Southern Ocean humpback whale trophic ecology. II. Influence of fasting and opportunistic feeding on skin stable isotope values of migrating whales

Many baleen whale species migrate between low-latitude breeding grounds and high-latitude feeding grounds, with increasing evidence that humpback whales Megaptera novaeangliae utilise supplemental feeding sites in temperate regions while migrating. The diet of whales while migrating is often unknown, and the impact that temperate feeding and/or fasting has on biochemical tracers used to investigate diet remains unclear. The aims of this study were to (1) determine whether prey consumption at supplemental feeding sites could be detected by carbon (δ¹³C) and nitrogen (δ¹⁵C) stable isotope analysis of skin; (2) obtain information on diet during migration; and (3) ascertain the impact of potential fasting on stable isotope values of baleen whales. Skin samples were taken from the eastern Australian humpback whale population on Antarctic feeding grounds and 2 sites on the southward migration route (a sub-tropical site and a temperate site) across 2 yr. At the sub-tropical site, δ¹³C and δ¹⁵N were consistent with the last place of foraging 5 mo earlier. One exception was the higher (0.5 %o) δ¹⁵N value in 2011, suggesting that in some years, potentially when blubber reserves are insufficient, δ¹⁵N may be influenced by fasting. In both years, skin δ¹³C and δ¹⁵N values at the temperate site were higher than those from the Antarctic and sub-tropical sites, indicating that a feeding signal from temperate zones had likely been incorporated, with whales feeding on fish and krill. Importantly, supplemental feeding while migrating could affect the interpretation of whale diet on feeding grounds if sampled early in the season.