Abstract
Growth and reproduction in grey seals (Halichoerus grypus Fabricius, 1791) from Iceland were examined. The oldest Icelandic grey seals obtained were a 36 year old female and a 23 year old male. The longest animals were a 255 cm 13 year old male, and a 230 cm 20 year old female. The heaviest grey seal was an 11 year old male weighing 310 kg. The heaviest female was a 20 year old female that weighed 240 kg. Females reached an asymptotic standard length and weight of 200 (95% CI 196 - 204) cm and 164 (95% CI 157 - 171) kg. Males attained an asymptotic standard length of 243 (95% CI 232 - 254) cm and mass of 279 (95% CI 254 - 306) kg. Investigations of the ovaries and testes indicate that, by the time that females were seven years old, over 90% were pregnant. The average age of sexual maturity of females was 4.0 years (95% CI 3.59 - 4.41) and the average age of first pupping was 5.3 (95% CI 4.95 - 5.72). Average age of sexual maturity for males was 4.9 (95% CI 4.43 - 5.40). Seven out of 8 grey seal males had fully developed testes at the age of 7. All males, 8 years of age and older were mature. Adult (5+ years) females and males are fattest in the summer right before breeding in the autumn, but leanest in the winter after breeding and mating, and in the spring after moulting.
Highlights
The grey seal (Halichoerus grypus Fabricius, 1791) is a large phocid, with males attaining over 300 kg in weight
The description Phoca grypus in 1791 by Fabricius is likely based on Ólafssons (Bonner 1981). Thienemann later described it as a species nova, in 1839, but Jónas Hallgrímsson showed that Thieneman was in error and that the Icelandic grey seal was P. grypus (Jónsson 1988)
The average age of sexual maturity of females was 4.0 years and average age of first pregnancy was 5.3
Summary
The grey seal (Halichoerus grypus Fabricius, 1791) is a large phocid, with males attaining over 300 kg in weight. In the early 1980’s, people in the Icelandic fishing industry were concerned about the possible detrimental effects of a growing unexploited grey seal stock on fisheries, both direct through predation and indirect through their role as the final host of the sealworm (Pseudoterranova decipiens) in Icelandic coastal waters (Ólafsdóttir 2001). Reproduction data for grey seals is available, from Norwegian waters (Wiig 1991), the Northwest Atlantic (Mansfield 1978, Hammill and Gosselin 1995), and from the British Isles and Faroe Islands (Bonner 1981, Boyd 1985). This paper examines recent information on growth and maturity of grey seals in Icelandic waters
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