Does disappearance mean extirpation? The case of right whales off Namibia

Abstract

Right whales off Namibia were severely depleted by early 19th century whaling, and rarely featured in modern whaling catches in the 1920s. Aerial surveys of the Namibian coastline from 1978 and onwards revealed increasing numbers of right whales, but few cow-calf pairs. Aerial surveys off South Africa since 2009 showed a major decline in the availability of animals without calves. Twenty individual matches were made between 94 whales photographed off Namibia/Northern Cape in 2003–2012 and 1,677 photographed off South Africa in 1979–2012. Eight were adult females that calved in South African waters, but only one was also seen with a calf off Namibia. Twelve out of 13 individuals off Namibia with distinctive dorsal pigmentation were first seen as calves off South Africa. These results strongly indicate connectivity between the two regions, while the presence off Namibia of three adult females from the South African population in the season in which they are believed to conceive suggests that there is unlikely to be any genetic differentiation between the two areas. We conclude that the reappearance of right whales off Namibia represents range expansion from South Africa rather than the survival of a few remnants of an originally separate stock.