Immature euphausiids do not appear to be prey for humpback whales (Megaptera novaeangliae) during spring and summer in Southeast Alaska

Abstract

Humpbacks whales (Megaptera novaeangliae) have shown a remarkable recovery in the North Pacific, raising concerns regarding their impact on marine communities. In Southeast Alaska, humpbacks feed heavily on euphausiids; however, it remains unclear whether they target immature individuals despite evidence that they do so elsewhere. I evaluate the hypothesis that humpbacks target immature euphausiids in late spring-summer in Southeast Alaska. Plankton samples were collected at random sites (n = 44) and near whales (n = 53) between 8 June and 9 September 2008 in Frederick Sound and Stephens Passage. The proportion of samples containing immature euphausiids, and immature euphausiid abundance within those samples, were compared between the two sample types. Similar analyses were conducted for adult euphausiids (prey) and calanoid copepods (nonprey) for comparison. I found no statistical difference between the whale and random samples with respect to the occurrence or numerical density of immature euphausiids, which is consistent with the hypothesis that whales did not target them in 2008. Smaller size, insufficient numerical densities and lower energy density of immature euphausiids are suggested as possible reasons. These findings can assist in resolving regional humpback abundance and distribution patterns, and can contribute to an understanding of the trophic interactions characterizing the local ecosystem.

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

“…The results from experiment 1 provide preliminary support to the hypothesis of a dietary time series in layers of the skin of cetaceans, specifically in humpback and sperm whales. 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 .…”

“…The difference in stable nitrogen isotope ratios between the inner and outer layers of the humpback whale could indicate diet shifts from higher trophic level herring in the spring to lower trophic level krill in the summer before it died. Humpback whales in southeast Alaska eat a mixed diet of herring, krill, and other forage fish, and have a lower trophic level diet overall than those in other areas of Alaska . The humpback whale sampled in this analysis was found dead on 28 July 2010, and while deposition rates are unknown for this species, incorporation of stable isotopes from prey into tissue has been estimated in other cetacean species to occur between 2.5 and 6 months .…”

“…This hypothesis rests on the assumptions that the cells within the same layer of skin were deposited closer to the same time on average than cells in the same core, and that the animal had undergone a detectable diet shift within the turnover time of the full skin thickness. 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 .…”

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

“…(Krieger and Wing , Whitehead and Carlson , Piatt et al . , Szabo ). Both species are known to switch between diets of zooplankton and forage fish depending on availability (Whitehead and Carscadden , Payne et al .…”

“…and Euphausia pacifica) and copepods (Neocalanus spp.) (Krieger and Wing 1986, Whitehead and Carlson 1988, Piatt et al 1989, Szabo 2015. Both species are known to switch between diets of zooplankton and forage fish depending on availability Carscadden 1985, Payne et al 1990).…”