Stable‐isotope ratios of nitrogen (δ15N) and lipid‐normalized carbon (δ13C′) were used to examine geographic and ontogenetic variability in the trophic ecology of a high latitude benthopelagic elasmobranch, the Pacific
sleeper shark Somniosus pacificus. Mean muscle tissue δ13C′ values of S. pacificus differed significantly among geographic regions of the eastern North Pacific Ocean. Linear models identified significant ontogenetic and geographic variability in
muscle tissue δ15N values of S. pacificus. The trophic position of S. pacificus in the eastern North Pacific Ocean estimated here from previously published stomach‐content data (4·3) was within the range of S. pacificus trophic position
predicted from a linear model of S. pacificus muscle tissue δ15N (3·3–5·7) for fish of the same mean total length (LT; 201·5 cm), but uncertainty in predicted trophic position was very high (95% prediction intervals
ranged from 2·9 to 6·4). The relative trophic position of S. pacificus determined here from a literature review of δ15N by taxa in the eastern North Pacific Ocean was also lower than would be expected based on stomach‐content data alone when
compared to fishes, squid and filter feeding whales. Stable‐isotope analysis revealed wider variability in the feeding ecology of S. pacificus in the eastern North Pacific Ocean than shown by diet data alone, and expanded previous conclusions drawn from analyses of stomach‐content
data to regional and temporal scales meaningful for fisheries management.