Abstract
Our current understanding of Southern hemisphere humpback whale (Megaptera novaeangliae) ecology assumes high-fidelity feeding on Antarctic krill in Antarctic waters during summer, followed by fasting during their annual migration to and from equatorial breeding grounds. An increase in the number of reported departures from this feeding/fasting model suggests that the current model may be oversimplified or, alternatively, undergoing contemporary change. Information about the feeding and fasting cycles of the two Australian breeding populations of humpback whales were obtained through stable isotope analysis of baleen plates from stranded adult individuals. Comparison of isotope profiles showed that individuals from the West Australian breeding population strongly adhered to the classical feeding model. By contrast, East Australian population individuals demonstrated greater heterogeneity in their feeding. On a spectrum from exclusive Antarctic feeding to exclusive feeding in temperate waters, three different strategies were assigned and discussed: classical feeders, supplemental feeders, and temperate zone feeders. Diversity in the inter-annual feeding strategies of humpback whales demonstrates the feeding plasticity of the species, but could also be indicative of changing dynamics within the Antarctic sea-ice ecosystem. This study presents the first investigation of trophodynamics in Southern hemisphere humpback whales derived from baleen plates, and further provides the first estimates of baleen plate elongation rates in the species.
Highlights
Southern hemisphere humpback whales (SHHW) exhibit one of the longest mammalian migration on Earth, travelling close to 10 000 kilometres between their equatorial breeding grounds and Antarctic feeding grounds [1,2,3]
We investigate the validity of the classical SHHW feeding model through analysis of baleen stable isotope records from twenty stranded adult individuals, with uncertain cause of deaths
There were minimal isotopic differences between the plates for each individual, with the main difference due to the length of the plates: a shorter plate covered a smaller temporal record than a longer one (D01, S1 Fig). This observation would suggest that the size differences between plates of a single individual are due to the rate at which there are eroded during feeding and not due to variations in elongation rate, as found in other cetacean species
Summary
Southern hemisphere humpback whales (SHHW) exhibit one of the longest mammalian migration on Earth, travelling close to 10 000 kilometres between their equatorial breeding grounds and Antarctic feeding grounds [1,2,3]. Populations assumes high-fidelity summer feeding on Antarctic krill (Euphausia superba) followed by fasting throughout the entire migration, with feeding only resumed in Antarctic waters the following summer [4,5,6,7] Because they are capital breeders, the humpback whales breed, calve and nurse during the migratory fast. A rise in such observations may be an artefact of the rapid, post-whaling reexpansion of the Southern Ocean baleen whale stocks [30, 31] These observations may signal changing dynamics within the Antarctic sea-ice ecosystem [13, 14, 32], or likely, a combination of both. We investigate the validity of the classical SHHW feeding model through analysis of baleen stable isotope records from twenty stranded adult individuals, with uncertain cause of deaths. This study further estimates elongation rates for baleen plates in SHHW, and provides insight into metabolic patterns associated with the migratory life-history of SHHWs, highlighting isotopic changes associated with fasting
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