Author/s Connor Bamford (1), Nat Kelly (2), Helena Herr (3), Elisa Seyboth (4), Jennifer A Jackson (5) (1) British Antarctic Survey, Cambridge, UK (2) Australian Antarctic Division, Kingston, Tasmania, Australia (3) Institute of Marine Ecosystem and Fishery Science, University of Hamburg, Hamburg, Germany (4) Mammal Research Institute Whale Unit, Department of Zoology and Entomology, University of Pretoria, South Africa (5) British Antarctic Survey, Cambridge, UK Brief Overview Baleen whales are predominately summer visitors to Antarctic waters, where they feed on large swarms of zooplankton, primarily Antarctic Krill (Euphausia superba). Over 2 million whales were killed in Southern Hemisphere whaling operations in the 20th Century 1. Many species were brought to the edge of extinction as a result of this commercial exploitation 2,3. The IUCN categorise the main Antarctic baleen whale species: (i) Antarctic blue whales (Balaenoptera musculus intermedia) as Critically Endangered; (ii) fin whales ( physalus) as Vulnerable; (iii) humpback whales (Megaptera novaeangliae) as Least Concern; (iv) southern right whales (Eubalaena australis) as Least Concern, although the sub-population in Chile and Peru is Critically Endangered; (v); sei whales (B. borealis) as Endangered; (vi) Antarctic minke whales (B. bonaerensis) as Near Threatened; (vii) common minke whales (B. acutorostrata) as Unknown; and (viii) pygmy blue whales (B. musculus brevicauda) as Data Deficient. Many Antarctic baleen whale populations are showing signs of recovery 4-6, but ongoing monitoring is essential so trends can be closely tracked because they face threats from climate change and direct anthropogenic impacts. Detailed Overview Eight species of baleen whale feed in Southern Ocean waters, south of the Polar Front, during the austral summer. Of these, three species feed predominantly in lower-latitude waters and visit Antarctic waters in low numbers (pygmy blue, sei and common minke whale), while five (humpback, fin, Antarctic minke, southern right and Antarctic blue whales) rely primarily on food from Antarctic waters for their diet. The most important prey for baleen whales in the Southern Ocean is Antarctic krill 7,8. There is evidence that different whale species have preferences in terms of the characteristics of the krill swarms they feed on and the depths that they feed at 9,10. Whales are most sighted in the Southern Ocean during the austral summer; most animals then migrate to lower latitudes to breed and calve in winter 11,12. However, some remain in the Southern Ocean through the autumn and winter 13,14, possibly related to their age or breeding status (hence we refer here to Southern Hemisphere whale populations). Humpback whale (Megaptera novaeangliae) on the Antarctic Peninsula. Credit: Elisa Seyboth. The past Before the turn of the 19th century, pre-modern whaling was predominately a shore-based activity. Catches were made and landed in coastal waters and processed at shore-based stations 3,15. However, as technologies developed, modern whaling effort shifted into the pelagic realm and previously out-of-reach species (i.e., the faster, larger whales) became a target of the industry 16. Industrialised modern whaling occurred through the twentieth century and caused the collapse of many of the large baleen whale populations in the Southern Hemisphere. The most recent estimates by Rocha, et al. 1 suggest that between 1900 and 1999 a minimum of 363,648 blue whales; 726,461 fin whales; 215,848 humpback whales; 204,589 sei whales; 117,213 minke whales; and 4,452 southern right whales were killed in the Southern Hemisphere. The lower catch of southern right whales during this period was due to populations already being depleted as a result of heavy exploitation through the 18th and 19th centuries 17. This relates to the historic (pre-modern) open-boat pelagic and land-based whaling operated from rowed catcher boats using hand-thrown harpoons 18. In the Southern Hemisphere, the main nations responsible for >90% of the catches were Norway, the USSR, Japan and the United Kingdom 19. Protection from whaling was introduced at different times during the 20th century; southern right whales were protected in 1935; humpback and blue whales protected in the mid-1960s; and fin whales in 1976. Commercial whaling for all species ended in 1986 when the International Whaling Commission’s (IWC) Moratorium came into effect 20. A short period of ‘under-objection’ whaling occurred in 1986-87, which was then followed by ‘special permit’ or ‘scientific whaling’, predominately for Antarctic minke whales by Japan, until 2019. In addition, it is worth mentioning that illegal whaling, primarily by the USSR, resulted in the killing of approximately 48,000 humpback whales; 26,000 sei whales; 32,000 minke whales; 9,000 blue whales; and over 3,000 southern right whales after protection was introduced 21,22. Through the 20th century, whalers hunted the quarry that was available to them. Initially this started with coastal populations of southern right and humpback whales; then once better technology developed, the industry shifted towards pelagic regions, including the Southern Ocean, where whalers targeted whales starting with the largest and proceeding through to the smallest through over the course of the 20th Century (Figure 1). Where population estimates are available, the scale of commercial whaling in the Southern Ocean is evident. Antarctic blue whales were reduced from an estimate pre-exploitation abundance of ~256,000 to 395 animals in the whole Southern Hemisphere 23. Fin whales were hunted from an estimated pre-exploitation abundance of 325,000 whales to 3,250 – 6,500 whales, 1-2% of estimated pre-exploitation abundance 3,24. Humpback whales were hunted from an estimated pre-exploitation abundance of between 111,833 – 197,781 animals down to ~6,582 animals in the Southern Hemisphere 25. Antarctic minke whales were hunted once the more profitable whales became uncommon 26, but catches were less prolific and likely to be less impactful than with the larger species. Populations of southern right whales were diminished prior to the onset of modern whaling with estimates suggesting that only 400 whales remained by 1920 27, and that pre-modern whaling had already reduced their populations from an estimated pre-exploitation population of between 53,466 – 75,882 whales 28.  These pre-exploitation population estimates are accompanied with a degree of uncertainty, which in this case is equivalent to 95% Probability Intervals (PI); i.e., that there is a 95% chance that the true value falls between the reported range.  See footnote *. Figure 1: Reported catch for five baleen whale species south of 40°S displaying the progression of the whaling industry from an initial coastal focus, where humpback whales were opportunistically targeted, through to pelagic operations where whales were targeted in successive size-order (from blue through to minke whales). Data sourced from Leaper, R. et al. (2008). The present Since the cessation of commercial whaling, many Southern Hemisphere whale populations are showing signs of recovery. In particular, humpback whales were estimated to have recovered to ~70% of their pre-exploitation abundance across all breeding stocks in 2015, with a combined estimated abundance of 96,657 (95% PI, 78,041 – 117,527) whales 25. Estimates of annual rates of Southern Hemisphere humpback whale population growth range up to 12% 29, but levels of recovery vary across regions; for example the population in the southwest Atlantic is >93% recovered 5 and the population in eastern Australia is assumed to be recovered, and may in the future exceed their pre-exploitation abundance. This recovery has led to their delisting as a threatened species in Australia 30. However, other populations are recovering more slowly 29. Circumpolar recovery of southern right whales was estimated to be between 20-25% of pre-exploitation abundance in 2009, with a combined estimated abundance of 11,984 whales 27; estimated annual growth rates are ~7%. Antarctic blue whales are still critically Endangered, as the last abundance estimate for this subspecies of 2,280 (in 1998; CV 0.36) was at less than 1% (95% PI 0.7 – 1%) of pre-exploitation numbers 23. There are also encouraging signs of population increase for fin whales, with high local densities around the Antarctic Peninsula, with mass feeding aggregations observed at Elephant Island (Figure 2 31), although contemporary abundance estimates and associated recovery levels are not available. The most recent estimate of Antarctic minke whale abundance was of whales south of 60°S during the Austral summer in 1998 (based on data collected from IWC’s Circumpolar (CP) III surveys between 1991/92 and 2003/04), although according to estimates from the last two circumpolar surveys, there is evidence of a decline in abundance of 30% between the late 1980s and 1990s 32. Figure 2: Aerial view of a fin whale feeding aggregation at Elephant Island, Antarctica. From: Herr et al. (2022). © BBC The future Although some whale populations have begun to recover in the Southern Ocean, the future of these recovering populations is uncertain due to alterations to the ecosystem that are resulting from climate change 4,33,34. Many baleen whales use the Southern Ocean as a seasonal feeding ground, and as such the abundance and spatial predictability of their prey is a crucial component of the recovery of these populations. The temperature of the Southern Ocean is increasing 35-37 and krill biomass, abundance and distributions seem to be shifting as a direct result of global climate change 38-40. There is evidence that regions of the Southern Ocean are undergoing more frequent anomalous weather episodes, such as an increased frequency of positive Southern Annular Mode events, which are associated with warmer water conditions, less sea ice and lower levels of krill recruitment 41-43. This interplay between temperature, sea ice and krill recruitment are concerning, particularly for regions such as South Georgia, which relies on oceanic currents transporting krill into the system to sustain both migratory and resident marine predators. Predictive models, which account for both climatic drivers and prey responses, have suggested that by 2100 baleen whale populations in the Southern Ocean are likely to decrease. If emission trajectories continue as forecast populations of Pacific blue, fin and southern right whales risk extinction 4. Globally, climate change is triggering poleward shifts in the distribution of multiple cetacean species; the consequences of such alterations currently remain uncertain44. As well as the risks posed by climate change, interactions with human activities can also impact recovering whale populations. These interactions are both direct and indirect. Entanglement in active and ghost fishing gear, particularly in coastal regions 45,46 has the potential to impact whale populations, and lethal interactions have been reported in the krill fishery 47. Greater numbers of ships increase the risk of ship strikes. Transit speed limits are an effective way of reducing this risk, and encouragingly are being implemented by some vessels in ‘high traffic’ regions of the Southern Ocean 48. Other threats such as pollution 49,50 and underwater noise 51,52 can also have an impact on population trends. Whilst these threats exist on their own, they frequently do not act alone. Cumulative impacts are particularly concerning, as the combination of multiple stressors are compounded, and we lack an overall understanding of these processes, along with the monitoring tools required to parameterise and predict the true extent of their combined impact 53. Additionally, the potential scale of these impacts becomes even greater if we consider their possible impact over the entire home range of baleen whale populations between the high and mid latitudes. Challenges There are several key challenges for providing conservation management advice regarding baleen whale populations in the Southern Ocean. These are: There is a lack of accurate and up-to-date population assessments for all baleen whale species in the Southern Ocean. To date, assessments have been carried out for humpback 54, southern right 27, Antarctic minke 32 and Antarctic blue whales 55. However, some of these are nearing two decades old, and assessments have also not yet been undertaken for fin or sei whales. As such there is an imperative to update this information. A lack of circumpolar survey effort is hindering the acquisition and acceptance of these much-needed assessments. Our understanding of baleen whale habitat use is biased towards the summer months when weather conditions permit easier research access. Evidence from both satellite telemetry deployments and acoustic observations have suggested that baleen whales, which were previously assumed to be almost wholly seasonal, stay in the Southern Ocean for some or all the winter months 11,13,56. In doing so, these animals are delaying or possibly even avoiding migration altogether; an occurrence which needs to be reflected in the management of the Southern Ocean krill fishery, notably in spatial overlap analyses and consumption estimates to account for the longer residence of whale populations. There is a need for an improved understanding of the potential competition between natural krill predators and krill fisheries. Current evidence suggests that baleen whales are some of the largest consumers of krill in the ecosystem 57, and estimates of their contemporary consumption of krill are factored into modelling to support the Commission for the Conservation of Antarctic Marine Living Resources’ (CCAMLR) management approaches 58,59. Given the potential for an increase in krill fishing pressure 60, alongside oceanic changes associated with climate change, understanding the spatial and temporal interaction between the fishery and krill-dependent whale populations is crucial for ongoing fishery management 61. Conclusion To the best of our knowledge, baleen whales are recovering from near extinction during the 20th century. However, present day populations are returning to a Southern Ocean environment that is facing systemic level changes over the next century. Efforts to mitigate against adverse impacts are beginning to be established in the Southern Ocean, including, but not limited to, two high-seas Marine Protected Areas, vessel speed restrictions and voluntary measures proposed by the Association of Responsible Krill harvesting companies (ARK 61). However, there needs to be a concerted effort to design science so that the results can be used to inform management; and that the resulting management measures are designed with longevity and resilience to withstand current and future climate change. At a global scale, the overwhelming priority is to reduce greenhouse gas emissions to avoid the worst projected climate scenarios, and the impacts these will have over the entire Hemisphere, not just high latitude habitats of the Southern Ocean. Fundamentally, research needs to keep pace, if not stay ahead, of ongoing environmental change. There is a need for systematic multi-season surveys to monitor whale distribution and abundance. These will aid in addressing the current uncertainties regarding circumpolar population recoveries, winter-time habitat use, and will shed light on the impact that recovering whale populations are having on other Antarctic species 62-65. Antarctica is uniquely positioned in that it provides globally important feeding grounds for many species, not only baleen whales, and as such, needs to be safeguarded from both climate change and over exploitation. With more precise and up-to-date data, spatial management efforts, such as High Seas Marine Protected Areas, IWC’s Southern Hemisphere Whale Sanctuaries, or regional IMMAs (Important Marine Mammal Areas) can be designed and situated where they will be most effective. Furthermore, given sufficient data, geopolitical instruments such as the Convention on Migratory Species (CMS) and Intergovernmental Instruments, such as CCAMLR, will be best placed to safeguard this region and the species that call it home.