lamna nasus). Regional Studies in Marine Science, 2024, 69, pp.103340. 10.1016/j.rsma.2023.103340. hal-04446621

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Submitted on 11 Jan 2025

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# Stable isotopes reveal intrapopulation heterogeneity of porbeagle shark (*Lamna nasus*)

Serre Sandrine <fnref n="1" />,*, Jung Armelle <fnref n="2" />,<fnref n="3" />, Cherel Yves <fnref n="4" />, Gamblin Caroline <fnref n="5" />, Hennache Cédric <fnref n="2" />,<fnref n="6" />,
Le Loch Francois <fnref n="1" />, Lorrain Anne <fnref n="1" />, Priac Anne <fnref n="1" />, Schaal Gauthier <fnref n="1" />, Stephan Éric <fnref n="2" />

## Abstract :

Porbeagle (*Lamna nasus*) is an ubiquist, highly mobile and long-living shark species with spatial and temporal sex and size segregation observed in catches. Porbeagle sharks were targeted by commercial fisheries in the European waters until the closure of the fishery in 2010. Most of the French fleet catches were located in Bay of Biscay, Celtic Sea and the English Channel. The aim of the study was to find out occurrences of spatial segregation within the shark population in the Northeast Atlantic Ocean based on the stable isotope method. Carbon and nitrogen stable isotopes (δ13C, δ15N) were measured in muscle of porbeagle sharks fished in the Bay of Biscay and the Celtic Sea between April and September 2008, and April and June 2009. Neither influence of sex nor ontogenic isotopic shift was detected in the sampled sharks, but muscle δ13C and δ15N values increased significantly from offshore towards coastal area, which reflected inter-areas variability. Realized isotopic niches were similar amongst offshore areas whilst in St Georges' Channel muscle isotopic composition exhibited higher inter-individual variability. This underpins the lack of information on life history of porbeagle sharks and underlines the need of more information on the species movements to support population management in European waters.

## Highlights

* δ¹³C and δ¹⁵N ratios in porbeagle shark do not vary with sex and size.
* δ¹³C and δ¹⁵N ratios in muscle vary across coastal and offshore areas.
* Coastal area exhibit higher δ¹³C and δ¹⁵N inter-individual variability.

<footnote n="1">¹ Univ Brest, CNRS, IRD, Ifremer, LEMAR, F-29280 Plouzané, France</footnote>

<footnote n="2">² Association Pour l'Etude et la Conservation des Sélaciens (APECS), 13 rue J-F Tartu, 29200 Brest,
France</footnote>

<footnote n="3">³ Des requins et des hommes, Bibliothèque la Pérouse, Technopole Brest Iroise, 15 rue Dumont d'Urville, 29280 Plouzané, France</footnote>

<footnote n="4">⁴ Centre d'Etudes Biologiques de Chizé (CEBC), UMR 7372 du CNRS-La Rochelle Université, 79360 Villiers-en-Bois, France</footnote>

<footnote n="5">⁵ CNPMEM, 134 avenue de Malakoff, 75116 Paris, France</footnote>

<footnote n="6">⁶ CAPENA, Site d'Oléron, Prise de Terdoux, 17480 Le Château d'Oléron, France</footnote>

<footnote>* Corresponding author : Sandrine Serre, email address : sandrine.serre@univ-brest.fr</footnote>
**Keywords** : Fisheries, Migration, Bay of Biscay, Celtic Sea, Isotopic niche
## Funding Information

Fisheries Directorate of French Ministry of Agriculture; National Committee for Marine Fisheries and Fish Farming; Joint Union for the Development of Aquaculture and Fishing in the Pays de la Loire region; Ile d'Yeu Producers' Organization; Ile d'Yeu Local Fisheries Committee; French Biodiversity Agency; French Ministry of Higher Education and Research; French National Research Institute for Sustainable Development.

# 1 Introduction

The porbeagle shark *Lamna nasus* (Bonnaterre 1788) is a widely distributed and long-lived shark species which occurs mostly in epipelagic waters from coastal areas to the open ocean (Campana and Joyce, 2004; Francis et al., 2008). During decades, it has been targeted by fisheries in the North Atlantic Ocean (Gauld, 1989; ICES, 2009), leading to a drastic decrease of populations (Campana et al., 2002). The species is listed as Vulnerable since 2006 in the IUCN Red List (Stevens et al., 2006). Fisheries targeting porbeagle shark are closed in the European waters since 2010 (Council Regulation (EU), 2010), after the joint ICCAT-ICES porbeagle assessment in 2009 (ICCAT, 2010; ICES, 2009). Porbeagle sharks are considered opportunists, feeding on teleosts and cephalopods from epibenthic to pelagic communities (Belleggia et al., 2021; Ellis and Shackley, 1995; Gauld, 1989; Joyce et al., 2002; Stevens, 1973). They undergo large seasonal movements across coastal and oceanic areas (Biais et al., 2017; Pade et al., 2009; Saunders et al., 2011), which are likely associated with spatial and temporal sex and size segregation. Indeed, from catches data over the North Atlantic Ocean, fishing area is the most important factor for size, while fishing period is the most important
factor for sex distribution (Campana et al., 2010; Campana and Joyce, 2004; Jensen et al., 2002; Jung, 2008; Mejuto et al., 2020). Thus, considering the wide distribution range of the species and its foraging movement patterns over different areas, feeding sources of porbeagle sharks may vary according to space and time, and between individuals.

Tracking feeding sources of free-ranging animals has been developed for decades using stable isotopes ratios occurring naturally, such as carbon (¹³C/¹²C, expressed as δ¹³C) and nitrogen (¹⁵N/¹⁴N, expressed as δ¹⁵N) (Trueman and St John Glew, 2019). Isotopes ratios are incorporating the food webs through primary producers and propagate along the food chain through metabolic processes (Boecklen et al., 2011; Ramos and González-Solís, 2012). δ¹³C values varies little along the food web, and, for piscivorous feeders such as porbeagle sharks, those values mainly reflect feeding sources in relation with primary production origin (Canseco et al., 2022). In the marine environment, δ¹³C values provide information on contribution of inshore-offshore gradient or pelagic-benthic gradient to feeding sources (Cherel and Hobson, 2007; Ramos and González-Solís, 2012). By contrast, δ¹⁵N values varies along the food chain with a stepwise enrichment in heavier stable isotope (¹⁵N), consequently δ¹⁵N values reflect trophic position of the consumer in the food web (McCutchan et al., 2003; Post, 2002). Nonetheless, in mobile species, isotopic composition is a dynamic process relying on turnover of tissues and reflects time integrated dietary inputs from multiple food webs (Fry and Arnold, 1982), providing clues on movements across environments and wandering behaviors (Trueman and St John Glew, 2019). Turnover of white muscle in elasmobranchs has been estimated to over a year for the low growth species, *Squalus acanthias* (Logan and Lutcavage, 2010); hence, it is expected to be similar in low growth porbeagle shark. Consequently, muscle stable isotope composition of porbeagle shark does
not depict the recent feeding strategy but reflects the long-term integrated assimilation of
food (MacNeil et al., 2005). Consequently, muscle values would be averaged across seasonal
movements of porbeagle sharks in a population depicting similar life histories across scales.
Stable isotopic compositions can be described in a bi-dimensional space (δ¹³C, δ¹⁵N), so called
realized isotopic niche (Post, 2002), thus providing a quantitative approach used to quantify
intra-specific variability.

L. nasus is known to perform seasonal movements from summer grounds on the continental shelf and shelf break to wintering offshore areas (Biais et al., 2017; Campana et al., 2010; Skomal et al., 2021). The aim of our study was to compare muscle isotopic composition in porbeagle sharks caught in offshore fishing grounds where seasonal movements have been studied (Biais et al., 2017) with porbeagle sharks from inshore fishing grounds. Hence, similar realized trophic niches were expected in relation with similar seasonal movements averaging muscle isotopic composition across the Northeast Atlantic Ocean.

# 2 Material and method

## 2.1 Sample collection

Individuals of porbeagle sharks were caught by French commercial longliners targeting this
species, from April to September 2008 (n = 58) and April to June 2009 (n = 93) (Table SI,
Supporting Information). The fishing grounds were located along the continental shelf in the
Bay of Biscay (South and North Bay of Biscay), around Belle-Ile and in the Celtic Sea (South
Ireland and St Georges' Channel) (Figure 1). The South Ireland, and North and South Bay of
Biscay are offshore areas, whilst St Georges' Channel and Belle-Ile are coastal areas. The
porbeagle sharks were identified on board with a label attached to the caudal fin and stored
in ice until they were landed at Les Sables d'Olonne (France). At the fish market, landed
porbeagles were measured (fork length, in cm) and sexed (Table 1). Dorsal white muscle was
sampled on individuals and stored in ethanol 70%. Sampled porbeagle sharks were all
mature and immature males and mainly immature females based on the estimated length-at-
maturity of 166 cm (males) and 208 cm (females) (Jensen et al., 2002; Natanson et al., 2002),
with only three individuals caught at size close to 70-90 cm, the estimated size range at birth
(Francis et al., 2008; Francis and Stevens, 2000). Overview on sampling size distribution can
be found in Supplementary materials (Figure SI) and further information about the fisheries
and sampling can be found in Hennache and Jung (2010, in French).

**2.2 Stable isotope analysis**

At the laboratory, muscle samples were dried at 50°C for 48h to remove ethanol. They were
then crushed into fine powder. As lipids are depleted in ¹³C (DeNiro and Epstein, 1978), they
were extracted using cyclohexane before analysis and samples were dried out during 12 h at
50°C. Stable isotope measurements were performed with a continuous-flow isotope-ratio
mass spectrometer (Delta V Advantage, Thermo Scientific, Germany) coupled to an elemental
analyzer (Flash EA1112 Thermo Scientific, Italy) at the Littoral, Environment and Societies
(LIENSs) Joint Research Unit stable isotope facility at the La Rochelle University, France.
Results were expressed in standard δ notation based on international standards, Vienna Pee
Dee Belemnite for δ¹³C and atmospheric nitrogen for δ¹⁵N, following the formula δX =
[(Rsample/ Rstandard) - 1] × 10³, where X is ¹³C or ¹⁵N and R is the ratio ¹³C/¹²C or ¹⁵N/¹⁴N.
Replicate measurements of five random samples indicate measurement errors <0.20‰ for both δ¹³C and δ¹⁵N values.

While several studies unveiled possible confounding effect of lipids and urea on elasmobranch muscle (Carlisle et al., 2017; Hussey et al., 2012; Kim & Koch, 2012), this study was conducted in 2008 and 2009 and did not address effects of urea. However, a correction was performed on δ¹⁵N values based on methodological study (unpublished data) using the following equation: $δ¹⁵N_{corrected} = 0.77 × δ¹⁵N_{cyclohexane} + 3.4$, where $δ¹⁵N_{corrected}$ is the δ¹⁵N value when sample underwent lipid and urea extraction as recommended by Li et al. (2016) and $δ¹⁵N_{cyclohexane}$ is δ¹⁵N value when sample underwent cyclohexane treatment.

## 2.3 Statistical analysis

Due to the low number a samples in Belle-Ile area (n = 4, Table 1), those samples were excluded from the following quantitative analysis.

Independence of categorical variables sex and area was tested using Chi² test. Normality of quantitative variables (length, δ¹³C, δ¹⁵N) was tested with Shapiro-Wilk tests and variance homogeneity with Bartlett's tests. Normality and homoscedasticity were assessed for individual length distribution solely. Therefore, Two-Way ANOVAs were performed to test effect of sex and area on individual length. Correlations between individual length and δ¹³C and δ¹⁵N values were performed using a Spearman's rank correlation. All results from tests were judged significant when *p* < 0.05. Values are means ± SD.

Since multivariate normality assumption was not met for stable isotope ratios, a Bayesian posterior distribution of muscle δ¹³C and δ¹⁵N values was estimated for each area using a
vague prior following a Normal Inverted Wishart distribution (R package *nicheROVER*, (Swanson et al., 2015)). Using Bayesian framework provided estimates of niche areas, known as corrected standard ellipse areas (Jackson et al., 2011), that are insensitive to sample size. Overlap metric of niches and associated Credible Intervals (CI) were calculated from wider niche areas provided by 100,000 draws from inferred posterior distributions. Values are means (CI95%).

Statistical analyses were conducted using R 4.2 (R Core Team, 2023).

# 3 Results

Muscle δ¹³C and δ¹⁵N values were measured on 151 porbeagle sharks (Table 1). Spatial sampling was unbalanced with 69 individuals analyzed from St Georges' Channel and only four from Belle-lle. Males and females numbered 74 and 77, respectively. Nevertheless, area had a significant effect on length (Two-way ANOVA p << 0.01), unlike sex compositions and combined variables (Two-way ANOVA p = 0.34 and 0.36, respectively). No significant correlation was found between isotopic values (δ¹³C and δ¹⁵N) and individual length (Spearman's rank test, ρ < 0.01 and ρ = -0.12, respectively, both p > 0.05). No significant effect of sex on δ¹³C and δ¹⁵N values were detected (Kruskal-Wallis test, p = 0.2 and 0.3, respectively).

Overall, muscle δ¹³C values ranged from -18.5 to -13.8‰, and δ¹⁵N from 11.3 to 15.7 ‰ (Table 1). Muscle δ¹³C value in the coastal area of St Georges' Channel (-16.9 ± 0.8 ‰) was significantly higher than in the offshore areas: South Ireland (-17.4 ± 0.5‰, Mann-Whitney p < 0.01), North Bay of Biscay (-17.5 ± 0.4 ‰, Mann-Whitney p << 0.01) and South Bay of
Biscay (-17.4 ± 0.5‰, Mann-Whitney p << 0.01). Biplot of stable isotopes ratios and their marginal distribution (Figure 2) indicated variations in muscle δ¹⁵N values distribution amongst sites whilst muscle δ¹³C values distribution exhibits less variation. Most of the muscle δ¹³C and δ¹⁵N values were packed in the normal distribution range delineated by ellipses. However, five immature females ranging from 126 to 170 cm and one immature male of 157 cm, all sampled in the coastal area of St Georges' Channel, were visibly outside ellipses due to higher δ¹³C values ranging from -15.5 to -13.8‰.

Offshore isotopic niches depicted strong overlap between areas. Hence, South Ireland's isotopic niche strongly overlapped North and South Bay of Biscay's niches, 96% with CI95% = [86,100]% and 92% with CI95% = [77,99]%, respectively (Figure 3). In contrast, coastal isotopic niche of St Georges' Channel exhibited lower overlap with other isotopic niches of South Ireland (58%, CI95% = [39,77]%), North Bay of Biscay (63%, CI95% = [45,81]%) and South Bay of Biscay (37%, CI95% = [19,59]%). These overlaps are likely driven by differences in δ¹⁵N values (Figure 2). Offshore Bay of Biscay areas were overlapping in the same magnitude of values: South Bay of Biscay isotopic niche encompassed 81% of North Bay of Biscay isotopic niche with CI95% = [68,97]% and North Bay of Biscay encompassed also 85% of South Bay of Biscay with CI95% = [63,94]%.

**4 Discussion**

**4.1 Sampling and method**

Porbeagles were sampled from spring to autumn during the species' fishing season of the French longline fleet. Therefore, our study provides information restricted to the fishing
season and fishing area based on fishers' empirical knowledge of optimal fishing grounds. Hence, it did not cover the whole year and did not represent the entire distribution range of the porbeagle shark population in the Northeast Atlantic. Indeed, this population is considered as homogeneous across the NE Atlantic Ocean, with limited exchanges with NW Atlantic Ocean population (González et al., 2021; Testerman, 2014). Another limitation is that only four porbeagle sharks have been sampled in Belle-Île coastal area; they can be considered as preliminary observations that merit further investigation.

Muscle samples were stored in ethanol 70%, which can potentially affect stable isotope values (Burgess and Bennett, 2017; Kim and Koch, 2012; Olin et al., 2014). Nevertheless, several studies on a variety of biological models, including elasmobranchs used the same sample conservation protocol mostly for logistical reasons (Argüelles et al., 2012; Chan et al., 2022; Lelièvre et al., 2018), and without any significant alteration of the isotopic composition of tissues (Hobson et al., 1997; Kelly et al., 2006; Kim et al., 2012b). Moreover, all our samples underwent the same treatment, which make unlikely the introduction of any comparative bias into the results.

## 4.2 No influence of sex and size

Muscle isotopic values of porbeagle shark reflect dietary inputs integrated over a period probably exceeding a year, considering its large size when compared to species on which estimates are available (Kim et al., 2012a; Logan and Lutcavage, 2010). Consequently, the stable isotope composition of this tissue is unlikely to be strongly affected by seasonal diet variations, even though the isotopic values of prey consumed over the last months should dominate the shark values (Ballutaud et al., 2022). The porbeagle shark is considered as a
generalist pelagic/demersal feeder, whose diet is generally dominated (up to 90%) by
teleosts, with cephalopods as a secondary prey throughout its distribution range (Belleggia
et al., 2021; Ellis and Shackley, 1995; Joyce et al., 2002). Around the coasts of Great Britain,
its diet is dominated by mackerels (*Scomber* spp.), clupeids, epibenthic teleosts and
cephalopods (Ellis and Shackley, 1995; Gauld, 1989). The diet of porbeagle shark has been
reported to be consistent among sexes in the NW Atlantic, with seasonal variability
associated to foraging in shallower waters in autumn (Joyce et al., 2002). Accordingly, no
isotopic difference was detected among sexes in any of the five sampling areas in the NE
Atlantic, which suggest that whenever they are present in sympatry, there is no sex-related
resource partitioning in porbeagle sharks. Temporary trophic specialization associated with
sex segregation, a feature of porbeagle shark ecology (Campana et al., 2010; Jensen et al.,
2002), yet cannot be totally ruled out, even though the time-integrating nature of stable
isotopes in muscle makes it unlikely on the analyzed population. The foraging behavior of
porbeagle is expected to vary with growth in relation with jaw gape size, as observed in other
pelagic species, such as blue shark *Prionace glauca* (Estupiñán-Montaño et al., 2019) or the
related white shark *Carcharodon carcharias* (Cortés, 1999). However, no isotopic shift
related to likely ontogenic dietary shift was found in porbeagle sharks in our study, which is
in line with results obtained in the Southwest Atlantic Ocean (Belleggia et al., 2021). This is
probably related to embryonic development of jaws and teeth in *L. nasus* with free-swimming
young exhibiting jaws and dentition with similar variations as large mature porbeagle sharks
(Bemis et al., 2015; Francis and Stevens, 2000; Shimada, 2002). Consequently, immature and
mature porbeagle have no morphological limitation to exhibit similar diets. Nonetheless, due
to our unbalanced sampling of sizes for both males and females, more information is needed
on small individuals of both sexes and on mature females to confirm the absence of ontogenic
dietary shift in porbeagle sharks in the NE Atlantic.

4.3 Spatial isotopic overlap

The main difference of porbeagle shark isotopic niches amongst areas was related to the
distance to the shore, with more positive muscle δ¹³C and δ¹⁵N values in coastal (St Georges'
Channel and Belle-Ile) than offshore individuals. Thus, porbeagle sharks spent enough time
(months) in these different areas to exhibit significant differences in their muscle isotopic
composition. Higher δ¹³C values in coastal than offshore sharks were likely related to the
well-known inshore/benthic-offshore/pelagic gradient in δ¹³C particulate organic matter
that propagate all along the food web (Cherel and Hobson, 2007; France, 1995; McMahon et
al., 2013). Indeed, high δ¹³C values are generally associated to coastal habitats driven by
benthic food webs, whilst offshore areas are more likely plankton-based food webs with
lower δ¹³C values (Chouvelon et al., 2012; France, 1995; Nerot et al., 2012).

The porbeagle sharks δ¹⁵N values were consistent with a relatively high trophic level in line
with their diet on small and medium teleosts and cephalopods (Layman et al., 2015; Post,
2002). However, Bay of Biscay is a spatio-temporally dynamic environment that affects the
δ¹⁵N baseline (Chouvelon et al., 2012; Nerot et al., 2012; Post, 2002) and therefore, δ¹⁵N
values should be interpreted with caution. Spatial models of variation in isotopic baselines,
namely isoscapes, do not have yet sufficient spatial resolution to support a *posteriori* baseline
estimates for our study (Magozzi et al., 2017; McMahon et al., 2013). Nevertheless, baseline
δ¹⁵N values generally decrease from coastal to offshore areas and this difference also
propagates along the food web (Chouvelon et al., 2012), in line with the variations of δ¹⁵N
values observed in our study. This support hypothesis of spatial intra-population differences
with long-term diets likely reflecting long-term associations to offshore or coastal areas.

Porbeagle sharks are considered as opportunist feeders (Belleggia et al., 2021; Ellis and Shackley, 1995), therefore the range of δ¹³C and δ¹⁵N values across individuals feeding in similar areas in the similar proportion of prey should be relatively narrow according to Matthews & Mazumder (2004). In addition, Bird et al (2018) suggest that shelf sharks such as *L. nasus* are generalist consumers feeding on distinct food webs but largely dependent on foraging within shelf areas with some individuals being more specialists than others. Indeed, in our study, the ranges of δ¹³C values in *L. nasus* are relatively narrow in the offshore areas (2.0 ± 0.5 ‰) with little inter-individual variations amongst areas supporting the hypothesis of similar diet across space and time. However, porbeagle sharks caught in St Georges' Channel exhibit greater δ¹³C range (4.5 ‰) in association with higher inter-individual variability. This inter-individual variability might be related to greater differences in their feeding habits within the area (Bird et al., 2018) or different past migration patterns (Abrantes and Barnett, 2011). Indeed, heterogeneity in stable isotope compositions within an area could also be related to staging arrival or to different previous individuals' life histories (Abrantes and Barnett, 2011). As shown by tracking studies (Biais et al., 2017; Pade et al., 2009; Saunders et al., 2011), *L. nasus* does not permanently reside in a given area and realizes seasonal migration out of the sampled areas for winter but returns to sampled shelf break next spring/summer. However, the migration of coastal porbeagle sharks from the St Georges' Channel remains unknown, as well as patterns of movement between strictly coastal and offshore areas in the Northeast Atlantic for porbeagle shark population. Seemingly, some immature porbeagle sharks from this area were exhibiting isotopic
compositions diverging from the isotopic composition of their mature and immature counterparts. Consequently, further analyses on different time-integrating tissues such as blood, liver and muscle (Logan and Lutcavage, 2010; MacNeil et al., 2005) should help elucidate the heterogeneity observed, in association with tracking studies focusing on porbeagle sharks frequenting coastal areas.
**CRediT authorship contribution statement**

**Sandrine Serre**: Methodology, Formal analysis, Writing – original draft, Writing – review & editing. **Armelle Jung**: Conceptualization, Methodology, Experimental design, Investigation, Writing – review & editing, Funding acquisition. **Yves Cherel**: Resources, Methodology, Experimental design, Writing – review & editing. **Caroline Gamblin**: Resources, Writing – review & editing, Project administration, Funding acquisition. **Cédric Hennache**: Investigation, Writing – review & editing, Project administration. **François Le Loc'h**: Supervision, Resources, Writing – review & editing. **Anne Lorrain**: Supervision, Resources, Methodology, Experimental design, Data analysis, Writing – review & editing. **Anne Priac**: Data analysis, Investigation, Writing – review & editing. **Gauthier Schaal**: Supervision, Resources, Writing – review & editing. **Eric Stephan**: Supervision, Resources, Writing – review & editing

**Declaration of competing interest**

The authors declare that they have no competing of interest in financial and personal relationship with other people or organization.

**Acknowledgments**

Thanks to all the volunteers from APECS who made EPPARTIY project possible by relentlessly sampling on board fisheries vessels and at the auction. Thanks to the fishers from Ile d'Yeu, the fishmongers and the staff of Sables d'Olonne fish market for their involvement.

Thanks to Ile d'Yeu Local Fisheries Committee, especially his director Sébastien Chauvet, and
finally to the Ile d'Yeu Producers' Organization. French Biodiversity Agency and French
Ministry of Higher Education and Research funded the PhD scholarship, hence the lead of this
manuscript.

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Figures

<figure><img src="image_3.png" /><figcaption>Figure 1. Sampling areas. Sampling areas of porbeagle sharks. S.G.: St Georges' Channel, S.I.: South Ireland, N.B.: North Bay of Biscay, S.B.: South Bay of Biscay, B.I.: Belle-Ile.</figcaption></figure>
Table 1. Sampling information. Size and muscle δ¹³C and δ¹⁵N values of porbeagle sharks from the five sampling areas with South Ireland, North and South Bay of Biscay as offshore areas and St Georges' Channel and Belle-Ile as coastal areas.

<table><thead><tr><th rowspan="2">Area</th><th colspan="3">Fork length (cm)<br>n samples</th><th rowspan="2">Muscle δ¹³C (‰)</th><th rowspan="2">Muscle δ¹⁵N (‰)</th></tr><tr><th>Females</th><th>Males</th><th>All</th></tr></thead><tbody><tr><td>South Ireland</td><td>145 ± 46<br>n = 7</td><td>163 ± 26<br>n = 15</td><td>158 ± 34<br>n = 22</td><td>-17.4 ± 0.5</td><td>13.4 ± 0.8</td></tr><tr><td>North Bay of Biscay</td><td>189 ± 37<br>n = 20</td><td>176 ± 31<br>n = 10</td><td>185 ± 35<br>n = 30</td><td>-17.5 ± 0.4</td><td>13.4 ± 0.7</td></tr><tr><td>South Bay of Biscay</td><td>181 ± 20<br>n = 11</td><td>169 ± 32<br>n = 15</td><td>174 ± 28<br>n = 26</td><td>-17.4 ± 0.5</td><td>13.0 ± 0.6</td></tr><tr><td>St Georges' Channel</td><td>154 ± 28<br>n = 38</td><td>149 ± 14<br>n = 31</td><td>152 ± 23<br>n = 69</td><td>-16.9 ± 0.8</td><td>14.4 ± 0.5</td></tr><tr><td>Belle-Ile</td><td>146<br>n = 1</td><td>143 ± 5<br>n = 3</td><td>144 ± 4<br>n = 4</td><td>-17.2 ± 0.3</td><td>14.7 ± 0.1</td></tr><tr><td>Total</td><td>166 ± 35<br>n = 77</td><td>159 ± 25<br>n = 74</td><td>163 ± 31<br>n = 151</td><td>-17.2 ± 0.7</td><td>13.8 ± 0.9</td></tr></tbody></table>

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Figure 2. Muscle δ¹³C and δ¹⁵N biplot. Muscle δ¹³C and δ¹⁵N values, 95%-ellipses and marginal distribution within each sampling area. S.G.: St Georges' Channel, S.I.: South Ireland, N.B.: North Bay of Biscay, S.B.: South Bay of Biscay, B.I.: Belle-Ile. Belle-Ile samples are only plotted in the biplot without ellipse due to low sample size (n = 4). Shades of blue are offshore areas while shades of orange are inshore areas.

<figure><img src="image_4.png" /></figure>

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Figure 3. Niche overlap between sampled areas. Niche overlap between offshore areas (blue shades) and inshore area (orange) based on Bayesian posterior probabilities. The overlap reflected the percentage of estimated isotopic niche from areas in row encompassed in the estimated isotopic niche from areas in column.

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<figure><img src="image_5.png" /></figure>

Overlap Probability (%) -- Niche Region Size: 95%

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27 Supplementary materials

28 Table SI. Number of male and female porbeagle sharks sampled per area and per year.

<table><thead><tr><th rowspan="2">Area</th><th rowspan="2">Year</th><th colspan="7">Number of individuals (Females/Males)</th></tr><tr><th>April</th><th>May</th><th>June</th><th>July</th><th>August</th><th>September</th><th>Unknown</th></tr></thead><tbody><tr><td>Belle-Ile</td><td>2009</td><td>-</td><td>1/3</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">North Bay of Biscay</td><td>2008</td><td>-</td><td>-</td><td>-/2</td><td>6/3</td><td>-</td><td>-</td><td>-</td></tr><tr><td>2009</td><td>2/1</td><td>-</td><td>12/4</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">South Bay of Biscay</td><td>2008</td><td>-</td><td>-</td><td>6/4</td><td>1/1</td><td>-</td><td>-</td><td>3/4</td></tr><tr><td>2009</td><td>-/5</td><td>1/1</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">South Ireland</td><td>2008</td><td>3/8</td><td>1/-</td><td>-</td><td>-</td><td>-</td><td>1/-</td><td>-</td></tr><tr><td>2009</td><td>-</td><td>-/2</td><td>2/5</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">St Georges' Channel</td><td>2008</td><td>-</td><td>-</td><td>-</td><td>-/1</td><td>2/2</td><td>6/4</td><td>-</td></tr><tr><td>2009</td><td>-</td><td>-</td><td>30/24</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

29 Figure SI. Size distribution of female (upper panels) and male (lower panels) porbeagle sharks sampled across fishing grounds. Dotted lines indicate length-at-maturity estimated for female and male porbeagle sharks. S.G.: St Georges' Channel, S.I.: South Ireland, N.B.: North Bay of Biscay, S.B.: South Bay of Biscay, B.I.: Belle-Ile.

<figure><img src="image_6.png" /></figure>

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Figure SII. Muscle δ¹³C (A) and δ¹⁵N (B) values of female (F: open circles) and male (M: solid circles) porbeagle sharks according to fork length (in cm) within each area.

<figure><img src="image_7.png" /></figure>

Figure SIII. Posteriori distribution of mean muscle δ¹³C (μ₁) and δ¹⁵N (μ₂) values and variance (Σ₁₂) for porbeagle sharks within each area, with the default Normal Inverted Wishart conjugate prior.

<figure><img src="image_8.png" /></figure>