The Southern Ocean Exchange: porous boundaries between humpback whale breeding populations in southern polar waters

Humpback whales (Megaptera novaeangliae) are a cosmopolitan species and perform long annual migrations between low-latitude breeding areas and high-latitude feeding areas. Their breeding populations appear to be spatially and genetically segregated due to long-term, maternally inherited fidelity to natal breeding areas. In the Southern Hemisphere, some humpback whale breeding populations mix in Southern Ocean waters in summer, but very little movement between Pacific and Atlantic waters has been identified to date, suggesting these waters constituted an oceanic boundary between genetically distinct populations. Here, we present new evidence of summer co-occurrence in the West Antarctic Peninsula feeding area of two recovering humpback whale breeding populations from the Atlantic (Brazil) and Pacific (Central and South America). As humpback whale populations recover, observations like this point to the need to revise our perceptions of boundaries between stocks, particularly on high latitude feeding grounds. We suggest that this “Southern Ocean Exchange” may become more frequent as populations recover from commercial whaling and climate change modifies environmental dynamics and humpback whale prey availability.

Connectivity among breeding stocks in the Southern Hemisphere. Analyses of mitochondrial DNA (mtDNA) and microsatellite loci from whales feeding in nearshore waters of the WAP determined that the natal areas of most whales are situated off Colombia (BSG) with > 94% of samples assigned to this breeding area, a few are associated with breeding grounds off French Polynesia and Samoa (breeding stock 'F'), yet no samples were genetically assigned to Brazil (BSA) 20 . Analysis of population connectivity between Colombia, Brazil, and the WAP using microsatellite loci also showed strong restrictions to gene flow between Brazil (BSA) and Colombia (BSG), with only ~ 1-2 animals estimated to migrate between the two breeding stocks each year 33 . In a broader comparison of all Southern Hemisphere breeding stocks, the level of maternally inherited gene flow was also lowest between breeding stocks A and G, reflecting strong and significant mtDNA differentiation between the two sites 12 and hinting that there may be long-term, natural barriers to oceanic mixing between the Atlantic and Pacific breeding stocks.
Photo-identification efforts in the WAP feeding area (between 1997 and 2003, 375 individuals photo-identified) resulted in no matches to whales in the Brazilian breeding area (between 1989 and 2000, 983 individuals photo-identified) 34 . The photo-identification catalogue built with images from the WAP is large and goes back to 1986. Satellite tracking of humpback whales in the WAP has shown some movement into the South Atlantic, but no further east than 50° W to date 35 . Nonetheless, an Ecuadorian whale has been photo-identified and re-sighted in the South Orkney Islands (45-46° W), consistent with the hypothesis that the BSG feeding range boundary may be in the South Orkney islands region 24 . Humpback whale sightings and historical catches also suggest a hiatus in distribution between the South Orkneys region and the South Atlantic feeding areas in the northern and eastern Scotia Arc 36 . Altogether, these catches, sightings, individual movements, and molecular evidence suggest that some level of segregation occurs not only between the breeding areas but also between the feeding areas associated with BSA and BSG 33,37 .
Despite the genetic and photo-identification patterns suggesting low population connectivity between oceans, photo-identified sightings of individual animals show that humpbacks can move between breeding grounds and migrate into different oceans from their natal breeding areas. Some examples of such breeding area interchange among populations in different ocean basins include: (1) a male identified by skin biopsy samples who transited from the Indian Ocean to the South Atlantic Ocean 38 , (2) a photo-identified female who moved from Brazil (BSA) to Madagascar (BSC) 39 , (3) a female photographed off Ecuador (BSG) who was later photo-identified off Brazil 40 and most recently (4) a whale photographed off Ecuador (BSG) who was later photo-identified off Brazil 41 . Previous photo-identifications also show that whales from BSG can feed in the same areas as whales from BSA; a single whale from BSG has been photographed both in Ecuador (2131 individuals photo-identified) and the South Sandwich Islands (36° W, 23 individuals photo-identified) 42 . These cases involve movements by individuals either between different Southern Hemisphere breeding areas, or into adjacent feeding areas that are believed to be used by a different population. www.nature.com/scientificreports/ , and feeding areas off the Antarctic continent (c). Putative feeding grounds for BSA are shown on the bottom map (c) in a shade of green [25][26][27][28][29] and for BSG in a shade of orange 24 . Colored symbols (circle for whales from BSA and triangles for whales from BSG) indicate where individuals were photo-identified. The green line within the BSG putative feeding area delimits where BSA whales are feeding in the WAP (this study) and the orange line indicates previous information of a BSG whale feeding on the putative BSA feeding area 42  www.nature.com/scientificreports/ The most recent case noted above was identified using a user-friendly platform that allows citizens to upload whale photographs and have their records compared to others from a large database-Happywhale 43 . Happywhale identified an opportunity to quickly expand data coverage globally by citizen science. Democratizing science through public participation and engagement brings benefits that include low cost data acquisition and increased collaboration network 44,45 while promoting universal and equitable access to scientific data and information 46 .
Here we further explore the Happywhale database to investigate where individuals co-occur at high latitudes in the Southern Hemisphere between the genetically distinct breeding stocks, mainly if individuals from Brazil (BSA) are occuring in the assumed feeding area of whales from the Pacific Central and South America (BSG).

Results
Here we present evidence from fourteen individuals from Brazil (BSA, n = 12) and Pacific Central and South America (BSG, n = 2) matched to putative feeding areas (Table 1, Figs. 1, 2) of BSG (Fig. 1c, showed in a shade of orange) and BSA (Fig. 1c showed in a shade of green). Happywhale's current dataset includes 4302 identified humpback whales from the WAP. Of these, 558 match to the west coasts of Central and South America (BSG), 12 match to Oceania (BSF) (T. Cheeseman, unpublished data), and six to Brazil (BSA; this study, Fig. 1 and Table 1). Two individuals from Brazil were matched to Shag Rocks, two to South Georgia, and two to the South Sandwich Islands, which are all known feeding areas for Brazilian whales, confirming the putative feeding area for the Brazilian population. But six other matches from Brazil were made with the WAP, which is the feeding area attributed to whales breeding in Pacific Central and South America (Figs. 1, 2). Additionally, two whales from Pacific Central and South America were matched at the eastern edge of the putative feeding area for BSG (South Orkney Islands). One of these two individuals (PAN-1055) was first sighted in 2007 in Central America (Panama, BSG), five years later was sighted in Colombia (Gorgona Island, BSG), matched to the WAP (Hovgaard Island) feeding area during the austral winter of 2019 and then, less than a year later, was found again in Antarctica off Signy Island, South Orkney Islands. This individual moved across the known range of BSG feeding areas within six months and may not have migrated during this time to low latitude waters (identified in Antarctica in July 2019 and in January 2020).
Photoidentification and humpback whale sexual maturity age. These photo-identification records also revealed additional information about age of sexual maturity. For instance, the female named Nina was first photo-identified in her birth year in Abrolhos, Brazil, and then photo-identified pregnant 11 years later along the WAP (pregnancy determined by hormone analysis of a skin biopsy sample 47 ). This time gap coincides with the mean age at first calving of humpback whales in Southeastern Alaska (11.8 years) 48 . However in this case we have no data to determine whether parturition was successful at the breeding area, nor whether this was the first pregnancy for this animal.

Discussion
We found new evidence of summer co-occurrence of two recovering humpback whale breeding populations from the Atlantic (Brazil, BSA) and Pacific (Central and South America, BSG) in the West Antarctic Peninsula feeding area. The temporal pattern of resightings of BSA individuals (older sightings in South Georgia and South Sandwich Islands and more recent sightings off the WAP) suggests BSA whales may be making more extended feeding area movements in recent years. Photo-identifications collected from South Georgia and South Sandwich Islands (Table 1) add to the previous evidence that this area is a known feeding area for whales belonging to BSA. For example, the oldest record of an individual photo-identified at Abrolhos Bank, Brazil in 1989 and matched to its feeding area was for whale IBJ-0261 (Table 1). This individual was photo-identified almost 30 years later in the South Sandwich Islands in 2019, the feeding area for BSA. Photo-identifications available in Happywhale from the South Georgia and South Sandwich Islands mostly come from recent efforts and presently include only 322 identified individuals, 7% of the WAP sample size, thus this effort offers a limited sighting history for photo-identification matching, perhaps explaining the lower number of matches compared to the matches seen between the WAP and wintering grounds in Pacific Central and South America.
Available photo-identification data from the South Orkney Islands is also limited in the Happywhale dataset, but of 17 individually identified humpback whales, two individuals (PAN-1055 and PAN-1834) have been resighted on BSG breeding areas off Ecuador, Colombia, and Panama (Table 1, Figs. 1, 2). Notably, individual PAN-1055 may have migrated north late (or not at all) during the 2019 winter providing information on recent unusual movements around southern polar waters. The breeding destination of the other 15 identified whales in the South Orkney Islands remains to be determined.
Despite differences in photo-identification effort among breeding areas, this study reveals the first evidence of whales moving from Brazil into BSG feeding areas, reinforcing the proposed "Southern Ocean Exchange". As such, the WAP feeding area is used by two different breeding stocks, BSG and BSA, which were previously strongly differentiated, and now co-occur in polar waters. Increasing photo-identification efforts in this region should provide further evidence of this assessment. The number of individuals that migrate between these two breeding stocks has been estimated to be 1-1.5 whales per season 33 . Two BSA individuals (named Nina and Raulzito Oleg) were photo-identified off the WAP, crossing the boundary between feeding areas attributed to BSA and BSG in the same period (January 2016, austral summer) and relatively close together (Fig. 1c). This is strong evidence that at least in recent years the use of BSG feeding areas by BSA whales is not a rare event.
While humpback whales are nearly global in their distribution, there is strong evidence that the equator represents a barrier to movement of this species 9  www.nature.com/scientificreports/ Atlantic), suggesting that gene flow is greater across the Southern Hemisphere oceans than across the equator 9,50 . This has led to a proposal to separate humpback whales into North Atlantic, North Pacific, and Southern Hemisphere subspecies 9,50 . Levels of genetic differentiation are weaker among Southern Hemisphere breeding stocks 12,13 but sufficiently strong that they have been considered demographically independent populations, with population assessments of recovery from exploitation carried out at the level of each breeding stock 51 . Within the Southern Hemisphere, inter-population genetic differentiation may be anticipated to decrease, as populations recover from whaling and migratory movements between populations increase. If the crossing of individuals from BSA into BSG feeding areas is a recent event, likely motivated by population recovery, the observations presented in this study support this hypothesis.
Mixing of Southern Hemisphere humpback whales on high latitude feeding areas (the Southern Ocean Exchange), sometimes leading to movements between breeding stocks, might be influenced by geographic features such as landmasses, and oceanographic barriers such as major ocean currents that restrict movements. Additionally, El Niño and La Niña climatic events influence the distribution of prey and therefore may potentially influence the migratory destination and timing of migration 41 . The observed long-range movements between Southern Hemisphere oceans may also result from increasing population size 52 and expanding distribution range, as whales colonize new areas and reoccupy historical breeding and feeding areas 5,53,54 .
Unique features that determine oceanographic and ecological characteristics of the Southern Ocean, such as climate and sea-ice extent variability that affect krill productivity and distribution dynamics, can also influence individual decision-making about migratory paths to these feeding destinations 55 . Observed shifts in migration timing of humpback whales from Antarctic feeding areas to earlier arrival at breeding areas in the Eastern Tropical Pacific off Colombia are also believed to be associated with population growth (e.g., high pregnancy rates detected in WAP 56 ) and reductions in sea ice coverage and pack mass during the austral autumn, which, in turn, should influence krill availability in Antarctica 57 . When the autumn ice sheet has a larger mass, more krill can find food in winter 58,59 and during the following summer, there are more prey available for whales. In years with larger autumn ice sheets, humpback whales arrive later to Colombian waters the following winter. Prey availability is probably used by whales as a cue to time their migration to wintering areas 57 .
Krill availability along the WAP correlates with reproductive rates in BSG whales, suggesting the importance of krill to the dynamics of this population 60 . Although krill abundance in the southwest Atlantic sector is one of the largest in the Southern Ocean, krill density in South Georgia is declining 61,62 and show interannual fluctuation linked to climate variability 63 , with population-level impacts on other krill predators using South Georgia waters 64 . High krill density values (> 30 g m −2 ) are interspersed with years with low density (< 30 g m −2 ) with fluctuations every 4-5 years 65 . The relationship between BSA whale population growth (12% per year 66 ) and krill availability is yet to be established but is assumed to be similar to that found for BSG. Increasing fluctuations in krill availability in the northern Scotia Arc may also be forcing humpback whales to venture further from their traditional feeding areas in order to feed. Melting sea ice around the WAP may open up new habitats and modulate krill densities and distributions across the Peninsula and into the Scotia Arc 67 which, in turn, may have a dampening effect on whale population growth while increasing the likelihood that humpback whales from different populations will venture into areas that were not traditionally utilized for feeding in that part of the ocean.
Behavioural traits such as humpback whale song and its evolution and transmission are also facilitated by the Southern Ocean Exchange. In humpback whales, songs are produced by males 68,69 . Molecular data also suggests most movements between breeding grounds, and associated gene flow, may be male-driven 33 . Male carriers of a particular song may therefore introduce it to another population. Cultural transmission of song patterns by males has been shown to occur between west and east coasts of Australia 70 and across the South Pacific Ocean 15 . The lower site fidelity of males therefore facilitates song sharing among breeding grounds in the Southern Hemisphere 16,71 . The co-occurrence of whales from different populations could also contribute to the spread of infectious diseases. In seabird populations, extensive winter mixing has been suggested as an important factor in disease transmission 72 . For example, morbillivirus was recently identified in the blow of humpback whales in Brazil 73 and it could be introduced to other breeding areas through whales that migrate away from their natal breeding and feeding grounds.
Here we have considered how population growth, climate change and oceanic productivity may be acting synergistically to increase the porosity of boundaries among feeding areas of different humpback whale breeding stocks. Underscoring the Southern Ocean Exchange hypothesis of porous Southern Ocean boundaries for humpbacks are the international collaborations among academics and citizens which have made these observations possible. Traditionally, fluke comparisons between regions were very time-consuming as they were carried out manually by researchers visually inspecting all images 74 . New algorithms and tools for automated fluke matching, such as the one developed and implemented by Happywhale 43 have enormously improved matching rates and also facilitate the detection of low-frequency movements. Photo-identification sample sizes used in humpback whale stock comparisons jumped 50-fold when citizens started to participate in biodiversity monitoring, facilitated by public campaigns, tourism, and the development of user-friendly platforms to engage and receive feedback on their contributions. In ours and other cases, these contributions add great complementary value to dedicated scientific research 75,76 . Global citizen engagement informs, at a much faster pace, stakeholders' actions towards conservation of marine life. The relative cost of citizen science compared to more academic approaches in wildlife monitoring may change how conservation science is made and perceived by society in the near future.

Methods
Digital images from research collaborator and citizen science contributors of the ventral surfaces of humpback whale tails (flukes) photographed in Central and South American breeding areas (~ 6476 photo-identified individuals in BSA and 3060 photo-identified in BSG) and feeding areas near South Georgia and the South Sandwich www.nature.com/scientificreports/