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Age and body length at sexual maturity of Harbour seals in the Netherlands.         Read, F L1, Pierce, G. J2 & Thijs K3 1&3 Department of Virology, Erasmus Medical Centre, Rotterdam, Netherlands 1&3 Seal Rehabilitation and Research Centre, Pieterburen, Netherlands 2 School of Biological Sciences, University of Aberdeen, Aberdeen, U.K.

Life history studies provide valuable information about the age structure, age at sexual maturity, and longevity of a population. For the East Atlantic harbour seal (Phoca vitulina) population, knowledge of the reproductive status is particularly important because of the large fluctuations in population size resulting from viral epidemics.  Life history samples were collected from 489 harbour seals that died during the 2002 phocine distemper epidemic in the Netherlands. Females were considered sexually mature if the ovaries contained at least one corpus luteum or albicans. Histological analysis of the testes failed to show any presence of spermatozoa, probably due to a combination of postmortem autolysis and freezing artefact. Therefore, based of previous research, males with an average testis weight of 15g were considered to be sexually mature. Age was determined from analysis of growth layer groups in the cementum. Age and length at maturity were predicted using Gompertz and von Bertalanffy growth curves. Total body length (TBL) and age ranged from 74-152 cm and 0-28 years in females and 74-178 cm and 0-21 years in males. The smallest mature female was 107 cm and the smallest mature male 124 cm. Asymptotic values obtained from growth curves were 136cm and 144cm for females and males respectively corresponding to an age of approximately 10 years for both sexes. These results indicate that harbour seals in the Netherlands are smaller than those in the Kattegat-Skagerrak area and Norway. Predicted sexual maturity from asymptotic length is obtained at 118cm (females) and 124cm (males). The present results suggest that female harbour seals in the Netherlands reach sexual maturity smaller and younger than other studied European populations but no different was found for the males.  Additionally, very few old animals were found in the present study due to the 1988 viral epidemic.

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