We have used data from 11 cohorts of female New Zealand sea lions on the Enderby Island rookery, tagged between 1987 and 2003 and observed between 1999-2000 and 2006-07, to estimate pupping rate.
In our previous criterion-based model it was assumed that all breeding animals are seen and positively identified as breeders during a season. However because most individuals’ breeding states cannot be categorically determined we developed a mixture model in which probabilities of breeding or not breeding are assigned and corresponding likelihoods are added.
Pupping rate was estimated as a domed function of age determined by five parameters. The maximum rate was reduced by a factor of 0.33 if a cow had not bred in the previous season. The population mean reached its maximum of 0.49 y-1 at age 12 years. Without the model our preliminary pupping rate calculation under-estimated breeders because not all of them produce breeder observations but also under-estimated live animals because some animals survive beyond their last sighting. The model estimate shows that the preliminary calculations tend to slightly over-estimate pupping rate.
First year survival varied considerably amongst cohorts. That for the 1998 cohort, which was affected by an epizootic, was estimated to be only 0.24 y-1, implying that considerable mortality occurred after the first two months, for which Chilvers et al. (2007) estimated survival of 0.58 y-1. The breeding rate of this cohort was also shown to be considerably lower than the estimated population mean.
Our pupping rate estimate suggests that sea lions may be less productive than previously thought and implies a lower maximum population growth rate. If our pupping rate estimate is used in a population model it should be done so that other parameters, especially population size, are estimated consistently with it.
This is the final report for this project.
Published by the Department of Conservation.
Report prepared for the Conservation Services Programme, Department of
Conservation by D.J. Gilbert and B.L. Chilvers.
NIWA Client Report: WLG2008 -77. November 2008. NIWA Project: DOC08302.