Distribution and population separation of Bryde's whale Balaenoptera edeni off southern Africa

Abstract

A review of available catch and biological data suggests that there are 3 populations of Bryde's whales in the southern African region. An inshore population (the South African Inshore Stock) occurs over the continental shelf of South Africa, south of about 30°S, and seems to be nonmigratory, although there is a movement up the west coast in winter. A pelagic population (the Southeast Atlantic Stock) occurs on the west coast of southern Africa, ranging from equatorial regions to about 34°S, and appears to migrate north in autumn and south in spring. Whales from the Southeast Atlantic Stock differ from the South African Inshore Stock in size, scarring, baleen shape, seasonality of reproduction, fecundity and prey types. Both occurred in the west coast whaling ground off Donkergat, but with differing seasonalities and distributions from the coast. Bryde's whales are rare on the east coast of southern Africa, but are found in summer in some numbers south of Madagascar. Whales from this population are clearly smaller than those from the Southeast Atlantic Stock, but are similar in size to, or even smaller than, those from the South African Inshore Stock. Their external appearance is unknown, but they differ in prey type from the South African Inshore Stock, and because of a clear discontinuity in distribution it is believed that they form a third (pelagic) population (the Southwest Indian Ocean Stock). This population may or may not move north as far as the Seychelles in winter, but seems to be separate from Bryde's whales in the Arabian Sea. From their size composition, length at sexual maturity and infrequent capture, Bryde's whales taken at Durban may have represented strays from either the South African Inshore Stock or the Southwest Indian Ocean Stock, and recorded stomach contents also indicate prey types common to either stock. The unusual degree of population differentiation shown by Bryde's whales may be a consequence of their limited seasonal migrations and apparent resource partitioning.