Trophic niche partitioning and diet composition of sympatric fin (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae) in the Gulf of Alaska revealed through stable isotope analysis

Abstract

Fin and humpback whales are large consumers that are often sympatric, effectively sharing or partitioning their use of habitat and prey resources. Stable carbon and nitrogen isotopes in the skin of fin and humpback whales from two regions in the western Gulf of Alaska, Kodiak, and Shumagin Islands, were analyzed to test the hypothesis that these sympatric baleen whales exhibit trophic niche partitioning within these regions. Standard ellipse areas, estimated using Bayesian inference, suggested that niche partitioning between species is occurring in the Kodiak region but not in the Shumagin Islands. Isotopic mixing models based on stable isotopes from whales and local prey samples, were used to estimate possible diet solutions for whales in the Kodiak region. Comparison of isotopic niches and diet models support niche partitioning, with fin whales foraging primarily on zooplankton and humpback whales foraging on zooplankton and small forage fish. The results of this study show that niche partitioning between sympatric species can vary by region and may be the result of prey availability, prey preferences, or both.

Evidence for dietary time series in layers of cetacean skin using stable carbon and nitrogen isotope ratios

“…We found significant differences in stable isotope ratios among different layers but not among cores in both sperm and humpback whales, which are known for their isotopically diverse diets . Furthermore, the stable isotope ratios did not differ among layers or cores in the skin of the fin whale, consistent with our expectations for a species which is thought to have a less variable diet . These results suggest that subsampling only the inner layer of cetacean tissue could provide a means for assessing recent diets, better suited for interpretation with observational feeding studies and comparative studies where prey is sampled as well.…”

“…In particular, higher latitude foraging is known to result in lower (more negative) δ 13 C values in the north Pacific Ocean . Witteveen and Wynne found δ 13 C values of near‐shore fin whales in a similar region to this study to be on average 1.5‰ higher than found in this study. Given similar latitude in the same ocean basin, this suggests that the fin whale in this study was foraging further offshore.…”

“…This was expected given the fairly homogeneous diet of this species . The fin whale's δ 13 C values were surprisingly low, 1–2‰ lower than the δ 13 C values observed from other studies of fin whales . However, two of these studies were conducted at either a much lower latitude or a combination of a lower latitude and different ocean basin, both of which can significantly affect δ 13 C values .…”

“…Fin whales ( Balaenoptera physalus ) forage in both near‐shore and offshore waters in the GOA. Fin whales worldwide have a fairly homogeneous lower‐trophic diet of euphausiid krill ( Euphausiacea ) and occasionally of small amounts of small schooling fish and copepods . Humpback whales ( Megaptera novaeangliae ) are generalist predators in coastal southeast Alaskan waters, feeding on a varied diet of krill, herring, and other small schooling fish, that have a broad range of isotopic ratios .…”

“…For the humpback whale and sperm whale, two generalist foragers with diverse and seasonal diets, this was probably the case . For the fin whale, with a more homogeneous diet, we expected there to be lower variability overall among layers . A second experiment measured the average differences among layers of a larger number of sperm whale skin samples, and tested if the differences between specific layers were consistent across individuals.…”