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Keywords:

  • sperm whale;
  • growth;
  • density dependence;
  • reproductive success

Abstract

Data from the North Pacific sperm whale (Physeter catodon Linnaeus, 1758) fishery were examined for a possible density dependent change in growth during 40 yr harvesting after World War II. Early in this period males from the eastern stock were 16.8 m or less in length. By the early 1970s the largest males in the catch exceeded 16.8 m in length and reached 18.9 m in the late 1970s. The proportion of males measuring over 16.8 m, among sexually mature males (≥14.0 m), increased from 0 to >20% during the 1970s. Increases in the maximum size of males were possibly preceded by a change in the frequency distribution of body lengths in the middle 1960s when only 10% of the postwar catch had been taken. Testis weights suggested an increase in body length at sexual maturity. Two of the three putative North Pacific stocks showed similar growth changes. Adult males taken in the Bering Sea did not show such changes during the exploitation which ended in 1972 because of overfishing. Females showed no detectable change in body size. It is concluded that: (1) density dependent effects on male growth are greater before sexual maturity than after it, (2) males may show density dependent changes even at a population level above 90% of the carrying capacity, (3) polygynous males acquire more mates and realize higher reproductive success because of increased body size, and (4) females appear to maximize production by maturing earlier and shortening calving intervals in response to density change.