Grey seal (<i>Halichoerus grypus</i>) from the Northwest Atlantic: female reproductive rates, age at first birth, and age of maturity in males

Abstract

Mean age at first birth and reproductive rates were examined in a sample of 526 female grey seal (Halichoerus grypus) reproductive tracts collected between 1968 and 1992. Age of first birth determined from reproductive tracts with a fetus was 5.5 ± 0.12 yr (mean ± SD). No trend over time was observed in mean age at first birth or in pregnancy rates. Pregnancy rates determined from reproductive tracts containing a fetus were 0.18, 0.86, and 0.88 for animals aged 4+, 5+, and &gt;6+ yr, respectively. Pregnancy rates calculated from the presence of a corpus luteum were 0.01, 0.45, 0.9, and 1 for ages 3+, 4+, 5+, and &gt;6+, respectively. Mean age of sexual maturity of males was 5.6 yr as estimated from changes in testes mass in a sample of 89 seals collected during August–September 1992. A marked increase in testes mass was observed among animals aged 3+ yr. By age 7+ yr, virtually all males had attained sexual maturity.