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

Tissue ¹³C of bowhead whale, Balaena mysticetus, varies seasonally in response to geographic variations in the ¹³C of prey organisms consumed along the annual migratory route between the Bering, Chukchi and Beaufort Seas. Seasonal changes in body ¹³C 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 ¹³C 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 ¹³C 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.