Reproductive histories of female humpback whales Megaptera novaeangliae in the North Pacific

Abstract

Reproductive histories of indnldually identified, female humpback whales were documented on both the summering and wintering grounds of an endangered but currently unexploited population. Interbirth or 'calving' intervals of mature females were on average longer and more variable than previously reported, ranging from 1 to at least 5 yr. In Hawaii, multiple sightings of 18 females provided an estimated calving rate (calves [mature female]-' yr-') of 0.58. In southeastern Alaska, multiple sightings of 41 females provided an estimated calving rate of 0.37 The survival of an individual through at least its first year of life was documented in 5 cases. Three of these, first identified as calves in southeastern Alaska, continued to return to this feeding region as juveniles. The possible weaning of a year-old whale was observed in Hawaii, and the apparent death of a calf was documented in southeastern Alaska. We suggest that the estimated calving rate from sightings of females in Hawaii is inflated by sighting biases and that the lower estimate from southeastern Alaska is a better measure of current reproductive rates. A comparison of this estimate with historical estimates of pregnancy rates from whaling records provides no evidence of a marked density-dependent increase in the reproductive rate of humpback whales.