Foraging niche, sex-specific resource partitioning, and mercury concentrations of male Steller sea lions in Alaska

Abstract

With their large size, the foraging behaviors of adult male Steller sea lions Eumetopias jubatus may have significant impacts on marine food webs and ecosystems. Limited studies from scat suggest that male and female Steller sea lions exhibit some dietary differences, and bycatch rates tend to be higher for males; however, most foraging ecology data are from adult females. Here we aimed to (1) characterize the isotopic foraging niches of adult male Steller sea lions in the Aleutian Islands and Bering Sea (n = 18), (2) assess sex-specific resource partitioning between male and female Steller sea lions (n = 18) in this region, and (3) identify male foraging strategies associated with greater total mercury concentrations ([THg]), a contaminant of concern in the region. Vibrissae collected from males bycaught in commercial fisheries from 1998-2017 were sectioned, and sequential segments were analyzed for bulk stable isotope ratios (δ15N and δ13C values) and [THg]. Individuals exhibited temporal oscillations in both δ15N and δ13C values that reflect significant summer to winter variation in foraging. Male Steller sea lions had larger isotopic niches than females, and limited overlap of isotopic niches suggests a moderate degree of sex-specific resource partitioning in this system. [THg] were positively associated with δ15N, and individuals exhibited evidence of bioaccumulation with age. These findings represent contemporary isotopic values and the first reported Hg levels for non-pup male Steller sea lions in a region where the population is considered endangered. Together, they provide a reference for assessing shifts in foraging patterns relative to future environmental fluctuations.