Original Articles

The effects of future consumption by the Cape fur seal on catches and catch rates of the Cape hakes. 3. Modelling the dynamics of the Cape fur seal Arctocephalus pusillus pusillus


Abstract

The current status of and future trends for the Cape fur seal Arctocephalus pusillus pusillus population are assessed using information from aerial counts of pup abundance. The model used in the analyses is age- and sex-structured, and assumes density-dependence in the survival rate of pups. Uncertainty is assessed by conducting sensitivity tests and estimating 90% confidence intervals by the percentile method. The results confirm the increase of the seal population since the introduction of legal controls at the end of the 19th century, and the estimate of 1,7 million animals of Age 1 and older at the start of 1993 is relatively robust to model assumptions and values for biological parameters. The food consumption by the seal population is estimated to have almost doubled between 1973 and 1993, and to be roughly 2 million tons annually at present. Future trajectories of seal numbers and annual consumption of food cannot be determined with great precision and are sensitive to the values assumed for the post-first-year survival rate and the degree of compensation in density-dependence. Depending on such assumptions, the seal population could already have reached its pre-exploitation equilibrium level or still be well below this level. Data from the 1993 aerial survey have considerable influence on the model results, and particularly on inferences concerning whether the population is now approaching its pre-exploitation level. This suggests that further surveys should be conducted to ascertain the reliability of the changed trend in pup abundance indicated by that 1993 count.

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