Abstract
Populations of many small pelagic fishes are known to fluctuate drastically. Small pelagic fishes off the coast of Japan boom in cycles, a phenomenon known as species replacement. The population of chub mackerel (Scomber japonicus) increased in the 1960s and decreased in the late 1970 s. The population of Japanese sardine (Sardinops melanostistus) increased in the 1980 s, and recently. Anchovy (Engraulis japonicus) has become abundant. The traditional idea of MSY relies on the assumption that stocks will settle into equilibrium if fishing pressure remains constant. The abundance of pelagic fishes, such as sardine or mackerel, changes drastically even under constant fishing pressure. Because of their variability, management of pelagic fishes with species replacement is difficult. In this paper, I investigated the effects of target switching among stocks that experience species replacement. This study revealed that target switching was effective for stocks with replacement, especially when the recruitment fluctuation was large. In some cases, the yield increased by 261% following target switching. Well-designed target switching is useful for multispecies management for non-equilibrium bioresorces.
