Optimal School Selection and Abundance Index in Purse Seine Fishery.

In purse seine fisheries which target pelagic fish in the daytime, the catch per shot represents the weight of the school, because schools are caught in one shot. Under the constraint of limited working time, purse seine fishery does target large schools to maximize the benefit. Because of this selectivity, the catch per shot is not proportional to the population abundance. In this paper we clarify the relationships between the catch per shot, the number of shots, the total catch, and the population abundance, assuming the ideal behaviour of fishermen maximizing the total catch. According to the result of a numerical example, the relationship between the catch and the population is a concave function. Thus the responses of total catch weight or catch per shot to changes in population abundance are not sensitive unless the population is highly depleted. In this situation, there is some risk that the detection of changes in population may be delayed when the change is observed only in terms of the catch per shot or the total catch.