Effects of alternative control rules on the conflict between a fishery and a threatened sea lion (<i>Phocarctos hookeri)</i>

Abstract

In New Zealand, a fishery for squid (Nototodarus sloanii) incidentally catches a threatened sea lion, Phocarctos hookeri. Bycatch is managed with an annual limit designed to ensure rebuilding of the sea lion population. We explore the conservation and cost effects of the current limit and two simple alternative rules, comparing them with no fishing and unrestricted fishing. We fitted an age-structured Bayesian model to sea lion pup estimates to obtain samples of the joint posterior distribution of parameters; from these we made 100-year simulations with five harvest control rules under six different sets of environmental conditions. The base-case fit suggests that the current sea lion population may be near its carrying capacity, although this may be sensitive to modelling choices. The fishery bycatch constitutes little risk to the sea lion population in the absence of catastrophes and generates small marginal risks when catastrophes are simulated. The current management rule does not minimise the marginal risk of extinction, is much more costly to the fishery than simple alternative rules, and incurs greatest cost when risk is smallest. The model appears to be a good tool for evaluating alternative management strategies against predefined objectives.