Summer diet of beluga whales inferred by fatty acid analysis of the eastern Beaufort Sea food web

Abstract

Beluga whales (Delphinapterus leucas) are the most abundant odontocetes in Arctic waters and are thus thought to influence food web structure and function. The diet of the Beaufort Sea beluga population is not well known, partly due to the inherent difficulty of observing feeding behaviour in Arctic marine cetaceans. To determine which prey items are critical to the Beaufort Sea beluga diet we first examine and describe the Mackenzie Delta and Beaufort Sea food web using fatty acid analyses. Fatty acid profiles effectively partitioned prey items into groups associated with their habitat and feeding ecology. Next, the relative contribution of various prey items to beluga diet was investigated using fatty acids. Finally, beluga diet variability was examined as a function of body size, a known correlate of habitat use. Beluga appeared to feed predominantly on Arctic cod (Boreogadus saida) collected from near shore and offshore regions. Size related dietary differences suggested larger sized beluga preferred offshore Arctic cod given the shared high levels of long chain monounsaturates, whereas smaller sized beluga appeared to feed on prey in their near shore habitats that included near shore Arctic cod. The presence of Arctic cod groups in shallow near shore and deep offshore habitats may facilitate the behavioural segregation of beluga habitat use as it relates to their size and resource requirements. Given Arctic cod are a sea ice associated fish combined with the accelerated sea ice loss in this region, beluga whales may need to adapt to new dietary regimes.

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.