Analysis of Fisheries and Aquaculture

Cyclic variation in the catch of Dungeness crab, Cancer magister, is being investigated with the goal of formulating alternative management policies for the crab fishery in northern California. One possible cause of the cycles is cannibalism. Gut contents of adult crabs collected near Trinidad, California indicated a high incidence of cannibalism on newly settled crabs in April, 1981. During the following months few juvenile crabs were captured in the trawl and the incidence of cannibalism was low. Laboratory analysis of rates of gut clearance following cannibalism were hampered by difficulty in collecting juvenile crabs. Another possible cause of the cycles is predation on eggs by the nemertean worm, Carcinonemertes errans. This egg predator is now as abundant in northern California as in central California. We are continuing to monitor the abundance of this worm and are working with a model of the interaction between the crab and worm populations. We have analyzed changes in catch-per-unit-effort within past fishing seasons to estimate the pre-season abundance of legal size male crabs. The analysis indicates that 60-90% of the crabs were harvested in most years but we identify two years in which the pre-season abundance was much greater than the catch. A manuscript describing this analysis has been prepared. Currently we are using multi-variate statistical techniques to test the hypothesis that the two large year-classes were the result of anomalous environmental conditions. These analyses and some of the current modeling activities are being done on the new PDP ll/03 computer at Bodega Marine Laboratory. An age-specific model of crab population dynamics (Botsford and Wickham, 1978) has been revised to include variation in growth. The cyclic behavior of this more elaborate model is similar to the cycles observed in the fishery. Other topics being studied are the decision-making process by which fishermen switch between the crab, salmon, and albacore fisheries and the effort dynamics within the crab fishery. In a new manuscript we investigate the possibility that changes in price and effort influence the cycles in population abundance though an effect on escapement. The lobster aquaculture model continues to be updated as new biological data and cost projections become available. A monograph on the economics of aquaculture is near completion.