Life history of the bowhead whale, Balaena mysticetus (Mammalia: Cetacea)

Abstract

This paper synthesizes data collected from bowhead whales (Balaena mysticetus) landed by Alaskan Eskimos between 1973 and 1982. From these data, and from the literature, length at birth has been estimated to be 4‐4.5 m, length at one year to be 8‐2 m, length at sexual maturity to be 14 m in females, and maximum length to be 20 m. Sexual activity, possibly mating, has been observed in March through May and the length of gestation is estimated to be 13 months. The calving period appears to extend from March through August, with the peak in births occurring in May. Lactation may extend for one year. Evidence has been provided for a pregnancy rate (percentage pregnant in mature female catch) of 015 and it has been suggested that calving occurs every three to six years. Gross annual reproductive rate (based on calf sightings) is estimated to be at least 3–6%. Sources of mortality are discussed. The sex ratio of animals taken by Eskimos is 0.83: 1.00 (females to males) and is not significantly different from unity.SummaryThe conclusions drawn in each of the preceding sections are likely to be re‐evaluated as more information becomes available. Still, it is suggested that the length at birth is 4–4.5 m, length at one year is 8.2 m, length at sexual maturity is 14 m in females, and the maximum attainable length is 20 m. Conception is most likely to occur in late winter while the whales congregate in Bering Sea polynyas, whereas the peak in calving appears to occur in May. Consequently, it has been hypothesized that the gestation time is greater than one year, but less than two. By analogy with other right whales and sparse data collected, the duration of lactation is assumed to be one year, although this is arguable. The pregnancy rate of mature females, calculated from ovarian data, is 0.15, projecting a calving interval of approximately seven years. The gross annual reproductive rate generated from field data is 3.6–12.4%. The sex ratio of the harvested animals is 0.83: 1.0, and it is assumed that this does not reflect the whole population. Whatever changes are yet to be made in the understanding of the bow‐head life history, all of the traits delineated are consistent with those predicted for slowly reproducing animals, the quintessential K‐strategist.We are most grateful to the scores of field biologists who have helped us count whales and sample those landed by the Eskimos, and who have contributed ideas and enthusiasm. In addition, we thank R. Tarpley for information on ovaries collected in 1982, J. Breiwick for running several sex ratio estimates, and H. Marsh for her generous assistance with the ovarian histology. Under contract to us R. Davis and W. Koski of L.G.L., environmental research associates, provided the photogrammetric measurements. An earlier, less complete version of our paper was submitted in 1982 as an unpublished report (document SC/34/PS1) to the International Whaling Commission, resulting in many valuable comments from members of the Scientific Committee. P. Best, D. Chapman, and an anonymous reviewer substantially improved the manuscript. Finally, we thank the people of Barrow, Point Hope, Wainwright, Gambell, Savoonga, and Kaktovik for their patience and assistance. Without their support this research would not have been possible.