Dietary patterns of humpback whales (Megaptera novaeangliae) in the Northwest Atlantic : evidence from 13C and 15N stable isotopes

Todd, Sean Kevin (1997) Dietary patterns of humpback whales (Megaptera novaeangliae) in the Northwest Atlantic : evidence from 13C and 15N stable isotopes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Previous dietary assessments of the northwest Atlantic humpback whale (Megaptera novaeangliae), mainly based on analysis of gut contents, may now be outdated because of recent substantial biological and oceanographic changes in the northwest Atlantic ecosystem. Within ecological studies, newer techniques are being developed that examine diet non-lethally. Stable isotope analysis (SIA), for example, utilizes prey-specific signatures preserved in the tissues of a consumer, and holds several advantages over traditional techniques of prey determination, including examination of longitudinal variation in diet. This thesis used SIA to assess diet in the Newfoundland and Labrador feeding substock of humpback whales. Two stable-isotope indicators—δ¹³C and δ¹⁵N—were used to interpret types of diet and trophic feeding level. -- Previous SIA studies used muscle tissue from dead, stranded cetaceans. However, recent advances in biopsy technique have provided a method to collect skin and blubber samples from free-ranging animals in a non-lethal manner. This study calibrated the two techniques using tissues from dead and live animals. Analyses demonstrated that isotopic ratios estimated for either tissue were statistically indistinguishable. Levels of δ¹⁵N and δ¹³C were measured with high accuracy and precision (± 02‰). -- A second part of this study is based on a collection of biopsied samples (n = 130) taken in 1988-1994 from humpback whales on their feeding grounds in the northwest Atlantic Isotopic ratios varied among regions, probably because of large scale oceanographic differences affecting phytoplankton isotopic composition, and because of differences in prey distribution. Isotopic ratios did not vary by sex. -- Examination of monthly data, correlated with prey availability, provided a first estimation of the integration time for skin tissue (~7-14 days). These data support the hypothesis that skin is a short-turnover tissue, and its isotopic composition reflects short-term diet (< 30 days). Large isotopic variations within and between years are attributed to differences in diet and large-scale oceanographic changes. Possible decreases in primary productivity in the early 1990s due to the North Atlantic Oscillation were correlated with increases in δ¹⁵N values. This finding suggested changes in humpback diet as prey species changed in distribution, availability and abundance in response to colder, less saline waters. -- In 1994, an opportunistic collection of biopsies from finback whales (Balaenoptera physalus) feeding in the same area as humpback whales confirmed that finback whales fed at lower trophic levels. Comparison of the isotopic values for humpback whales with other mysticete species suggests that the humpbacks whale is piscivorous. -- In summary, biopsy samples may provide useful indicators of diet using stable-isotope methods. Stable-isotopic analysis provides researchers with a relatively non-intrusive and non-lethal method that, if used in conjunction with other techniques, can provide an accurate assessment of diet.

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