Return of the giants: Summer abundance of fin whales in the Scotia Sea

Among large cetaceans in the Southern Hemisphere, fin whales were the most heavily exploited in terms of numbers taken during the period of intense industrial whaling. Recent studies suggest that, whilst some humpback whale populations in the Southern Hemisphere have almost completely recovered to their estimated pre-whaling abundance, much less is known about the status of Southern Hemisphere fin whales. Circumpolar estimates in the 1990s suggest an abundance of about 5 500 animals south of 60° S, while the IDCR/SOWER-2000 survey for the Scotia Sea and Antarctic Peninsula areas estimated 4 670 fin whales within this region in 2000. More recent studies in smaller regions indicate higher densities, suggesting that previous estimates are overly conservative and/or that fin whales are undergoing a substantial recovery. Here we report findings from a recent multi-vessel single-platform sightings survey carried out as part of the 2019 Area 48 Survey for Antarctic krill. While fin whales were encountered throughout the entire CCAMLR Management Area 48, they were particularly abundant around the South Orkney Islands and the eastern Bransfield Strait. Large feeding aggregations were also encountered within the central Scotia Sea between South Orkney Islands and South Georgia. Average fin whale density throughout the Scotia Sea was 0.0256 (CV=0.149) whales per km2, which agrees well with recent density estimates reported from smaller sub-regions within the Scotia Sea. The best estimate of total fin whale abundance was 53,873 (CV=0.152, 95% CI: 38,122 - 69,625), which is at least an order of magnitude greater than the previous estimate from the same region based on the IDCR/SOWER-2000 data. These estimates suggest that fin whales are undergoing a substantial recovery, which may have important implications for the assessment of cetacean population trends, but also for the CCAMLR risk assessment process and efforts to implement a Feedback Management system for Antarctic krill. Our abundance estimate suggests an annual summer krill consumption by fin whales in the Antarctic Peninsula and Scotia Sea area of 5.7 (95% CI: 4.2-7.6) million tonnes, which would represent around 15 times the total krill catch taken by the commercial fishery in Area 48 in the same season. This highlights the crucial importance of including cetacean krill predators in assessment and management efforts for living marine resources in the Southern Ocean, and the urgent need for a re-appraisal of abundance, distribution and ecological role of Southern Hemisphere fin whales.