EVIDENCE FOR A SEX-SEGREGATED MIGRATION IN THE HUMPBACK WHALE (MEGAPTERA-NOVAEANGLIAE)

Existing population models for humpback whales assume that all individ uals within a population under take the annual migration from feeding areas in high latitudes to breeding areas in tropical waters. An exces s of males was recorded in the commercial whaling catches near breedin g areas in the southern hemisphere, but no account of this was taken i n developing population models, because it was believed that this bias was a result of whalers selecting against females with young calves. Here we demonstrate that the sex ratio of migrating humpback whales ne ar a breeding area is highly skewed towards males. A biopsy study carr ied out in 1992 throughout the northward and southward migrations reve aled a sex ratio of 2.4 males:1 female in the population of humpback w hales migrating along the east Australian coast (n = 180). A reanalysi s of the catches made during commercial whaling in this and other area s of the southern hemisphere gave a sex ratio of the same order. The m ost plausible explanation, supported by some evidence, is that some fe males remain in the feeding areas throughout winter. The results repor ted here show that existing management models require major revision t o take account of these findings.