Research Document - 2009/091

Timing of grey seal pupping on Hay Island

By M.O. Hammill, W.D. Bowen and W. Blanchard

Abstract

The temporal distribution of births on Hay Island was examined using two approaches. One model assumed that births followed a normal distribution and used the change in the proportion of pups in 3 identifiable age-dependent stage classes as the season advances to develop the birthing ogive. The second approach assumes that the birth rate in a year can be adequately described by a continuous function of time. Animals pass through a series of 5 identifiable age-dependent stages of which the duration can be described by a semi-Markov process, i.e., the transition intensities depend only on the current stage and the time so far spent in that stage. The Normal model estimated that 90% of the young of the year on the island are estimated to be weaned by 9 February (se=1.7) and have reached the beater stage by 15 February (se=3.1). This was slightly later than the 5 stage-model which estimated that 90% of the animals had reached the beater stage by 11-12 February. The timing of pupping, weaning, when animals reached the beater stage and when they might disperse were examined together. The available information suggest that a harvest on or shortly after 9 February would encounter few adults still lactating, but a large proportion of beaters would be present and few beaters would  have dispersed from the breeding colony.

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