Summer diet of beluga whales inferred by fatty acid analysis of the eastern Beaufort Sea food web
Introduction
Beluga whales (Delphinapterus leucas) are the most abundant odontocetes in Arctic waters (Brodie, 1989). As a result, they play a critical role in the Arctic marine food web structure and function. Arctic cod (Boreogadus saida) is thought to be an important forage species for some beluga populations (Dahl et al., 2000, Seaman et al., 1982, Welch et al., 1993); however, redfish (Sebastes marinus), halibut (Reinhardtius hippoglossoides), and shrimp (Pandalus borealis) were found in Greenlandic beluga stomach contents (Heide-Jorgensen and Teilmann, 1994), whereas Pacific salmon (Oncorhynchus spp.) were dominant prey items for Alaskan beluga populations (Frost and Lowry, 1981). These observations suggest beluga diet may be population and habitat specific.
The diet of the Beaufort Sea beluga whale population is not well known partly due to the inherent difficulty of observing feeding behaviour in Arctic marine cetaceans. Understanding the diet of this population is of high priority to contaminant research, conservation management and the local Inuvialuit communities who lead subsistent lifestyles. This population has had the highest mercury contaminant loads in the Canadian Arctic (Lockhart et al., 2005), thus a comprehensive understanding of diet is required to identify contaminant sources.
The Beaufort beluga population segregates with habitat use of sea ice and bathymetry, varying with life stages (Loseto et al., 2006), a common theme among marine mammals (Stevick et al., 2002). As a result, sex and life stage are common factors driving differences in diet composition and feeding behaviour (e.g. the size dimorphic grey seal (Halichoerus grypus) (Beck et al., 2007)). The dietary biomarkers, fatty acids and stable isotopes, relate to beluga size and thus suggest diet differences among size classes and habitat use groups (Loseto et al., 2008a). However, fatty acids have not been used to determine prey items in the diet of the Beaufort Sea belugas. Fatty acids have successfully identified predator diets because they transfer from prey to predator adipose or blubber tissue with little modification (Iverson, 1993, Iverson et al., 2004). This biomarker approach has characterized trophic links within and among species (Budge et al., 2002, Iverson et al., 1997, Richoux et al., 2005, Stevens et al., 2004a, Stevens et al., 2004b) as well as determined predator diets in both marine and terrestrial mammals (Bradshaw et al., 2003, Iverson et al., 2001).
The biomarker δ15N along with the contaminant mercury, were used to examine food webs and estimate the Beaufort Sea beluga diet as it related to their size and habitat use (Loseto et al., 2008b). They may feed on one or many of the available prey in the eastern Beaufort Sea and Mackenzie Delta that include several coastal, anadromous and marine fish, in addition to invertebrates and bottom-feeding fish. Loseto et al. (2008b) hypothesized the following beluga size differentiated diets based on habitat use: 1) smaller sized beluga using shallow, near shore open water habitats fed on near shore estuarine prey in the Mackenzie Delta; 2) medium sized males and large females selecting ice edge habitats fed on offshore Arctic cod; and 3) largest male belugas selecting heavy sea ice concentrations in deep offshore waters, fed on the epibenthic food web. Although these feeding groups were supported by δ15N and mercury results, not enough information was available to validate the beluga diet.
To determine which prey items are important to the Beaufort Sea beluga diet we first investigate food web relationships among the potential prey collected in the beluga summering region. Food web relationships are evaluated using prey fatty acid profiles. Next, we determine which prey items are most important in the beluga diet using a multivariate approach that combines beluga and prey fatty acid profiles, followed by an assessment of the relative importance of prey items across the beluga body size range.
Section snippets
Beluga
Beluga blubber samples were collected during local harvests at Hendrickson Island near the community of Tuktoyaktuk, and at Browns Harbour near the community of Paulatuk, in Northwest Territories Canada (Fig. 1). A total of 43 samples were collected in July, from Tuktoyaktuk in 2004 (n = 19) and 2005 (n = 13) and from Paulatuk in 2005 (n = 11). Two of the youngest beluga were collected from Paulatuk, one of which was a young of the year (1.9 m) and the other was 8 years old and sexually immature (
Beaufort Sea food web and prey differentiation
The first two axes of the food web PCA explained 62% of the variance (Fig. 2). The first PCA axis on the score plot (prey plot) separated fish by placing pelagic fish on the negative side of the PCA, opposite to bottom-feeding fish on the positive side (Fig. 2A; PC1 39% variance explained). Fish collected in brackish waters such as Arctic cisco and rainbow smelt were centrally placed whereas saffron cod and least cisco were closer to the bottom-feeding fish. The second PCA axis separated fish
Beaufort Sea food web and prey differentiation
Results provide new information about the habitat use and feeding ecology of poorly studied Arctic marine species collected from the eastern Beaufort Sea and Mackenzie Delta region. Prey items were partitioned based on similarities and differences of their fatty acid profiles. High levels of the long chain C20 and C22 monounsaturates grouping Arctic cod and Pacific herring away from other prey, suggest they thrived on a copepod based food web. Those fatty acids, specifically 20:1n-9 and
Conclusion
Arctic cod is the most important summer diet item to the Beaufort Sea beluga population. Given the accelerated sea ice loss in the Arctic (Kerr, 2007, Serreze et al., 2007), an understanding of predator–prey ecology will have significant consequences to predicting future success and failure of both predator (beluga) and prey (Arctic cod) and marine food web dynamics. The beluga size dietary gradient may be driven by habitat preferences as it relates to Arctic cod distribution and ecology, as
Acknowledgements
This project was supported by a Natural Science and Engineering Research Council–Industrial Post-Secondary Scholarship sponsored by Devon Corporation Canada to LL. Funding and project support was provided by CASES (Canadian Arctic Shelf Exchange Study), ArcticNet, Fisheries Joint Management Committee, Northern Students Training Program, Northern Contaminants Program and by NSERC Discovery Grants to DD and SHF. We are grateful to N. Kenkel, M. Yunker for statistical assistance. We thank S. Budge
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