Bowhead whale (Balaena mysticetus) growth and feeding as estimated by δ13C techniques

Tissue δ13C of bowhead whale, Balaena mysticetus, varies seasonally in response to geographic variations in the δ13C of prey organisms consumed along the annual migratory route between the Bering, Chukchi and Beaufort Seas. Seasonal changes in body δ13C of whales taken in 1986 from Alaskan waters provide a means of estimating energy intake from the different habitats in which the whales feed. Adult bowheads do not show significant seasonal shifts in the δ13C of muscle and visceral fat. Their enrichment in 13C relative to subadult whales suggests that they acquire most of their food from fall and winter feeding or from unsampled parts of the summer range where zooplankton are enriched in 13C. Young individuals however, undergo marked seasonal shifts indicating that they feed heavily both in the summer and autumn or winter. Oscillations in δ13C along the length of the baleen also provide a measure of age. Baleen growth rates decline as the whales age and provide a means of correcting for wear loss and allow aging of individuals less than ∼ 12 yr old. Although body length is a poor indicator of age in young bowhead whales, baleen length is closely correlated. Growth rates of bowheads after Year 1 are slow (∼ 0.4m yr-1) and up to 20 yr are required to reach the assumed length of sexual maturity at 13 to 14m.

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