Spatial and long-term trends in the strandings of humpback whales in Brazil breeding ground
Marcondes, M.C.C.; Barreto, A.S.; Colosio, A.C.; Kolesniskovas, C.K.M.; Ramos, H.G.C.; Secchi, E. ; Seyboth, E. ; Wedekin, L.L.
The humpback whale (Megaptera novaeangliae) population that breeds in Brazil (Breeding Stock A - BSA) is recovering from depletion caused by whaling. This stock was heavily hunted in the last century, reaching an estimated low abundance of 450 in mid-1950s (Zerbini et al., 2019). Recently, the population reached growth rates of 12% per year (Wedekin et al., 2017), having increased from 3,396 (CV = 14.15%; CI 95% = 2562-4501) in 2002 (Andriolo et al., 2010) to 24,900 (95% PI = 22,400?27,000) in 2019 (Zerbini et al., 2019). Humpback whales distribute across the whole Brazilian coast, from the temperate southern coast to the northeast and northern tropical coasts. However, the main breeding concentration of humpback whales is the Abrolhos Bank (16? 40? to 19?30? S, 37?25? to 39?45?W), an extension of the continental shelf located in the south of the state of Bahia and the north of Esp?rito Santo state with an area of 882 km? (Andriolo et al., 2010; Martins et.al., 2013). The humpback whales are found in this region from July to November for the breeding season, with the peak of the season between late August and early September (Morete et al., 2008). Whales usually leave from the Brazilian coast from late October to late December between 20? and 25?S, gradually moving away from the South American coast as they travel towards high latitudes to the feeding area around the South Sandwich Islands (Zerbini et al., 2011). Long term datasets are invaluable to study the population dynamics of endangered populations and their recovery, such as is the case of humpback whale populations. The humpback whales that breed in Brazil recovered from the whaling depletion and now are closer to an equilibrium state, where fluctuations may be governed by climate and availability of their main prey (krill) in the Southern Ocean. Studying the mortality spatial and temporal patterns, and what age classes are more affected, may help to understand this species demography (and a possible transition to a lower growth phase) and its vulnerability to modern human threats. In this paper we present data on stranding records of humpback whales in Brazil during 20 years (2002 to 2021), correlate these data with population growth and discuss on possible causes for the unusually high mortalities, especially that occurred in 2021.