Trends in age-at-maturity and growth parameters of female Northeast Atlantic harp seals, Pagophilus groenlandicus (Erxleben, 1777)

Abstract

AbstractWe analyzed and compared trends in age-at-maturity and body growth in the Greenland Sea and Barents Sea stocks of harp seals, Pagophilus groenlandicus, from the early 1960s to the early 1990s. Mean and median age at sexual maturity (MAMPM and MdAM) were estimated from Richards curves fit to age-specific proportions mature. No long-term trends were found in the Greenland Sea seals, where a common value of MAMPM (5.6 years) and MdAM (4.8 years) could be fit to samples from 1959 through 1990. There were also no significant changes in length-at-age of molting females between 1964 and 1987. For Barents Sea harp seals, MAMPM increased significantly from 5.4 years in the period 1962–1972 to 6.6 years in 1976–1985 and 8.2 years in 1988–1993, concurrently with a decline in body growth rates. Tests on MdAM also showed an increasing trend, but the grouping of samples was slightly different. Estimates of MAMPM for the Barents Sea stock were similar to previously published back-calculated values of MAM, but simulations showed that this method is sensitive to the age distribution of the sample, thus complicating comparisons between samples with different age structures. The high values of MAMPM and low growth rates in the Barents Sea stock in the late 1980s to early 1990s coincided with severe depletion of important prey species in the Barents Sea, reports of mass invasions of harp seals along the Norwegian coast and indications of reduced body condition. All these are consistent with a hypothesis of reduced per-capita resource levels within the distribution area of Barents Sea harp seals at that time, but no cause-and-effect relationship for the long-term trend in age-at-maturity can be established.