Commercial krill fishing within a foraging supergroup of fin whales in the Southern Ocean.

Industrial whaling in the 20th century reduced the global biomass of marine mammals by over fourfold (Bar-on et al., 2018). The majority of this overexploitation occurred during a 70-year period in the Southern Ocean (1904–1978) (Rocha et al., 2014). Fin whales (Balaenoptera physalus), the second largest species of all time, accounted for half of all baleen whales harvested in the Southern Ocean, 726,461 individuals in total (Rocha et al., 2014). A recent survey estimated that ~1300 fin whales were in the southwestern Scotia Sea (Elephant Island and South Orkney Islands region) (Viquerat & Herr, 2017). This recovering population subsists primarily on Antarctic krill (hereafter “krill”; Euphausia suberba) (Herr et al., 2022; Viquerat & Herr, 2017). The extent of competition between recovering whales and the growing krill fishery is of great interest but is poorly understood. On 13 January 2022 (17:10–19:10 local time), we observed a remarkably large aggregation of foraging fin whales 15 km northwest of Coronation Island between 60°24.44′ S, 46°04.45′ W and 60°18.29′ S, 45°53.51′ W (Figure 1 and Video S1). Eye height was 12 m above sea level, providing a visible area of 185 km2 over the passage. Viewing conditions were favorable: visibility >20 km, swell height 1–2 m, Beaufort 3 wind speed. The charted depth was between 294 and 465 m, and the nearest point of land was 14 km away (Figure 1A). The sighting occurred at the tail end of a large spring phytoplankton bloom in the southern Scotia Sea (Figure 2A,B, Appendix S1). Evidence of a mix of behavioral modes was observed, including surface feeding (distended buccal cavity) and steeply arched deeper dives (presumably nonsurface feeding). The mean (±SD) blow duration was 8.3 ± 2.9 s, and the total number of instantaneous blows visible was 143 (Figure 1E). When combined with estimates of hourly blow rate (Kopelman & Sadove, 1995), we obtained a group size estimate of 979 (95% CI: 830–1153; Figure 1D, Appendix S1) for the mixed-behavior—surface and nonsurface feeding—aggregation of fin whales. Among the fin whales were two juvenile humpback whales (Megaptera novaeangliae), a blue whale (B. musculus), Antarctic fur seals (Arctocephalus gazella), and thousands of seabirds, including prions, petrels, penguins, and albatrosses. Within this multispecies aggregation of krill predators there were also four commercial krill vessels actively harvesting krill (Figure 1B). This observation of a supergroup of ~1000 foraging fin whales is among the largest aggregations of baleen whales ever recorded (Herr et al., 2022; Viquerat & Herr, 2017). While large aggregations of baleen whales—specifically fin whales—are not new for these waters, our observation is larger than most postwhaling sightings by an order of magnitude (Herr et al., 2022; Joiris & Dochy, 2013; Santora et al., 2014; Viquerat & Herr, 2017). Further, our sighting highlights that krill fishing and high densities of foraging whales can co-occur in the Southern Ocean (Figure 1B and Video S2). This scenario presents both welfare and conservation concerns for whales as the fishery grows. In 2020 and 2021, three humpback whale fatalities were reported from krill fishing operations in the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Area 48 (CCAMLR, 2021). Specifically, South Orkney West—the Small-Scale Management Unit (SSMU) in CCAMLR Subarea 48.2 where we made our observation—is the global epicenter of krill fishing. Nearly 30% of all krill taken annually is harvested from this SSMU (Trathan & Grant, 2020). Under the present scenario of increasing whale densities (Zerbini et al., 2019) and krill fishing effort (CCAMLR Secretariat, 2021), whale–fishery interactions, including bycatch and direct competition for krill, are likely to increase unless action is taken. The recent combination of technological advances in fishing, reduced sea ice extent and duration, and increasing demand for krill in aquaculture feed and nutritional supplements has led to the expansion of the krill fishery (CCAMLR, 2022). Three of the four observed vessels were using continuous trawling, where the krill is transported from the cod end of the net to the vessel without having to remove the net from the water, while the remaining vessel was using conventional trawling, where the net is removed from the water. Each krill fishing vessel typically harvests 250 t day−1 but can harvest up to 800 t day−1 via continuous trawling or 400 t day−1 via conventional trawling (Nicol & Foster, 2016). Thus, while the observed combination of fishing vessels typically harvests ~1000 t day−1, they can extract 2800 t day−1. Field-based estimates of baleen whale prey consumption (Savoca et al., 2021) suggest a fin whale aggregation this size likely consumes 4000 t day−1 of krill. Thus, these fishing vessels may have harvested krill at rates similar to those of the entire supergroup of whales. Industrial krill harvesting in direct competition with foraging fin whales is concerning because the regional abundance of fin whales and krill remains far below the prewhaling baseline. Paradoxically, krill abundance in the Atlantic sector of the Southern Ocean has likely declined since the end of commercial whaling (Atkinson et al., 2004), which may be due to the loss of nutrient recycling services of the missing whales (Savoca et al., 2021; Smetacek, 2008). Fin whales and other mysticetes play a crucial role in enhancing the productivity of the Southern Ocean by catalyzing nutrient availability to the base of marine food webs (Roman et al., 2014; Savoca et al., 2021; Smetacek, 2008). Therefore, threats to whale recovery, survival, or ability to forage optimally can disrupt ecosystem functioning. Prior to krill fishing, whales and phytoplankton blooms were often spatially separated perhaps due to local top-down control of phytoplankton by krill grazing (Hardy, 1967). In contrast, our observation of an overlapping phytoplankton bloom, vast krill swarm, and supergroup of foraging whales raises the possibility that simultaneous harvesting of krill by commercial fishing vessels and a supergroup of whales depressed krill abundance and grazing pressure locally. Such potential changes to ecosystem dynamics from commercial fishing should be closely monitored. Climate change also looms large in the Southwest Atlantic, where sustained surface warming has led to ecological shifts over the past several decades (Whitehouse et al., 2008). The whale aggregation and fishing activity reported here was associated with a large spring phytoplankton bloom (Figure 2A,B,E) following an exceptionally warm austral spring and a near-record low in regional sea ice extent (Figure 2C,D). Although isolated, our observation brings into focus interactions between recovering baleen whales and the intensifying Southern Ocean krill fishery. CCAMLR is mandated to ensure “maintenance of the ecological relationships between harvested, dependent and related populations of Antarctic marine living resources” (CCAMLR Convention Article II.3 b). Regarding the krill fishery specifically, CCAMLR is responsible to ensure that krill predators are not harmed due to fishing. CCAMLR has typically deferred cetacean conservation and management to the International Whaling Commission (IWC), but IWC has no jurisdiction in the management of the krill fishery or bycatch. The potential for industrial-scale krill harvesting to disrupt the positive feedback loop in ocean productivity, yielded by the dynamic interplay between phytoplankton, krill, and whales (Roman et al., 2014; Savoca et al., 2021), warrants further evaluation by CCAMLR and the IWC. CCAMLR can use its management mandate to protect krill-dependent predators, including baleen whales, from adverse interactions with the krill fishery (Meyer et al., 2020). Ultimately, more research is needed to quantify how commercial krill harvesting alongside foraging predators will impact whale and krill recovery in the 21st century. Conor Ryan, Maya Santangelo, and Brent Stephenson made field observations. Conor Ryan, Matthew S. Savoca, Trevor A. Branch, and Earle A. Wilson designed the research, analyzed the data, and wrote the paper. Thanks go to Captain Aaron Wood amd the officers and crew of National Geographic Endurance. The assistance of our colleagues, videographer Eric Wehrmeister in particular, is greatly appreciated. Satellite and Argo profiling float data were collected and made freely available by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project funded by the National Science Foundation, Division of Polar Programs (NSF PLR -1425989 and OPP-1936222), supplemented by NASA, and by the International Argo Program and the National Oceanic and Atmospheric Administration (NOAA) programs that contribute to it. The Argo Program is part of the Global Ocean Observing System. This work was supported by MAC3 Impact Philanthropies. The authors declare no conflict of interest. Processed data and analysis code (Ryan et al., 2023) are available in Zenodo at https://doi.org/10.5281/zenodo.7511123. We use sea ice data from the NOAA/National Snow and Ice Data Center Climate Data Record (CDR) of Passive Microwave SIC (Meier et al., 2022) and sea surface temperature from the NOAA Optimum Interpolation (OI) SST V2 product (Reynolds et al., 2002). The OI SST V2 data were accessed via the NOAA PLS website at https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.html. Appendix S1. Video S1. Video S2. Video S1 legend. Video S2 legend. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.