Introduction
Grouping behaviour has many benefits for animals, such as better protection from predators, access to food, cooperation, and communication (
Krause and Ruxton 2002;
Silk 2007). Formation of distinct group types from a specific composition of individuals (e.g., based on sex, age, or kinship) within a population is considered social segregation (
Conradt 2005;
Ward and Webster 2016). Habitat segregation occurs when group types differ in their spatial distribution and thus in their use of the habitat (
Conradt 2005). To explain habitat segregation, the foraging selection hypothesis asserts that individuals with higher energy requirements (e.g., lactating females, growing individuals, and larger individuals with higher metabolic rates) select habitats based on the availability of high-quality food (e.g.,
Breed et al. 2006;
Staniland and Robinson 2008). The predation risk hypothesis states that more vulnerable animals choose a habitat that offers better protection even if the food is of lower quality, whereas less vulnerable animals choose a habitat with better food quality but higher predation risk (e.g.,
Grignolio et al. 2007;
Hay et al. 2008).
Social and habitat segregation is observed in many animal species (
Ruckstuhl and Neuhaus 2002;
Wolf et al. 2005;
Wearmouth and Sims 2008;
Kock et al. 2013;
Cleasby et al. 2015), including in marine mammals (
Michaud 2005). Sperm whales (
Physeter macrocephalus (Linnaeus, 1758)) are a thoroughly studied example of social segregation in which distinct groups are observed: groups of females with calves and juveniles, groups of all males of similar size, and single large males (
Best 1979;
Whitehead 2003;
Whitehead 2018). The distribution of each group type varies, sometimes creating extreme latitudinal segregation (
Rice 1989;
Lyrholm et al. 1999;
Mizroch and Rice 2013). Humpback whales (
Megaptera novaeangliae (Borowski, 1781)) exhibit habitat segregation, in which females and groups with calves tend to stay in shallow areas nearshore for better protection against predators, aggressive males, or turbulent oceanic conditions, while groups of adults select deeper, offshore areas (
Smultea 1994;
Guidino et al. 2014;
Lindsay et al. 2016). During the spring migration to the Beaufort Sea, bowhead whale (
Balaena mysticetus Linnaeus, 1758) mother-calf pairs or groups with calves are segregated from groups with no calves, delaying travelling to nurse calves, while adults are already feeding in more productive waters (
Clarke et al. 2022).
Beluga whales (
Delphinapterus leucas (Pallas, 1776)) are social whales that aggregate into various group types. Groups typically range from two to 20 whales and the age composition can be described as mother-calf dyads, juveniles-only, adults-only, or mixed-age groups (
O'Corry-Crowe et al. 2009;
Krasnova et al. 2012;
McGuire et al. 2020;
O'Corry-Crowe et al. 2020). Large seasonal aggregations can also reach up to hundreds of belugas (
Harwood and Norton 1996;
Clarke et al. 2011;
Clarke et al. 2012). Many environmental variables have been used to define habitat for beluga populations, such as bathymetric features, sea surface temperature (SST), distance to shore, tides, currents, turbidity, and sea ice measures (
Loseto et al. 2006;
Goetz et al. 2007;
Hauser et al. 2017;
Hornby et al. 2017;
Ouellet et al. 2021;
Noël et al. 2022). However, rarely has social structure been considered in analyses of beluga habitat.
The Eastern Beaufort Sea (EBS) beluga whale population migrates seasonally from their wintering grounds in the Bering Sea to the southeastern Beaufort Sea. This population uses an extensive range of habitat types within its summering grounds, including the offshore waters of the Beaufort continental shelf, the shallow bays of the Mackenzie Delta, and the heavy ice-concentrated Arctic Archipelago, in Viscount Melville Sound and M'Clure Strait (
Richard et al. 2001;
Harwood et al. 2014a;
Storrie et al. 2022). It is hypothesized that the EBS beluga population migrates to and aggregates in the Mackenzie Delta to benefit from the warm freshwater, which facilitates epidermal moult and calving activity (
St Aubin et al. 1990;
Scharffenberg et al. 2019;
Noel et al. 2022). EBS belugas are also sustainably harvested by Inuvialuit communities during the summer and represent an important cultural and subsistence harvest (
Harwood et al. 2020). Sexual and habitat segregation has been observed from tagged belugas, where females select open-water habitats close to shore, while males select areas with heavy sea ice concentrations located farther from the mainland (
Richard et al. 2001;
Loseto et al. 2006;
Hauser et al. 2017). Foraging opportunity and predation risk hypotheses have been used to explain beluga habitat selection (
Loseto et al. 2006;
Hauser et al. 2017). If foraging and protection from predators affect grouping behaviour, then habitat use could differ among beluga group types.
The objective of this study is to model the habitat preference of EBS belugas in their summering grounds in July and August, accounting for the variability among social group types. In this study, following the definition of
Beyer et al. (2010), we refer to habitat preference as the habitat use relative to a sample of the available environment described by statistical methods. We expect to see a difference in the habitat use between individuals and groups, and between groups of different age compositions. We hypothesize that groups of belugas, especially groups with young whales, prefer shallow environments closer to shore to reduce predation risk and facilitate nursing, while solitary belugas are expected to prefer habitats in deeper and colder waters to access higher quality food. Understanding variability in habitat use across the distinct group types and their relationship to the environment can provide useful information for management, conservation, and assessment of climate change impacts for this population (
Robinson et al. 2017).
Results
A total of 277 belugas were observed during the July survey and 426 during the August survey on days with available environmental data (
Table 1). In July, 210 individual belugas were sighted, representing 93 transect segments on which belugas were present. For groups of adults and groups with calves, we observed 37 and 30 groups (respectively), accounting for 31 and 20 segments with presence. In August, 315 observations of individual belugas were used, representing 162 transect segments on which belugas were present. Additionally, 54 groups of adults and 57 groups with calves were sighted, for a total of 44 and 45 segments with presence. The difference between the total number of transect segments and the presence represents the number of absences used in the model.
The correlation test showed a high positive correlation (+0.7) between bathymetry and slope (Fig. S2). The other variables were weakly or moderately correlated. Considering the collinearity and to compute ecologically significant models, we only tested the models with either bathymetry or slope.
Global functional response
The best model for July included the interaction between SST and slope (REML = 441.83) (
Table 2). The next best model included the interaction between SST and bathymetry (REML = 443.35). For August, the best model included the interaction between SST and bathymetry (REML = 709.71) (
Table 3). The second-best model for August included bathymetry only (REML = 731.94). The model, including SST and slope, did not perform as well for August (REML = 783.79) as for July. Each model showed a global response for the observed population, that is, a similar functional response to the covariate (
Pedersen et al. 2019). In the July model, preference for SST peaked around 6–10 °C for all group types (
Fig. 3). In August, all beluga groups showed a high preference for areas with a temperature around 2–4 °C and bathymetry of 300–500 m (
Fig. 4). All groups also had a low preference for shallow waters (<250 m) (
Fig. 4). Details on the functional response curves for each environmental variable can be found in the Supplementary Material (Figs. S3, S4, and S5). The best model for each month included group type as a significant factor, also resulting in a unique response for each social group type for both environmental variables.
Group-specific response
We examined the group-level response for each type of social group for each month and the relative preference for specific environmental conditions. In July, slope generated the most variation between group types. Individual adults and groups with calves had a higher preference for high slope (>2°), while groups of adults preferred areas of low slope (<1°). Individual belugas were found mostly at the continental slope, in waters of 6–8 °C. In comparison to the other group types, individual belugas had higher preferences for colder waters (<2 °C) (
Fig. 3A). Groups of adults used habitat on the continental shelf with a wider range of preferred temperatures compared to the other two groups, between 6 and 12 °C (
Fig. 3B). As with individual belugas, groups with calves also had a higher preference for high slopes and a narrower range of preferred temperatures between 6 and 8 °C (
Fig. 3C).
We also investigated results from the second-best model for July, because it included SST and bathymetry, the same significant covariates as the August best model (see below) (Figs. S4 and S6). A similar global response with a high preference for SST of 7–10 °C combined with a depth of < 200 m in all group types is observed. For individual adults and groups with calves, the model also showed a high preference for a bathymetry of 400–600 m with temperatures around 4–7 °C.
In August, individual belugas had high preferences for very deep and cold waters (>1500 m, 1–2 °C), which, considering the survey coverage, is associated with the continental slope and the start of the Canadian Basin (see Fig. S1E and S1F). As mentioned above, individual adults were also found near the 500 m isobath, where temperatures were a little warmer (2–4 °C), but habitat preference was lower than for the two other groups (
Fig. 4A). Groups of adults showed a high preference for the habitats preferred by individual belugas, but the former had a higher preference for the 500 m isobath (
Fig. 4B). Groups with calves had a higher preference for the area around the 500 m isobath and SST of 2–4 °C (
Fig. 4C). Except for one sighting, groups with calves were not found in the Canadian Basin, past the continental slope (
Fig. 4F).
Acknowledgements
For the July survey, the authors would like to acknowledge the logistical support provided by Polar Continental Shelf Program, Fisheries Joint Management Committee, and Inuvialuit Game Council. They thank the July 2019 aerial survey team, including the local observers from Inuvik (Andrew Gordon and Bertha Joe) and Ulukhaktok (Byron Okheena and Roy Inuktalik), the DFO crew (David Yurkowski, Carie Hoover, Dana Neumann, and Cassandra Debets), and pilots and crew from Kenn-Borek (Mike Whitley, Kim Turner, Chantelle Callaway and Alex de Boer). For the August surveys, the authors would like to thank the Alaska Outer Continental Shelf Region of Bureau of Ocean Energy Management and the Alaska Fisheries Science Center of National Oceanic and Atmospheric Administration for logistical and financial support, and the Project Manager, Janet Clarke. They would like to thank Craig George and Dr. Robert Suydam as well, retired from North Slope Borough Department of Wildlife Management; DFO; Fisheries Joint Management Committee; and the Inuvialuit Game Council. Our dedicated observers included Corey Accardo, Lisa Barry, Rachel Hardee, Suzie Hanlan, and Amy Willoughby. Clearwater Air’s pilots and crew (Jake Creglow, Andrew Harcombe, and Jake Turner), and Kenn Borek’s pilots and crew (William Allen, Chantelle Callaway, Joel Consaul, Alexander de Boer, Andre Martineau, and Markku Vanonen) safely kept us airborne.