Growth, physical maturity, and mortality of fin whales (Balaenoptera physalus) inhabiting the temperate waters of the northeast Atlantic

Abstract

Data obtained from the examination of the ear plugs of 665 specimens were used to estimate age-related parameters for the fin whale population inhabiting the temperate waters of the northeastern Atlantic. Samples were collected from 1979 to 1984 at the Spanish whaling stations. The length of the ear plug core strongly correlated with the number of growth layers in whales younger than 25 years of age but showed no correlation in older whales. Readability of ear plug growth layers decreased with age in females but showed no clear trends in males. Minimum size limits of the fishery produced a selective fishing mortality rate in whales up to 5 years of age. Hence, only age-classes older than 5 years have been included in the fitting of the growth curves. Fully grown females are 1.47 m longer than males, but their growth rate is slower. Thus, males reach 95% of asymptotic length at age 9 years, while females reach it at age 13. Fin whales of both sexes attain physical maturity when they are between 20 and 30 years old. Despite growing relatively slower, females are always larger than males and recruitment to the fishery occurs at an age of 5 years in females and 6 years in males. Apparent mortality rates are similar in young whales of both sexes, but are higher in adult females than in adult males; the most plausible explanation for this difference is an increase in mortality associated with reproduction in sexually mature females.