# Intra-species differences in impacts of climate change: A case study of the hooded seal (Cystophora cristata)

Jade Vacquié-garcia, Kit M Kovacs, Christian Lydersen, Arnoldus S Blix, Lars P Folkow, Tore Haug, Martin Biuw, Marianna Chimienti, Jérôme Spitz, Tiphaine Jeanniard Du Dot

► To cite this version:

Jade Vacquié-garcia, Kit M Kovacs, Christian Lydersen, Arnoldus S Blix, Lars P Folkow, et al. Intra-species differences in impacts of climate change: A case study of the hooded seal (Cystophora cristata). Ecosphere, 2025, 16 (12), pp.e70482. 10.1002/ecs2.70482. hal-05412996

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ARTICLE

Climate Ecology

# Intra-species differences in impacts of climate change: A case study of the hooded seal (Cystophora cristata)

Jade Vacquié-Garcia<fnref n="1" />

Kit M. Kovacs<fnref n="2" />

Christian Lydersen<fnref n="2" />

Arnoldus S. Blix<fnref n="3" />

Lars P. Folkow<fnref n="3" />

Tore Haug<fnref n="4" />

Martin Biuw<fnref n="4" />

Marianna Chimienti<fnref n="5" />

<figure><img src="image_2.png" /></figure>

Jérôme Spitz<fnref n="1" />,<fnref n="6" />

Tiphaine Jeanniard du Dot<fnref n="1" />

<figure><img src="image_3.png" /></figure>

**Correspondence**

Jade Vacquié-Garcia

Email: jadevacquiegarcia@gmail.com

**Handling Editor:** Hunter S. Lenihan

**Abstract**

The Arctic is warming nearly four times faster than the global average, but the rate of change is not uniform across the region. Wide-ranging Arctic species are exposed to variable environmental conditions across their range, but it remains unclear how different climate change regimes are affecting populations within the same species that have become exposed to different conditions and how each population may respond in the future. In this study, we investigated how environmental changes in the Greenland Sea (GS) have influenced foraging behavior and diet of GS hooded seals (Cystophora cristata) over recent decades, and evaluated how future climate scenarios may affect them. We then compared these findings with recently published results on hooded seals from the Northwest Atlantic (NWA) stock, placing them in the broader context of Arctic ecosystem dynamics. Specifically, we analyzed changes in (1) foraging distribution and habitat use based on a two-decade tracking time series (1992–1993 and 2007–2008) from GS hooded seals and (2) diet, using carbon and nitrogen stable isotopes from blubber samples collected in 2007 and 2022. We also projected future suitable foraging habitats under the SSP5-8.5 climate scenario. Our results show that while GS hooded seals have maintained consistent habitat preferences and diets over the past two decades, their foraging areas have shifted eastward, rather than northward as was seen for the NWA seals—despite northward shifts in their breeding and molting grounds. Suitable foraging habitats of the future are projected to expand and continue shifting eastward. This pattern is unexpected for an

This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Ecosphere* published by Wiley Periodicals LLC on behalf of The Ecological Society of America.

<footnote n="1">¹Centre d'Etudes Biologiques de Chizé, UMR 7372 CNRS – La Rochelle Université, Villiers-en-Bois, France</footnote>

<footnote n="2">²Norwegian Polar Institute, Fram Centre, Tromsø, Norway</footnote>

<footnote n="3">³Department of Arctic and Marine Biology,UiT - The Arctic University of Norway, Tromsø, Norway</footnote>

<footnote n="4">⁴Institute of Marine Research, Fram Centre, Tromsø, Norway</footnote>

<footnote n="5">⁵School of Ocean Sciences, Bangor University, Menai Bridge, UK</footnote>

<footnote n="6">⁶Observatoire Pelagis, UMS 3462 CNRS – La Rochelle Université, La Rochelle, France</footnote>
ice-associated Arctic species, and contrasts projections for the NWA stock. Although it is uncertain whether hooded seals possess sufficient behavioral or physiological plasticity to cope with future climate-driven changes, our findings underscore the importance of considering population-level behavioral heterogeneity when assessing species-wide responses to environmental change.

### KEYWORDS

Arctic, ecological shift, foraging, future projections, ice-associated seal, populations, suitable habitat, time series, warming

## INTRODUCTION

Climate change is reshaping ecosystems globally, driving range shifts, population fluctuations, niche diversification, food web restructuring, and even causing local extinctions (IPCC, 2022; Pacifici et al., 2015; Scheffers et al., 2016; Stanton et al., 2014). In the Arctic, environmental transformations are among the fastest and most pronounced on Earth (AMAP, 2021; IPCC, 2022; Meredith et al., 2019). Air and sea surface temperatures (SST) are increasing nearly four times faster than the global average (Carvalho & Wang, 2020; Previdi et al., 2021; Rantanen et al., 2022; Shu et al., 2022), while sea ice extent, thickness, and seasonal duration continue to decline, making an ice-free Arctic possible within the next decade (Kim et al., 2023; Kwok, 2018; Meredith et al., 2019). Concurrently, the upper ocean's mixed layer is deepening in the region (Deb & Bailey, 2023; Pecuchet et al., 2020). However, these changes are not uniform across the Arctic. The Northeast Atlantic sector of the Arctic—including the Greenland and Barents Seas—is warming faster than other regions (Efstathiou et al., 2022; Laidre et al., 2015; Taylor et al., 2022; Urbanski & Litwicka, 2022). This region shows clear signs of “Atlantification,” with northward shifts of temperate species and major restructuring of marine ecosystems (Bengtsson et al., 2021; Brandt et al., 2023; Dalpadado et al., 2020; Descamps et al., 2017; Fossheim et al., 2015; Ingvaldsen et al., 2024). These transformations are likely to affect all Arctic-endemic marine mammals (Kovacs et al., 2021; Laidre et al., 2015; Tynan & DeMaster, 1997). However, for species with wide distributions, population-specific environmental conditions and varying degrees of behavioral or physiological plasticity may lead to divergent responses (Gilg et al., 2012). In this context, understanding intraspecific variation is critical to anticipating species-level responses and guiding conservation efforts.

The hooded seal (*Cystophora cristata*) is an Arctic-endemic, drift-ice-associated pinniped that depends on sea ice for pupping, molting, and resting (Folkow & Blix, 1995; Hamilton et al., 2022; Hammill & Stenson, 2006).

Two distinct management stocks are recognized: (1) the Greenland Sea (GS) stock and (2) the Northwest Atlantic (NWA) stock. These two stocks have similar breeding and molting phenologies, but they are spatially segregated in where they breed, molt, and forage (Andersen, Wiersma, et al., 2013; Folkow et al., 1996; Kovacs, 2018; Vacquié-Garcia et al., 2017). NWA hooded seals follow well-defined migratory routes connecting three breeding areas (Gulf of St Lawrence [GULF], northern Newfoundland [FRONT], and Davis Strait) with a common molting area off southeastern Greenland (Appendix S1: Figure S1). Their distribution is influenced primarily by the Labrador Current, and by the Arctic-derived West and East Greenland Currents that transport cold, low-salinity water southward. In contrast, GS hooded seals breed between the east coast of Greenland and Jan Mayen Island, and they molt further north on drift ice along the northeast coast of Greenland. They undertake wide-ranging and asynchronous post-molt movements throughout the Northeast Atlantic and sub-Arctic returning intermittently to the drifting ice east of Greenland (Appendix S1: Figure S1; Folkow et al., 1996; Vacquié-Garcia et al., 2017). Their range is shaped by both the cold East Greenland Current and the warmer North Atlantic Current that transports relatively warm, saline water northward along the west coast of Spitsbergen (Svalbard, Norway) and into the GS and Fram Strait via topographically steered branches of the current.

Despite differences in timing, both stocks have experienced significant population declines in the recent decades. The NWA stock experienced a >60% drop in pup production between 2005 and 2012, with no recovery observed in subsequent years (Hamilton et al., 2024). The GS stock has declined by ~85% since the mid-1940s and is currently classified as “Endangered” on the Norwegian Red List (Eldegard et al., 2021; ICES, 2023). While historical overharvest initiated the GS decline, commercial hunting was banned in 2008 and recovery has not yet occurred. Climate change impacts on their breeding habitat, increased predation rates from polar
bears (*Ursus maritimus*) and killer whales (*Orcinus orca*), and potentially also changes to their prey base, have all been suggested as possible drivers for this lack of recovery (Foote et al., 2013; McKinney et al., 2013; Øigård et al., 2014; Stenson & Hammill, 2014).

To date, foraging ecology studies of hooded seals have typically focused on one stock at a time and over limited time spans (Andersen et al., 2009; Andersen, Skern-Mauritzen, et al., 2013; Andersen, Wiersma, et al., 2013; Bajzak et al., 2009; Folkow & Blix, 1999; Hammill & Stenson, 2006; Vacquié-Garcia et al., 2017). A long-term, comparative analysis encompassing both stocks has not yet been undertaken. However, such a comparison may be key to understanding how differing environmental drivers shape the foraging strategies and resilience of each population. A recent study (Vacquié-Garcia et al., 2024) showed how changing ocean conditions in the NWA have altered the foraging and dietary patterns of NWA hooded seals over the last three decades, and how suitable habitat is likely to shrink in the future under continued climate forcing. In this study, we apply the same methodological approach as Vacquié-Garcia et al. (2024) to data from the GS hooded seal stock in the Northeast Atlantic, in order to assess and compare changes across the full distributional range of the species. The GS stock dataset includes biotelemetry from 1992–1993 and 2007–2008, and tissue samples from 2007 and 2022. More precisely, we investigate (1) how environmental changes in the GS have affected foraging (distribution and habitat use) and dietary habits (trophic level of prey and their habitat location) of GS hooded seals over time and (2) how they will likely affect them in the future. Finally, we compare findings from both the NWA (detailed in Vacquié-Garcia et al., 2024) and the GS stocks (this study) to explore broader implications for species-level responses to Arctic environmental change within the framework of ecosystem dynamics and conservation planning.

# MATERIALS AND METHODS

## Movement data collection and processing

In this study, we compiled all available biotelemetry data for adult hooded seals from the GS stock (N = 31). The tracking data span two time periods: 1992–1993 (N = 19) and 2007–2008 (N = 12). Seals were tagged during the pupping/breeding season (March) or the molting season (July) (see Table 1 for more details). Tagging methodologies from each field season can be found in the previously published articles referenced in Table 1. Even though the limited duration of each deployment period may capture

atypical environmental or behavioral conditions compared to adjoining years, for the purposes of this study, tracks were grouped into two decadal periods: early 1990s (1992–1993) and late 2000s (2007–2008).

Track processing was performed using the R statistical framework (R Development Core Team, 2024). Regardless of the year of deployment, tags collected and transmitted Argos locations (via the Argos satellite system, Fedak et al., 2002). All tracks were first standardized and cleaned as described in Vacquié-Garcia et al. (2024). A speed, distance, and angle filter (SDA filter; Freitas et al., 2008) from the R package “argosfilter” (Freitas, 2013) was subsequently applied to the tracks from 1992 and 1993 to homogenize the tracking datasets. This filter deletes locations related to unrealistic swimming speeds (>2 m/s) or turning angles (angles smaller than 15° or 25° if their lengths were greater than 2.5 or 5 km; Andersen, Skern-Mauritzen, et al., 2013; Andersen, Wiersma, et al., 2013; Freitas et al., 2008). Then, the cleaned tracks from both decades were processed further using a Bayesian switching state-space model fitted under a hierarchical framework (hSSSM, package bsam; Jonsen et al., 2005, 2013 and see Vacquié-Garcia et al., 2024 for more details on the procedure) assuming that individuals are “samples” of a population (which allowed us to analyze short/incomplete tracks). The filtered trajectories provided an estimate of seal behavioral states along the tracks at fixed time intervals. Behavioral states range from 1 to 2. Behavioral states <1.25 correspond to transiting behaviors, >1.75 to foraging behaviors, and between 1.25 and 1.75 to uncertain behaviors (Jonsen et al., 2007). Tracks with data gaps >4 days were split into different segments, and tracks or track segments shorter than 4 days or with fewer than 20 locations (excluding aberrant ones) were removed from the dataset. Given the resolution of the Argos-derived location data, we fitted the hSSSM using a time step of 4 h (e.g., the average time between locations). Location and behavioral state of seals every 4 h along the tracks of the animals were inferred from hSSSM outputs (see Vacquié-Garcia et al., 2024 and Appendix S1: Table S1 for details on the hSSSM output statistics per decade).

Five environmental variables, (1) bathymetry (in meters; 0.01-degree resolution ETOPO 1 Arc-Minute global relief data set from the National Geophysical Data Center, NOAA47); (2) minimum distance to the shore (in kilometers; land coastline (1:10 m) shown at www.naturalearthdata.com); (3) SST (in degrees Celsius; 0.05° daily grid resolution from the Global Ocean OSTIA Sea Surface Temperature and Sea Ice Reprocessed); (4) ice concentration (in percentage; 0.05° daily grid resolution from the Global Ocean OSTIA Sea Surface Temperature and Sea Ice Reprocessed); and (5) salinity (in practical
<table><caption>**TABLE 1** Summary information of satellite tag deployments on 31 Greenland Sea adult hooded seals instrumented between 1992 and 2008.</caption><thead><tr><th>Seal ID</th><th>Sex</th><th>Mass (kg)</th><th>Deployment season</th><th>Starting date</th><th>Track duration (days)</th><th>Decade</th><th>Haul-out data</th><th>Publications</th></tr></thead><tbody><tr><td>9663</td><td>F</td><td>100</td><td>Molting</td><td>16/07/1992 10:57</td><td>191.09</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9664</td><td>M</td><td>101</td><td>Molting</td><td>16/07/1992 16:58</td><td>264.14</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9665</td><td>F</td><td>110</td><td>Molting</td><td>16/07/1992 14:16</td><td>293.10</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9666</td><td>F</td><td>98</td><td>Molting</td><td>17/07/1992 08:25</td><td>264.55</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9667</td><td>M</td><td>120</td><td>Molting</td><td>17/07/1992 10:35</td><td>274.45</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9668</td><td>F</td><td>112</td><td>Molting</td><td>17/07/1992 15:30</td><td>295.07</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9669</td><td>F</td><td>112</td><td>Molting</td><td>19/07/1992 11:17</td><td>278.61</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9670</td><td>M</td><td>159</td><td>Molting</td><td>20/07/1992 07:47</td><td>188.20</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9671</td><td>M</td><td>134</td><td>Molting</td><td>16/07/1992 17:32</td><td>200.17</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9672</td><td>F</td><td>167</td><td>Molting</td><td>24/07/1992 15:19</td><td>213.81</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9870</td><td>F</td><td>156</td><td>Molting</td><td>19/07/1992 09:37</td><td>196.25</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9871</td><td>F</td><td>171</td><td>Molting</td><td>19/07/1992 14:39</td><td>193.71</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9872</td><td>F</td><td>104</td><td>Molting</td><td>19/07/1992 19:46</td><td>74.87</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9873</td><td>M</td><td>144</td><td>Molting</td><td>26/07/1992 09:00</td><td>168.36</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>9874<sup>a</sup></td><td>F</td><td>68</td><td>Molting</td><td>16/07/1992 08:33</td><td>339.55</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>17606</td><td>F</td><td>175</td><td>Breeding</td><td>19/04/1993 18:58</td><td>22.87</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>17607</td><td>F</td><td>155</td><td>Breeding</td><td>31/03/1993 19:09</td><td>108.76</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>17608</td><td>F</td><td>150</td><td>Breeding</td><td>31/03/1993 20:49</td><td>114.88</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>17609</td><td>F</td><td>235</td><td>Breeding</td><td>31/03/1993 22:28</td><td>82.53</td><td>1992-1993</td><td>No</td><td>Folkow et al. (1996)</td></tr><tr><td>2007-1</td><td>M</td><td>98</td><td>Molting</td><td>14/07/2007 20:52</td><td>320.51</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2007-3</td><td>M</td><td>171</td><td>Molting</td><td>01/08/2007 00:20</td><td>280.47</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-2</td><td>F</td><td>150</td><td>Breeding</td><td>23/03/2008 13:58</td><td>95.15</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-3</td><td>F</td><td>175</td><td>Breeding</td><td>22/03/2008 12:32</td><td>88.12</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-4</td><td>F</td><td>110</td><td>Breeding</td><td>23/03/2008 08:58</td><td>91.95</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-5</td><td>F</td><td>119</td><td>Breeding</td><td>22/03/2008 17:36</td><td>112.50</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-6</td><td>F</td><td>115</td><td>Breeding</td><td>22/03/2008 15:13</td><td>96.58</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-9</td><td>F</td><td>93</td><td>Breeding</td><td>22/03/2008 10:13</td><td>90.16</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-11</td><td>M</td><td>225</td><td>Breeding</td><td>22/03/2008 12:04</td><td>96.06</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-13</td><td>F</td><td>154</td><td>Breeding</td><td>25/03/2008 09:11</td><td>88.06</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-15</td><td>F</td><td>147</td><td>Breeding</td><td>23/03/2008 16:44</td><td>95.94</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr><tr><td>2008-16</td><td>F</td><td>180</td><td>Breeding</td><td>23/03/2008 10:41</td><td>92.02</td><td>2007-2008</td><td>Yes</td><td>Vacquié-Garcia et al. (2017)</td></tr></tbody></table>

Note: Deployment occurred between the east coast of Greenland and Jan Mayen during the breeding season (March) or off the northeast coast of Greenland during the molting season (July).

Abbreviations: F, female; M, male.

aIndividual not used in the analyses after the selecting and cleaning processes.

salinity unit; 0.083° daily grid resolution from the Global Ocean Physics Reanalysis) were determined at each (filtered) seal location from https://resources.marine.copernicus.eu/, using the most precise product available for each variable (see Vacquié-Garcia et al., 2024 for more details). The salinity product was not available prior to 1993, so 1993 salinity values were assigned to our 1992 locations.

## Distribution and habitat use models

### Model designs

First, breeding and molting areas were compared between the two decades. For each decadal period (1992–1993 and 2007–2008), these areas were estimated as the mean of the first cleaned Argos locations recorded for each
individual during its tagging period (the first cleaned Argos location per individual used only). Mean breeding and molting positions (latitude and longitude) were then compared between decades using a Wilcoxon rank-sum test.

To investigate potential shifts in foraging distribution over time, we then compared the movement patterns associated with foraging activities between the two decades (cf. distribution model). Foraging behavior was inferred by identifying area-restricted search patches (ARS), which are defined as periods of reduced swimming speed and increased track sinuosity—commonly used as a proxy for foraging activity (Benhamou & Bovet, 1989; Charnov, 1976). ARS patches were identified along individual tracks as beginning after 3 or more successive locations associated with a behavioral state >1.75 and ending after 3 or more consecutive locations with a behavioral state ≤1.75, along the tracks of the animals (Jonsen et al., 2007). We then modeled the temporal changes in ARS locations (longitude, latitude) and in the closest distance to the central places (estimated by a line connecting the averaged breeding and molting areas) in each decade. These spatial variables were analyzed as a function of time of year (expressed as days since January 1). To account for differences in the explanatory power of each ARS event, the duration of each ARS patch was included as a weighting factor in the models.

Changes in the within-year timing of foraging activity and shifts in habitat use between decades were assessed by analyzing variation in behavioral states along the seal tracks from 1992–1993 and 2007–2008. Two modeling approaches were used: (1) a temporal model, which examined behavioral changes in relation to the day of the year and (2) a habitat model, which assessed behavioral responses to oceanographic habitat variables (defined above). In both models, the response variable was the behavioral state, a continuous value ranging from 1 to 2, used as a proxy for foraging activity—higher values indicate a greater likelihood of foraging behavior (Jonsen et al., 2007). For the purposes of the models, behavioral state values were averaged per day (as is also the case for oceanographic parameters), and behavioral states were logit transformed prior to analysis.

The distribution, temporal, and habitat models were run using available yearly data, irrespective of season or the sex of the tagged animals. We acknowledge that habitat use likely varies seasonally and that spatial and vertical segregation between males and females has been documented in the Greenland Sea hooded seal stock (Vacquié-Garcia et al., 2017). However, due to the unbalanced temporal coverage between decades—with most 1992–1993 data collected between March and June, and most 2007–2008 data collected between July and February (Appendix S1: Table S1)—a season-specific analysis was not feasible.

The number of males and females was similar across decades (Appendix S1: Table S1), reducing concerns of sex bias. Given the constraints, we opted to model general population-level patterns across years and sexes as the most robust and interpretable approach for this dataset.

## Modeling approach

Modeling methods followed Vacquié-Garcia et al. (2024). To summarize, we used generalized additive mixed models (GAMMs [“uGamm” function in the R package “MuMln” using “gamm” function in the R package “mgcv”]) to build the distribution, temporal and habitat models with Gaussian error distributions. Day of the year and/or oceanographic parameters were included by default as a smooth term (knot value *k* was set at 5, 10, and 4 for, respectively, distribution, temporal, and habitat models) unless relationships were linear (i.e., *edf* = 1) in which case, they were included as a linear term. When day of the year was used as a smooth term, a cyclic cubic regression spline was specified to account for the cyclical effect of the full year. We included decade (1992–1993 and 2007–2008) as a “by” variable in the day of the year and/or oceanographic parameter terms in order to compare patterns over decades. The distribution (1 and 1bis models), temporal (2 model) and habitat use (3 model) models were built as follows:

$$ \text{LonARS} \sim s(\text{day, by} = \text{Decade}) + \text{Decade, random} \\ = \text{list(ID} = \sim 1), \text{correlation} = \text{corAR1}() \quad (1) $$

$$ \text{LatARS} \sim s(\text{day, by} = \text{Decade}) + \text{Decade, random} \\ = \text{list(ID} = \sim 1), \text{correlation} = \text{corAR1}() \quad (1bis) $$

$$ \text{LogitBS}_{\text{day}} \sim s(\text{day, by} = \text{Decade}) + \text{Decade, random} \\ = \text{list(ID} = \sim 1), \text{correlation} = \text{corAR1}() \quad (2) $$

$$ \text{LogitBS}_{\text{day}} \sim s(\text{bathymetry, by} = \text{Decade}) \\ + s(\text{SST, by} = \text{Decade}) + s(\text{ice, by} = \text{Decade}) \\ + s(\text{minimum distance to shore, by} = \text{Decade}) \\ + s(\text{salinity, by} = \text{Decade}) + \text{Decade, random} \\ = \text{list(ID} = \sim 1), \text{correlation} = \text{corAR1}() \quad (3) $$

where LonARS, LatARS refer to the longitude and latitude values for the identified ARS patches, and where LogitBS\_day refers to the logit-transformed behavioral state averaged per day.

We included individual ID as a random effect and as a grouping factor in the first-order autoregressive structure (corAR1). Correlation between explicative variables
was checked when needed (<0.8; Appendix S1: Table S2) and explanatory variables were standardized (Zuur et al., 2009). We selected models (including interaction terms) using Akaike information criterion (AIC)—keeping the most parsimonious model when difference in AIC between models (ΔAIC) was smaller than 2. We determined differences between decades using the 95% CIs of the corresponding smooth curves (Zuur et al., 2009). We used diagnostics and temporal autocorrelation assessment plots to validate final models.

## Predicted future suitable foraging habitat in response to global warming

We applied the best-performing habitat model identified for the GS hooded seal stock to assess how suitable seal habitats are expected to change over time under both (1) historical oceanographic conditions and (2) future conditions predicted under the SSP5-8.5 climate warming scenario. Because the only available genetics study considered the two stocks to be one panmictic population (Coltman et al., 2007), animals from one stock could theoretically be distributed over the entire range of the other stock. Therefore, we investigated how suitable seal habitats are expected to change over time across the entire range of hooded seals (e.g., of both stocks). To do so, we overlaid a 25 × 25-km grid across the full species range (see Vacquié-Garcia et al., 2024 for more details) and extracted monthly oceanographic parameters for each grid cell on the 15th of each month in: (1) 1993 (representing 1992–1993), (2) 2007 (representing 2007–2008), and (3) 2100. Extracted parameters included all of our habitat variables (see above). Historical values were extracted using Copernicus data (see Movement data collection and processing), while future projections (for 2100) were extracted from the INM-CM5-0 model under the SSP5-8.5 scenario (50-km precision), that is, the CMIP6 “worst-case scenario” (because the rate of ongoing climate change has been increasing over the last two decades) of the plausible future forcing pathways, to extract predicted SST, ice, and salinity for 2100. We then predicted daily behavioral states for the seals for each geographic cell, month, and year.

Given that habitat models can only reflect animal preferences within ranges of previously measured oceanographic parameters, we assigned not applicable (NA) (i.e., uncertain habitat) to predicted daily behavioral states when the cell values of one or more extracted environmental parameters were below or above the corresponding parameter range in the models (see Vacquié-Garcia et al., 2024 for more details). Finally, dynamic oceanographic variables were extracted from different data products (Copernicus, CIMP6) with inherent differences in uncertainties around

their estimated oceanographic variables. We incorporated a ±5% buffer on the observed range of each dynamic environmental variable to reflect these potential differences (i.e., we increased/decreased the highest and lowest limits in the ranges of dynamic variables by 5%). This allowed for more robust comparisons across time periods while accounting for known uncertainties in ocean model outputs.

## Tissue data collection and dietary habits

We estimated changes in the dietary habits of adult hooded seals from the GS stock over time using stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) in 22 blubber samples collected in 2007 (10 males and 8 females) and 2022 (4 females) in the Northeast Atlantic. The integration time for blubber is ~1 year (Kiszka et al., 2014), so isotope ratios provide information on the overall dietary habits in the year before samples were taken. Stable isotope analyses (given in mean ± SD‰ in Results) were performed at the “Littoral Environnement et Sociétés” laboratory (cf LIENs) at La Rochelle Université in France following the same protocols as those detailed in Vacquié-Garcia et al. (2024).

To compare δ¹³C and δ¹⁵N ratios over time (2007 vs. 2022—and between sexes), linear models were used. All combinations of variables were tested, and AIC was used to select the best models (keeping the most parsimonious model when ΔAIC was smaller than 2); %C to %N ratios were checked (<4; Kiszka et al., 2014; Lesage et al., 2010) prior to modeling and δ¹³C ratios were corrected for the Suess effect (Teixeira et al., 2022), using the SuessR package in R.

## RESULTS

We obtained 34,731 raw locations from the 31 seals tagged from 1992 to 2008 (N = 19 in 1992–1993—13,715 raw locations and N = 12 in 2007–2008—21,016 raw locations). These data are available at https://doi.org/10.21334/NPOLAR.2025.AD93905D. Deployments lasted from 23 to 337 days (171 ± 16 days; mean ± SE; Table 1). The selecting and cleaning processes resulted in the loss of 30% of the locations (this translates to 58% of raw locations from 1992–1993—mostly duplicated locations from historical raw files—and 13% from 2007–2008) and all of the data from one seal (ID9874 from 1992–1993) (see Appendix S1: Figure S2 for information on the data structure after selecting and cleaning processes). All this combined, allowed us to account for 61% of the total tracking duration period (overall time that the seals carried the satellite tags) in this study (ratio between the sum of the conserved track segments
durations and the sum of days that all deployments lasted), so results need to be interpreted accordingly. Outputs from the hierarchical switching state-space model, run on the remaining data, provided 19,768 locations and behavioral states (transiting vs. foraging) at 4 h-intervals (10,535 and 9233 locations in 1992–1993 and 2007–2008, respectively; Figure 1; Appendix S1: Table S1). The cleaning process did not show any spatial bias where locations were removed within and between years.

## Distribution shifts over time

Defined breeding and molting areas shifted northward ($W = 1, p < 0.010$ and $W = 0, p = 0.015$, respectively) and slightly eastward ($W = 0, p < 0.01$ for breeding areas only) between 1992–1993 and 2007–2008 (Figure 2). We identified 162 ARS patches along the seals' tracks in 1992–1993 (duration: 3.55 ± 0.32 days) and 53 in 2007–2008 (duration: 11.51 ± 1.39 days; see Appendix S1: Table S1). Typically, seals foraged in areas north of their breeding area between the Denmark Strait and Jan Mayen, close to their molting areas along the northeast

<figure><img src="image_4.png" /><figcaption>FIGURE 1 Filtered tracks from 30 hooded seals from the Greenland Sea stock instrumented with satellite tags in the North Atlantic Arctic area in 1992–1993 and 2007–2008. Yellow and red tracks correspond to 4-h seal locations inferred from hierarchical switching state-space models (hSSSMs) in 1992–1993 and 2007–2008, respectively. The map was generated from publicly available land coast product available at www.naturalearthdata.com.</figcaption></figure>

coast of Greenland following breeding and then they moved southward again during the post-molting season (Figure 3). Distribution models showed that foraging occurred somewhat further north in July and further south in March in 2007–2008 compared to 1992–1993, but there were no significant differences (i.e., curves overlapping; Figure 3a; Appendix S1: Table S3). However, the seals clearly moved further east to forage in the more recent decade (Figure 3b; Appendix S1: Table S3)—and traveled longer distances from their breeding and molting locations to find food—compared to the 1992–1993 decade, especially in March–April (Figure 3c; Appendix S1: Table S3).

## Habitat use over time

Daily behavioral states of the seals ranged from 1 ("transiting") to 2 ("foraging") (1.48 ± 0.0073; Figure 3), daily average bathymetry from 35 to 4318 m (1390 ± 18 m), daily SST from -1.91 to 11.00°C (2.11 ± 0.06°C), daily ice concentration ranged from 0% to 100% (17 ± 0.45%), daily salinity from 29.04 to 35.40 psu (34.11 ± 0.02 psu), and daily distance to coastline from 3 to 731 km (184 ± 2 km) along the tracks of the seals in all years sampled. Foraging occurred mostly along the east coast of Greenland, from the Denmark Strait to Jan Mayen, but also took place north of Svalbard in 1992–1993 (Figure 4). Foraging areas included the east coast of Iceland, around the Faroe Islands and along the shelf break between Bjørnøya and the Norwegian mainland, as well as along the coast of the Norwegian mainland in 2007–2008 (Figure 4).

The temporal model showed that seals foraged more (i.e., had significantly higher daily behavioral state values) in May (post-breeding) and August (post-molting) than during the rest of the year (Appendix S1: Figure S3, Table S4). This seasonal pattern to foraging was only found in 2007–2008, while no effect of season was found for foraging in 1992–1993 (Appendix S1: Figure S3). The habitat model showed that ice concentration, salinity, bathymetry, and SST all affected the daily behavioral state values significantly (a 5% threshold)—except for bathymetry in 1992–1993 (Figure 5; Appendix S1: Table S4). Seals tended to forage more when ice concentration and SST increased, and when salinity and bathymetry decreased (Figure 5). The seals used similar types of habitats over the entire study period with model curves overlapping for the two decades (Figure 5).

Haul-out times can be misinterpreted in hSSSMs because both foraging and haul-out are characterized by a decrease in swimming speed and an increase in sinuosity. However, haul-out time was not recorded by tags
<figure><img src="image_5.png" /><figcaption>**FIGURE 2** Latitudes and longitudes of the estimated breeding (a) and molting (b) geographical areas in 1992–1993 and 2007–2008—these areas were estimated as the mean of the first cleaned Argos locations of individuals tagged, respectively, in each decade. ** = *p* < 0.01 and * = *p* < 0.05. Within each box, the horizontal black line represents the median value. The box itself spans from the 25th to the 75th percentile of each group's distribution of values. Vertical lines (whiskers) extend to the most extreme values that fall within 1.5 times the interquartile range from the 25th and 75th percentiles. Points beyond these whiskers represent observations outside the range of adjacent values.</figcaption></figure>

deployed in 1992–1993, so this parameter could not be
processed in our models (Table 1). Nevertheless, based
on available haul-out information from 2007–2008
(N = 12; Table 1), we found that seals hauled out
primarily in June, July, and September (21.2 ± 2.4%,
41.7 ± 31.9%, and 21.1 ± 1.7% of their time, respectively,
vs. <8.6 ± 2.2% for the other months, N = 12; Table 1)
and that these were the months with the highest propor-
tion of misinterpreted locations (35.2 ± 5.7%, 51.6 ± 48.3%,
and 20.3 ± 10.2% of locations with a behavioral state
>1.75 correspond to haul-out periods, respectively, vs.
<16.9 ± 6.9% for the other months). Consequently, to
confirm that haul-out periods do not significantly bias
outputs from our habitat model, we compared results
from habitat models computed with or without June,
July, and September. The results showed no significant
differences (Appendix S1: Figure S4). Therefore, we con-
cluded that the habitat model was reliable for inferring
foraging preferences.

Predicted future suitable foraging habitat

The best habitat model (Appendix S1: Table S4) was used
to spatially project past and future suitable seal habitats
across the entire range of hooded seals in response to
a scenario of environmental change. Since we did not
detect any significant change in the selection of environ-
mental parameters in our habitat model between decades
(Figure 5, overlapping of the curves), we decided to com-
bine data from all decades and build a global habitat
model (without any new model selection). This model
was subsequently used to predict future suitable habitats
(Appendix S1: Figure S5).

Spatial predictions suggested that suitable foraging
habitats for GS hooded seals expanded over time, from
the early 1990s (1992–1993) to 2100, across all months,
while the extent of unfavorable habitats decreased
(Figure 6; Appendix S1: Figure S6). Some areas that were
already favorable in 2007–2008—such as the east coast of
<figure><img src="image_6.png" /><figcaption>**FIGURE 3** Temporal variation in (a) latitude, (b) longitude, and (c) distance from central places (i.e., a line connecting breeding and molting areas) of the area-restricted search patches (ARS, interpreted as potential foraging spots) identified along the hierarchical switching state-space model (hSSSMs) using filtered Greenland Sea seal tracks. The sizes of dots are proportional to number of consecutive days seals were in ARS mode. Solid lines and shaded areas represent the fitted estimates and their respective CIs as calculated from the variance-covariance matrices of the fitted models' random effects. Dark and light gray rectangles represent, respectively, the breeding and molting periods.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**FIGURE 4** Average daily behavioral states used in the temporal and habitat models for Greenland Sea hooded seals tagged in 1992–1993 and 2007–2008. Light to dark purple represent the mean daily behavioral states from <1.25 (i.e., transiting) to >1.75 (i.e., foraging).</figcaption></figure>

Iceland, the Faroe Islands, and the shelf break between Bjørnøya and the Norwegian mainland—are expected to remain suitable until 2100, particularly during March–April

and July. However, the most notable trend is a marked eastward shift in suitable habitat over time. Areas within the triangle formed by Iceland, Svalbard, and
<figure><img src="image_8.png" /><figcaption>**FIGURE 5** Effects of (a) ice concentration, (b) salinity, (c) bathymetry, and (d) sea surface temperature (SST) on the daily behavioral states (logit-transformed) of Greenland Sea hooded seals based on the habitat model. The higher the logit transformed behavioral state value, the closer the behavioral state is to foraging. Lines and shaded areas represent the fitted estimates and their respective CIs, as calculated from the variance-covariance matrices of the fitted model's random effects. Solid lines indicate a significant relationship (at a 5% threshold) while dashed lines indicate non-significance. Hatched bars above the x-axis represent the available oceanographic parameter values in each decade.</figcaption></figure>

mainland Norway, which were not important foraging zones in the past, are projected to become increasingly favorable in the future (Figure 6).

## Diet habits over time

Stable isotope ratio data (C and N) from the 22 blubber samples collected in 2007 (10 males and 8 females) and 2022 (4 females) in the Northeast Atlantic are available at https://doi.org/10.21334/NPOLAR.2025.E65BA2A1. Blubber $\delta^{13}$C and $\delta^{15}$N were higher in males ($\delta^{13}$C: -18.04 ± 0.46‰ and $\delta^{15}$N: 17.52 ± 0.75‰) than in females ($\delta^{13}$C: -18.98 ± 0.39‰ and $\delta^{15}$N: 16.29 ± 0.25‰) (output of the model—comparison with females): estimate $\delta^{13}$C males = 0.94, $t$ = 5.21 and $p$ < 0.005 and estimate $\delta^{15}$N males = 1.24, $t$ = 5.35, and $p$ < 0.005, respectively. However, no

significant decadal effect was detected on Suess-corrected $\delta^{13}$C or $\delta^{15}$N values in blubber when comparing individuals of the same sex (i.e., Figure 7a for females).

## DISCUSSION

In this study, we examined how two decades of rapid environmental change in the GS have affected the foraging behavior and diet of GS hooded seals. By combining these findings with comparable analyses of the NWA stock (Vacquié-Garcia et al., 2024), we conducted a long-term, species-wide meta-analysis to investigate how two distinct populations of this Arctic ecosystem sentinel are responding to environmental shifts—considering their differing foraging strategies, ranges, and oceanographic contexts.
<figure><img src="image_9.png" /><figcaption>**FIGURE 6** Suitable habitat for the Greenland Sea hooded seals based on predicted daily behavioral states obtained from the habitat model for January–December in 1993, 2007, and for the future in 2100. Shades of purple represent likelihood of foraging suitability based on local oceanographic parameters and habitat preferences: Dark purple shows areas where foraging is the most likely to occur (i.e., predicted daily behavioral state >1.75, i.e., suitable foraging habitat), while lighter purple areas identify where seals are increasingly less likely to display foraging (representing transiting, i.e., predicted daily behavioral state <1.25, i.e., unsuitable foraging habitat). Geographic cells in yellow represent predictions outside the model parameter ranges (assigned to NA for Not Applicable/Uncertain habitat). Predictions for 2100 are based on predicted oceanographic parameters from the CMIP6 SSP5-8.5 scenario (https://cds.climate.copernicus.eu).</figcaption></figure>

## Foraging and dietary habit changes of GS seals in the past 20 years and predictions for future suitable foraging habitats

GS hooded seals undertake unsynchronized annual foraging trips, typically heading eastward from their breeding

ice to distant open waters. Their migratory pattern within the east Greenland drift ice involves a northward movement from March (breeding season) to July (molting season), followed by a generally more southward return between July and March (Folkow et al., 1996; this study). Between 1992–1993 and 2007–2008, both
<figure><img src="image_10.png" /><figcaption>**FIGURE 7** Stable isotope ratios (mean ± SD‰) for nitrogen (δ¹⁵N) and Suess-corrected carbon (δ¹³C) in female hooded seals from both stocks (Greenland Sea and Northwest Atlantic). (a) The ratios for 22 Greenland Sea blubber samples in 2007 and in 2022 and (b) the ratios for 45 serum samples of hooded seals from the Northwest Atlantic Arctic stock breeding in the Gulf of St Lawrence in 1995 and in 2018–2019 indicated for comparison (in *Discussion* only). Data from Northwest Atlantic come from Vacquié-Garcia et al. (2024) (2019 was used as the reference year for the correction of the Suess effect on Northwest Atlantic stock data).</figcaption></figure>

breeding and molting grounds shifted northward, a pattern consistent with other Arctic ice seals such as harp seals (*Pagophilus groenlandicus*; Stenson et al., 2020; Stenson & Hammill, 2014), likely in response to reduced sea ice in South Greenland waters (Fossheim et al., 2015; Laidre et al., 2015).

Given the change in environment, we expected foraging areas to shift northward, in line with patterns observed in other Arctic marine mammals seeking cold-water refugia (e.g., Carlyle et al., 2021; Chambault et al., 2020; Koenigstein, 2020; Kovacs et al., 2011; Shuert et al., 2022; Vacquié-Garcia et al., 2024). However, our results revealed a surprising pattern: GS hooded seals maintained similar foraging latitudes, but they shifted their foraging areas eastward. The cleaned location used in the present analysis suggests that in 1992–1993, foraging occurred mainly along Greenland’s east coast, from the Denmark Strait to Jan Mayen and north of Svalbard. By 2007–2008, core foraging zones had moved east—toward Iceland’s coast, the Faroe Islands, and the shelf break between Bjørnøya and mainland Norway. This eastward shift, which was particularly marked in April–May and November–December, is, to our knowledge, unique among Arctic endemic species.

The changed movement patterns result in an increase in the distance between foraging and central places (breeding/molting grounds), especially post-breeding in April–May—a critical time for energy recovery (Bajzak et al., 2009). This likely reduces foraging efficiency by

increasing travel costs (Shuert et al., 2022). According to optimal foraging theory (Charnov, 1976), one way to offset increased travel costs is by extending time spent in food patches. Although dive depth data were unavailable for this study, we observed that ARS durations were longer in 2007–2008 than in 1992–1993 (55 ± 0.32 days vs. 11.5 ± 1.4 days), suggesting prolonged foraging activity. However, caution is warranted due to lower tracking resolution in 1992–1993. Consequently, the suggested patterns need to be further investigated using dive behavior metrics.

Despite substantial environmental changes (Efstathiou et al., 2022; Isaksen et al., 2022; Laidre et al., 2015), GS hooded seals were found to maintain consistent preferences for oceanographic conditions over time. They have altered their distribution in order to remain in places with their preferred oceanographic conditions.

Stable isotope analysis (for females) revealed no major dietary shift between 2007 and 2022, suggesting that prey selection has remained stable. Hooded seals appear to target prey with narrow environmental niches, which have themselves shifted eastward in recent decades. However, the absence of tissue samples from 1992–1993 limits our ability to make conclusions about dietary changes that might have taken place prior to 2007. Indeed, a shift in diet could have occurred earlier without our being able to detect it.

Across decades, GS hooded seals consistently foraged in deep (~1000 m) pelagic waters along productive shelf edges, favoring low salinity and high SST—conditions
supporting temperate schooling prey such as Atlantic mackerel (*Scomber scombrus*), Atlantic cod (*Gadus morhua*), blue whiting (*Micromesistius poutassou*), and herring smelt (*Argentina silus*), which spend most of the summer and autumn feeding in the upper water layers in warm areas (ICES, 2008; Langøy et al., 2012; Nøttestad et al., 2016). Some of these fish species have been reported as dietary items for hooded seals between 1994 and 2004 (Haug et al., 2007; Tucker et al., 2009), though ice-associated species such as Greenland halibut (*Reinhardtius hippoglossoides*), squid (*Gonatus fabricii*), redfish (*Sebastes* sp.), and polar cod (*Boreogadus saida*) have been reported to be their main prey.

Despite spending ~55% of time in ice-free waters (Folkow et al., 1996; Vacquié-Garcia et al., 2017), GS hooded seals also showed a preference for high ice concentrations. Foraging in areas with both high ice concentration and high SST appears counterintuitive and does not align with known habitat preferences of some previously mentioned fish prey. However, it is important to consider that this study examined GS hooded seal habitat use year-round. The observed selection of high ice and warm SST areas may therefore reflect seasonal shifts in oceanographic conditions across different foraging periods. While relatively warm waters seem favorable for foraging, the preference for high ice concentration may be partly explained by opportunistic access to ice-associated prey or the availability of sea ice as resting platforms. To better assess potential seasonal dietary shifts, future studies would benefit from biological samples with shorter integration times (e.g., blood), which were not available in this study.

The climate model used to project future foraging habitat changes of GS hooded seals reflects a current “worst-case” scenario. Our predictions should be interpreted accordingly. Nevertheless, if GS hooded seals maintain their current foraging preferences, suitable habitat is expected to expand in extent and shift eastward by 2100 compared to 1992–1993 and 2007–2008. While habitat expansion under climate change has been observed or predicted for temperate whale species (e.g., Moore et al., 2003, Moore & Huntington, 2008; Stafford et al., 2007; Storrie et al., 2018), Arctic specialists like harp seals, narwhals, and bowhead whales have generally experienced—and are projected to continue experiencing—shrinking habitat ranges (Carlyle et al., 2021; Chambault et al., 2020; Koenigstein, 2020; Kovacs et al., 2011; Shuert et al., 2022; Vacquié-Garcia et al., 2024). In this context, GS hooded seals appear to be an exception. However, habitat expansion does not necessarily translate to ecologically viable or realistic foraging locations—especially if these lie far from breeding or molting areas. For instance, the northeast coast of

Greenland is classified as suitable habitat currently, but has not been used by GS seals in past decades, suggesting that they might continue to avoid it in the future. Furthermore, if central places continue to shift northward, seals may face continuously increasing energetic costs (travel costs from and to central places) and elevated pup predation risk from polar bears (McKinney et al., 2013).

It remains uncertain whether GS hooded seals possess sufficient behavioral or physiological plasticity to cope with the mounting challenges posed by climate change. These include increased competition with temperate species and fisheries, heightened risks of predation and disease, and potential changes in contaminant toxicity in a warming Arctic (e.g., Arctic Council, 2009; Burek et al., 2008; Harvell et al., 1999; Jensen et al., 2010; Letcher et al., 2010; Moore & Huntington, 2008; Reeves et al., 2014). The sharp decline in the GS hooded seal population—estimated at ~85% since the mid-1940s (ICES, 2023)—suggests that environmental changes are already having detrimental effects (Øigård et al., 2014). However, it is currently difficult to determine which specific pressures—increasing distances to central places, pollution, disease, competition, predation, or a combination—are most responsible for this decline.

## Comparison between stocks and perspectives within an ecosystem dynamic context

Our results indicate that GS hooded seals selected similar oceanographic features for foraging—shallow waters, low salinity, and high SSTs—in both 1992–1993 and 2007–2008 (Table 2). NWA hooded seals also maintained consistent preferences over time (Vacquié-Garcia et al., 2024), but notable differences emerged between NWA seals from different breeding sites. Seals breeding in the GULF foraged in colder, nearshore waters, while those breeding on the FRONT preferred warmer, more offshore habitats (Vacquié-Garcia et al., 2024; Table 2). Interestingly, NWA FRONT hooded seals exhibited foraging preferences more closely aligned with GS seals than with their NWA conspecifics from the GULF, despite being part of the same stock. These significant intra- and inter-stock differences in habitat use underscore the importance of accounting for behavioral heterogeneity when assessing species-level responses to environmental change.

In recent decades, the Northeast Atlantic has experienced a more rapid decline in sea ice cover than the Northwest, accompanied by strong signals of ecosystem change (Brandt et al., 2023; Fossheim et al., 2015; Heide Jørgensen et al., 2022; Ingvaldsen et al., 2024). Climate models project that this trend will intensify, with substantial future warming expected in the Northeast
<table><caption>TABLE 2 Summary of the behavioral changes that occurred in recent decades for hooded seals from the Northwest Atlantic (NWA) (from Vacquié-Garcia et al., 2024) and the Greenland Sea (GS) (this study) hooded seal stocks. NA = Not Applicable.</caption><thead><tr><th rowspan="2">Characteristic</th><th rowspan="2">GS stock</th><th colspan="2">NWA stock</th></tr><tr><th>FRONT</th><th>GULF</th></tr></thead><tbody><tr><td>Habitat use</td><td>Constant over decades</td><td>Constant over decades</td><td>Constant over decades</td></tr><tr><td>Characteristics of foraging areas</td><td>Warm but icy, small salinity, and bathymetry</td><td>Warm but icy, small bathymetry, distant to shore</td><td>Cold and icy, small salinity, and bathymetry, close to shore</td></tr><tr><td>Shifts of their foraging areas</td><td>Eastward</td><td>Northward</td><td>Northward</td></tr><tr><td>Diet</td><td>Constant over decades</td><td>NA</td><td>Decrease in δ<sup>13</sup>C and δ<sup>15</sup>N</td></tr><tr><td>Historical foraging areas in the future</td><td>Remain</td><td>Diminish</td><td>Disappear</td></tr><tr><td>Favorable foraging areas in the future</td><td>Extension</td><td>Shrinkage</td><td>Shrinkage</td></tr></tbody></table>

Abbreviations: FRONT, northern Newfoundland; GULF, Gulf of St Lawrence.

Atlantic. One might therefore expect GS hooded seals to shift their diet toward prey adapted to warmer waters, while NWA seals might retain an Arctic prey base. However, our data show no change in the isotopic signatures of GS hooded seals between 2007 and 2022 (Table 2), whereas NWA seals breeding in the GULF exhibited a marked decline in nitrogen isotopic values between 1995 and 2018–2019 in both sexes (Figure 7b). Although the time span of GS hooded seal dietary data is shorter, this divergence suggests that either the NWA food web has shifted significantly or NWA hooded seals have transitioned to smaller or lower trophic level prey—unlike GS hooded seals. It also implies that the NWA ecosystem may have begun shifting from a cold- to a warm-dominated system more recently, while such changes in the GS ecosystem may have occurred prior to 2007. This could help explain the early and steep population decline observed in the GS stock. These scenarios remain unresolved and would require further investigation using archival trophic markers from both seals and their prey.

Although both the GS and NWA hooded seal stocks maintained consistent preferences in foraging conditions over time, they shifted their distributions to remain within similar oceanographic habitats (Table 2). This indicates a greater degree of plasticity in spatial distribution and movement patterns than in habitat preference itself (see Gilg et al., 2012). Hooded seals are generalist foragers whose diets vary between, but not within, regions (Haug et al., 2007; Tucker et al., 2009), suggesting that they are more likely to follow their preferred prey to new locations than to switch prey types. Nonetheless, we observed a shift in isotopic signatures consistent with a dietary change in the NWA stock between 1995 and

2018–2019 (Vacquié-Garcia et al., 2024; Table 2). Given that NWA seals undertake long seasonal migrations between breeding and molting sites—unlike the more spatially resident GS seals—they may have less flexibility to adjust their movements in response to shifting prey distributions, which could constrain their ability to maintain previous foraging strategies.

The observed—and projected—distributional shifts differ between the hooded seal stocks (Table 2). GS hooded seals appear to have progressively shifted their foraging areas eastward, a trend projected to continue alongside an expansion of suitable habitat. In contrast, NWA hooded seals have shifted their foraging areas northward, with future models predicting a contraction of their favorable habitat (Table 2). This divergence suggests that geographical segregation between the two stocks will likely increase over time. Such a trend has important implications for population structure, as continued divergence in foraging ranges could lead to reduced overlap and increased niche segregation between stocks and potentially result in greater genetic isolation in the future.

Given direct habitat changes and indirect ecological pressures, both hooded seal stocks are likely to face increasing challenges under future climate scenarios. GS hooded seals will likely have to contend with the impacts of foraging in increasingly warm waters closer to European coasts—creating higher risks of disease and pollution, intensified competition with temperate species, longer travel distances to central places, and more frequent interactions with fisheries (Arctic Council, 2009; Jensen et al., 2010; Letcher et al., 2010; Noyes et al., 2009; Reeves et al., 2014). In contrast, NWA hooded seals are expected to face greater
competition for food and space from other Arctic species being pushed into shrinking cold-water refugia, along with heightened predation pressure from polar bears (Kovacs et al., 2011; Lameris et al., 2021). As the breeding habitat in the Gulf of St Lawrence is predicted to disappear first, GULF-breeding NWA seals will confront a stark choice: relocate—potentially merging with FRONT animals—or face reproductive failure. Whether such a distributional shift is feasible will depend on their behavioral plasticity.

While the long-term fate of hooded seals remains uncertain, this study contributes key insights into behavioral diversity and plasticity across stocks and breeding areas—knowledge that is vital for anticipating species responses to rapid environmental change and informing conservation strategies.

## AUTHOR CONTRIBUTIONS

Jade Vacquié-Garcia and Tiphaine Jeanniard du Dot conceived the study. Tiphaine Jeanniard du Dot, Kit M. Kovacs, Christian Lydersen, Arnoldus S. Blix, Lars P. Folkow, Martin Biuw, and Tore Haug provided data and conducted fieldwork. Jade Vacquié-Garcia conducted the statistical analyses with inputs from Tiphaine Jeanniard du Dot, Marianna Chimienti, and Jérôme Spitz. Jade Vacquié-Garcia and Tiphaine Jeanniard du Dot conducted the isotope analyses. Jade Vacquié-Garcia and Tiphaine Jeanniard du Dot drafted the manuscript. All authors contributed to discussions of the results and writing of the final manuscript.

## ACKNOWLEDGMENTS

We are grateful to all those involved in data collection, particularly the teams working in the field. We also thank our colleagues at the LIENSs laboratory, La Rochelle Université, for their valuable expertise and assistance with the stable isotope analyses. We also would like to thank the ECOMM Excellence Chair, awarded by the Région Nouvelle-Aquitaine and La Rochelle Université to the CEBC-CNRS, which funded J. Vacquié-Garcia during this project.

## CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

## DATA AVAILABILITY STATEMENT

Data (raw movement data and extracted stable isotope ratio; Vacquié-Garcia et al., 2025a, 2025b) are available from the Norwegian Polar Data Center: https://doi.org/10.21334/NPOLAR.2025.AD93905D and https://doi.org/10.21334/NPOLAR.2025.E65BA2A1.

## ORCID

Marianna Chimienti https://orcid.org/0000-0002-8236-9332

Tiphaine Jeanniard du Dot https://orcid.org/0000-0003-1985-8325

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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**How to cite this article:** Vacquié-Garcia, Jade, Kit M. Kovacs, Christian Lydersen, Arnoldus S. Blix, Lars P. Folkow, Tore Haug, Martin Biuw, Marianna Chimienti, Jérôme Spitz, and Tiphaine Jeanniard du Dot. 2025. "Intra-Species Differences in Impacts of Climate Change: A Case Study of the Hooded Seal (*Cystophora Cristata*)." *Ecosphere* 16(12): e70482. https://doi.org/10.1002/ecs2.70482