Southern Ocean humpback whales are shifting to an earlier return migration

The Southern Ocean ecosystem is undergoing unprecedented environmental changes, which have led to shifts in the primary food source of baleen whales, Antarctic krill. Additionally, many humpback whale populations have rebounded from near extirpation due to historical whaling, increasing pressure on now vulnerable krill populations. Since humpback whales rely on energy reserves built up during their feeding season to sustain them during migration, changes in their food supply are likely to influence their migration strategies. In this study, the timing of the 'return to feeding grounds,' or southern migration, of the eastern Australian humpback whale population was tracked over a 21-year period. Both land-based and acoustic surveys were used to estimate the timing of the migratory peak in southern Queensland, i.e., the week with the highest number of whales sighted or recorded. Land-based surveys recorded all whales, including females with newborn calves, while acoustic surveys recorded singing adult males. Both datasets revealed that the peak of the southern migration has shifted earlier by approximately three weeks from 2003 to 2024. Although this study does not establish causation, it highlights a strong correlation between changes in migratory timing and sea ice coverage in the whales' Antarctic feeding grounds. The observed decline in sea ice area, combined with the large increase in the humpback whale population, are discussed as potential factors contributing to the shift in migratory timing.