Estimated summer abundance and krill consumption of fin whales throughout the Scotia Sea during the 2018/2019 summer season

Summary

This 2019 whale survey in the Southern Ocean found dramatically more fin whales than expected. Scientists counted approximately 53,873 fin whales - at least 10 times more than found in 2000 (4,670 whales).The whales were most common around the South Orkney Islands, eastern Bransfield Strait, and in large feeding groups in the central Scotia Sea. These whales eat about 7.97 million tonnes of krill each summer - roughly 20 times more than commercial fishing takes from the same area.This increase shows fin whale populations are recovering well from past whaling. The findings highlight that whale populations must be considered when managing Antarctic fisheries, as whales are now major consumers of krill - a key food source in Antarctic waters.

Map showing the routes of three research vessels - R/S Kronprins Haakon (KPH), F/V Cabo de Hornos (CDH) and RRS Discovery (DIS) - that participated in the 2019 Antarctic krill survey in Area 48. The thick lines show when the vessels were actively collecting data (KPH in blue, CDH in light brown and DIS in green). Orange circles show where fin whales were spotted, with larger circles indicating more whales seen together.

Key Findings

Fin whale abundance estimated at 53,873 in the Scotia Sea region, at least 10 times more than found in 2000

Fin whales were most common around the South Orkney Islands and eastern Bransfield Strait

Annual krill consumption by fin whales estimated at 7.97 million tonnes - 20 times more than commercial fishery catch

Fin whale krill consumption represents about 12.7% of the 2019 summer krill standing stock

Evidence of substantial recovery of fin whale populations following historical whaling

Abstract

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 appears to 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 5500 animals south of 60° S, while the IDCR/SOWER-2000 survey for the Scotia Sea and Antarctic Peninsula areas estimated 4670 fin whales within this region in the year 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 increase. 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 survey area, which covered the majority of 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. Distance sampling analyses suggest an average fin whale density throughout the Scotia Sea of 0.0256 (CV = 0.149) whales per km², which agrees well with recent density estimates reported from smaller sub-regions within the Scotia Sea. Design-based distance sampling analyses resulted in an estimated total fin whale abundance of 53,873 (CV = 0.15, 95% CI 40,233–72,138), while a density surface model resulted in a slightly lower estimate of 50,837 (CV: 0.136, 95% CI 38,966–66,324). These estimates are at least an order of magnitude greater than the previous estimate from the same region based on the IDCR/SOWER-2000 data, suggesting that fin whales are undergoing a substantial abundance increase in the South Atlantic. This may have important implications for the assessment of cetacean population trends, but also for CCAMLRs spatial overlap analysis 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 7.97 (95% CI 4.94–11.91) million tonnes, which would represent around 20 times the total krill catch taken by the commercial fishery in Area 48 in the same season, or about 12.7% of the 2019 summer krill standing stock estimated from data collected during the same survey. This highlights the crucial importance of including cetacean krill predators in assessment and management efforts for living marine resources in the Southern Ocean, and particularly stresses the urgent need for a re-appraisal of abundance, distribution and ecological role of Southern Hemisphere fin whales.

Published in

Scientific Reports

2024

Authors

Martin Biuw, Ulf Lindstrøm, Jennifer A. Jackson, Mick Baines, Nat Kelly, George McCallum, Georg Skaret & Bjørn A. Krafft

Institutions

Institute of Marine Research, Fram CentreNERCWildscopeAustralian Antarctic DivisionWhalephoto Marine PhotographyInstitute of Marine Research

Methods

DataAcousticBiological sampling Field

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