Diets of Mature Male and Female Steller Sea Lions (Eumetopias jubatus) Differ and Cannot Be Used as Proxies for Each Other

Abstract: Disturbance of otariid breeding sites (rookeries) to determine diet from fecal remains (scats) could be eliminated if the diets of males using adjoining bachelor haulouts could be used as a proxy for diets of breeding females. We collected scats from sexually mature Steller sea lions (Eumetopias jubatus) at one male resting site (haulout) and three female dominated breeding sites (rookeries) at Forrester Island, southeast Alaska (June and July, 1994 to 1999) to test whether the diets of bachelor bulls differed from that of breeding females. Female diets were fairly evenly distributed between gadids, salmon, and small oily fishes (forage fish) and contained lesser amounts of rockfish, flatfish, cephalopods, and other fishes. The female diet did not differ significantly between the three rookeries, but it did differ significantly from that of males. Males consumed significantly fewer salmon and more pollock, flatfish, and rockfish compared to females. The males also consumed larger pollock compared to females. These dietary differences may reflect a sex-specific difference in foraging areas or differences in hunting abilities related to the disparity in physical sizes of males and females. The similarity of the female diets between rookeries suggests that female diets can be determined from samples collected at a single site within a rookery complex. Unfortunately, summer diets of breeding females cannot be ascertained from hard parts contained in the scats of mature male Steller sea lions.

Key Words: Steller sea lion, Eumetopias jubatus, prey, diet, size, diet diversity, diet composition

Document Type: Research article

DOI: 10.1578/AM.34.1.2008.25

Page Numbers: 25-34

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