Abstract
Southern hemisphere humpback whales are classified as high-fidelity Antarctic krill consumers and as such are vulnerable to variability and long-term changes in krill biomass. Evidence of heterogeneous feeding patterns of east coast of Australia migrating humpback whales has been observed, warranting a comprehensive assessment of interannual variability in their diet. We examined the lipid and fatty acid profiles of individuals of the east coast of Australia migrating stock sampled between 2008 and 2018. The use of live-sampled blubber biopsies showed that fatty acid profiles varied significantly among all years. The two trophic indicator fatty acids for Antarctic krill, 20:5ω3 and 22:6ω3 remained largely unchanged across the 10-year period, suggesting that Antarctic krill is the principal prey item. A distance-based linear model showed that 33% of the total variation in fatty acid profiles was explained by environmental variables and climate indices. Most of the variation was explained by the Southern Annular Mode (23.7%). The high degree of variability observed in this study was unexpected for a species that is thought to feed primarily on one prey item. We propose that the observed variability likely arises from changes in the diet of Antarctic krill rather than changes in the whale’s diet.
Highlights
Under the classical feeding ecology paradigm, southern hemisphere humpback whales are assumed to be highfidelity Antarctic krill (Euphausia superba) consumers[1], feeding in Antarctic waters during the austral summer
Our results show that E1 humpback whales, their potential temperate prey species (N. australis, E. nitidus, S. neopilchardus, T. declivis) and their potential Antarctic prey species (T. macrura and M. gregaria) all occupy a more carnivorous feeding niche than Antarctic krill, as they have lower 18:1ω7c/18:1ω9c ratios
Our results show that the prominent interannual variability is mainly driven by dietary fatty acids, which are generally thought to be deposited unmodified into the adipose tissue of marine mammals, in similar proportions to that consumed in the diet[15]
Summary
Under the classical feeding ecology paradigm, southern hemisphere humpback whales are assumed to be highfidelity Antarctic krill (Euphausia superba) consumers[1], feeding in Antarctic waters during the austral summer. Rates during migration than previously assumed under the classical feeding paradigm, which is in accordance with findings based on isotopic signatures of stranded E1 humpback whales, but contrary to findings based on the fatty acid profiles of free-swimming E1 individuals. The biochemically evidenced departure from the classical feeding paradigm indicated in this study may, not be the behaviour of healthy individuals, as baleen whale stranding events are biased towards old, young or sick individuals This assumption is reinforced by a study utilising blubber fatty acid profiles of free-swimming E1 humpback w hales[1]. A difference in the fatty acid composition of E1 humpback whales compared to Antarctic krill was observed, indicating that species-specific metabolism impacts blubber fatty acid profiles of E1 humpback w hales[1]. The difference between the fatty acid composition of E1 humpback whales and Antarctic krill found by Waugh et al.[1] indicates that direct deposition of fatty acids from diet is not the sole pathway of fatty acid incorporation into the adipose tissue of whales, which precludes direct delineation of the relative influence of diet on the whale’s adipose tissue composition
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