COS 184-3 - High movement rates of Antarctic blue whales (Balaenoptera musculus intermedia) on Southern Ocean feeding grounds estimated from historic mark-recovery data

Abstract: Baleen whales undertake some of the longest migrations of any mammal on earth and it is generally believed that they migrate between polar high-latitude feeding grounds and warm low-latitude breeding and calving grounds. Antarctic blue whale (Balaenoptera musculus intermedia) migration patterns remain somewhat of a mystery. There is evidence of annual migrations between low-latitudes in the Southern Hemisphere and high-latitude feeding grounds in the Southern Ocean, but there is also evidence that some Antarctic blue whales remain in the Southern Ocean year-round, and that some skip migration and remain in temperate waters during the summer. In other species, maternally-inherited fidelity to migration routes has led to population structure between breeding and feeding areas, even when the area lacks obvious geographic boundaries. Understanding population structure is important for conservation, especially in the case of Antarctic blue whales, which were hunted to the brink of extinction by 20^th century whaling. We tested the long-standing assumption that Antarctic blue whales are a single circumpolar population in the Southern Ocean by analyzing historic mark-recovery data using a Bayesian multistate model that incorporated spatially-explicit abundance estimates, movement between ocean basins, intrinsic population growth, and the probability of mark loss. Using this model, we estimated inter-season movement rates among the three ocean basins in the Southern Ocean (Atlantic, Indian, and Pacific), predicting high probabilities of inter-season movement in almost all directions. For blue whales in the Atlantic basin of the Southern Ocean, each year 16% (95% interval: 6-33%) were estimated to move to the Indian and 25% (7-34%) to the Pacific; for those in the Indian, 10% (2-26%) moved to the Atlantic and 26% (15-34%) to the Pacific; and for those in the Pacific, 20% (11-32%) moved to the Indian, and 10% (3-26%) to the Atlantic. Overall, these results indicate that Antarctic blue whales regularly vary their migration routes between low and high latitude areas over time. This suggests that there is little evidence for population structure arising from Southern Ocean basin-specific migratory fidelity by Antarctic blue whales. However, despite model predictions no direct movement between the Atlantic and Pacific was observed, which suggests there could be barriers to movement in the Southern Ocean on a finer spatial scale than captured within our model.