Abstract
Sperm whales interact with commercially important groundfish fisheries offshore in the Gulf of Alaska (GOA). This study aims to use stable isotope analysis to better understand the trophic variability of sperm whales and their potential prey, and to use dietary mixing models to estimate the importance of prey species to sperm whale diets. We analysed tissue samples from sperm whales and seven potential prey (five groundfish and two squid species). Samples were analysed for stable carbon and nitrogen isotope ratios, and diet composition was estimated using Bayesian isotopic mixing models. Mixing model results suggest that an isotopically combined sablefish/dogfish group, skates and rockfish make up the largest proportion of sperm whale diets (35%, 28% and 12%) in the GOA. The top prey items of whales that interact more frequently with fishing vessels consisted of skates (49%) and the sablefish/dogfish group (24%). This is the first known study to provide an isotopic baseline of adult male sperm whales and these adult groundfish and offshore squid species, and to assign contributions of prey to whale diets in the GOA. This study provides information to commercial fishermen and fisheries managers to better understand trophic connections of important commercial species.
Highlights
Understanding top predator diets and their role in marine food webs is important to managing fisheries and mammals from an royalsocietypublishing.org/journal/rsos R
Females and calves primarily inhabit warm equatorial regions between 40° S and 40° N latitude, while males are thought to leave their natal groups after age 12, when they move to high-latitude feeding grounds and roam widely [1,2]. One of those high-latitude foraging grounds is in the Gulf of Alaska (GOA), where sperm whales were historically killed in large numbers during commercial whaling [3,4]
Three samples, collected from the NOAA GOA longline survey in 2006, were taken in the central GOA, outside the study area. These samples fell within the range of δ13C and δ15N values of samples collected in the eastern GOA and isotope ratios were not significantly different than the rest of samples (MANOVA, F8,56 = 2.19, p = 0.05), and so they were included in the analysis to help improve our sample size
Summary
Understanding top predator diets and their role in marine food webs is important to managing fisheries and mammals from an royalsocietypublishing.org/journal/rsos R. Females and calves primarily inhabit warm equatorial regions between 40° S and 40° N latitude, while males are thought to leave their natal groups after age 12, when they move to high-latitude feeding grounds and roam widely [1,2]. One of those high-latitude foraging grounds is in the Gulf of Alaska (GOA), where sperm whales were historically killed in large numbers during commercial whaling [3,4].
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