Abstract
Walruses in Canada are currently subdivided into seven stocks based on summering areas; Western Jones Sound (WJS), Baffin Bay (BB), Penny Strait-Lancaster Sound (PS-LS), North Foxe Basin (N-FB), Central Foxe Basin (C-FB), Hudson Bay Davis Strait (HB-DS) and Southern and Eastern Hudson Bay (SE-HB). In this study, walrus were sampled from six of the seven stocks (SE-HB samples were not available) and genotyped at 10 microsatellite loci. All stocks were genetically diverse (average heterozygosity of 0.58) with no evidence of inbreeding (average FIS of 0.03). We detected significant genetic differentiation among the stocks and a pattern of genetic spatial autocorrelation that suggests a moderate effect of geographic distance on gene flow among stocks. Bayesian clustering suggested the six recognized stocks were elements of two larger genetic clusters - a northern Arctic population (containing BB, WJS, and PS-LS stocks) and a central Arctic population (containing C-FB, N-FB, and HB-DS stocks). These populations are moderately differentiated (FST = 0.07), but based on evidence of contemporary movement from assignment tests, are not completely isolated. There was support for maintaining the WJS stock and a combined BB+PS-LS stock, although the latter conclusion is based on a small sample size. Similarly, there was some evidence suggesting separation of the Foxe Basin stocks from the HB-DS but not the N-FB from the C-FB stock. However, given that there are morphological and chemical differences between N-FB and C-FB stocks, there is currently insufficient evidence to support a revision of the current stock designations.
Highlights
Walruses (Odobenus rosmarus rosmarus) occur in Canada from James Bay to Smith Sound and from the Canada–Greenland international boundary in Davis Strait to the longitudinal center of Canada (Fig. 1)
We addressed three questions: 1) Is there genetic differentiation among Stewart’s (2008) designated stocks? 2) Is there genetic differentiation, spatially and temporally, within stocks? and 3) What is the rate of genetic exchange between stocks? Following Stewart (2008), we employed Pianka’s (1988) definition of a population, an intraspecific group with a higher probability of interbreeding than breeding with members of other groups, and used wintering areas as surrogates for population identification because breeding takes place in winter (Sjare and Stirling 1996)
Walruses in Arctic Canada are divided into two large genetic groups: the central Arctic population that contains the North Foxe Basin (N-FB), Central Foxe Basin (C-FB), and Hudson Bay Davis Strait (HB-DS) stocks, and the high Arctic population that contains the Baffin Bay (BB), Penny Strait–Lancaster Sound (PS-LS), and Western Jones Sound (WJS) stocks (Fig. 4)
Summary
Walruses (Odobenus rosmarus rosmarus) occur in Canada from James Bay to Smith Sound and from the Canada–Greenland international boundary in Davis Strait to the longitudinal center of Canada (Fig. 1). Within this range, walruses are subdivided into seven stocks based on summering areas for the purpose of making management decisions that affect walrus and walrus habitat (Stewart 2008). In the absence of definitive information, it is more precautionary to assume greater subdivision than exists in nature rather than to assume less (Taylor 1997, Taylor and Dizon 1999). Overly conservative subdivision can lead stock managers to overestimate the risk of stock extirpation, potentially leading to negative effects on resource users.
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