Population Structure of Humpback Whales from Their Breeding Grounds in the South Atlantic and Indian Oceans

Abstract

Although humpback whales are among the best-studied of the large whales, population boundaries in the Southern Hemisphere (SH) have remained largely untested. We assess population structure of SH humpback whales using 1,527 samples collected from whales at fourteen sampling sites within the Southwestern and Southeastern Atlantic, the Southwestern Indian Ocean, and Northern Indian Ocean (Breeding Stocks A, B, C and X, respectively). Evaluation of mtDNA population structure and migration rates was carried out under different statistical frameworks. Using all genetic evidence, the results suggest significant degrees of population structure between all ocean basins, with the Southwestern and Northern Indian Ocean most differentiated from each other. Effective migration rates were highest between the Southeastern Atlantic and the Southwestern Indian Ocean, followed by rates within the Southeastern Atlantic, and the lowest between the Southwestern and Northern Indian Ocean. At finer scales, very low gene flow was detected between the two neighbouring sub-regions in the Southeastern Atlantic, compared to high gene flow for whales within the Southwestern Indian Ocean. Our genetic results support the current management designations proposed by the International Whaling Commission of Breeding Stocks A, B, C, and X as four strongly structured populations. The population structure patterns found in this study are likely to have been influenced by a combination of long-term maternally directed fidelity of migratory destinations, along with other ecological and oceanographic features in the region.

A quarter of a world away: female humpback whale moves 10 000 km between breeding areas

“…Previously documented movement between humpback whale breeding grounds was made by males [14,15,29]. Using genetic signals, males in the Indian and South Atlantic Oceans had higher evidence of movement between breeding stocks [30], and the only other whale to have been identified in both of these oceans was a male [29]. Instances of extreme long-distance movement may represent exploration of a new habitat or a navigational miscue, and the observation presented here could be explained by either of these plausible explanations.…”

“…Alternatively, even in populations where there is strong within-group selection for philopatry to breeding grounds, between-group selection may favour emigrants [2], providing incentive for occasional exploration of new breeding habitats. Whatever brings them to these new breeding areas, evidence from genetics suggests that interbreeding has occurred between animals from the western and eastern South Atlantic, and between those from western Africa and Madagascar [30] showing that at least some long-distance movements lead to successful reproduction.…”

First circumglobal assessment of Southern Hemisphere humpback whale mitochondrial genetic variation and implications for management

“…BSB1) substructure. Two substocks, BSB1, which breeds off Gabon, and BSB2, which seasonally feeds off and migrates past western South Africa (and which shows lower than expected direct connectivity with, and some genetic differentiation from, BSB1 based on the mtDNA control region and 10 microsatellites), are recognized (Rosenbaum et al 2009, Carvalho et al 2014, Kershaw et al 2017. Fine-scale temporal genetic substructure has been observed within BSB, consistent with the temporal segregation of migrating whales on the basis of age, sex, and reproductive status (Carvalho et al 2014).…”

“…Such variation may influence the interpretation of the number of demographically distinct populations due to temporal sampling effects (Carvalho et al 2014). BSC is currently divided by the IWC into 4 substocks (BSC1−BSC4), although evidence of considerable migrant exchange between 3 of the substocks has emerged based on mtDNA control region and microsatellite data as well as satellite telemetry (Rosenbaum et al 2009, Ersts et al 2011, Fossette et al 2014, Kershaw et al 2017, suggesting relatively fluid exchange between the Mascarene region (BSC4), the Madagascar ridge (BSC3), and the Mozambique channel (BSC2). Gene flow between BSB and BSC is also apparent (Best et al 1998, Pomilla & Rosenbaum 2005, Rosenbaum et al 2009, Kershaw et al 2017, and further sampling is needed to explore the possibility of a westward directional bias (i.e.…”

“…Finally, the ASHW population is considered to be small and extremely isolated and shows the greatest genetic differentiation from all the other Southern Hemisphere breeding stocks based on mtDNA control region sequences and microsatellite data (Rosenbaum et al 2009, Pomilla & Amaral et al 2014.…”