甲殻類の増殖と資源管理に行動生態学を生かす

Abstract

Japanese crab fisheries for king crabs (Paralithodes spp.), snow crab (Chionoecetets opilio), and hair crab (Erimacrus isenbeckii), have declined in recent years. Various fishing regulations have been adopted to increase and maintain the stocks for these fisheries, however they have been ineffective. Here I propose behavioral ecological approaches that might help increase crab stocks through aquaculture and resource management, particularly through seed production at hatcheries and resource management of wild stocks. I present three case examples: 1) the operational sex ratio at crab hatcheries, 2) determination of male maturity size in the wild, and 3) resource management of wild crab stocks. Male crabs guard ripe females for several days before mating. This guarding behavior results in a great physiological cost for males. In such mating systems, a female-biased operational sex ratio, which is often observed in space-limited hatcheries, may reduce the potential reproductive ability of females. A more even operational sex ratio should be maintained to improve reproductive success in crab hatcheries. In the wild, male maturity should be determined based on the functional (behavioral) maturation size (defined as the size at which copulation begins), and size limits for fishing should be based upon on functional maturity. Selective fishing for larger male crabs, which is a common practice in these fisheries, may lead to female biased stocks on the fishing grounds. Removing the functionally mature males may lead to a serious depression in crab productivity by reducing the number of viable males available for copulation with recently molted receptive females. Furthermore, smaller male crabs usually have a lower fertilizing ability due to their small amounts of sperm (sperm limitation). Therefore, fishing regulations in large decapod fisheries should be enacted that ensure effective reproduction in order to realize the potential spawning capacity of unfished females. These examples illustrate that behavior-ecology approaches can be effective in the conservation of crab populations, and should be more frequently applied to crab aquaculture and fisheries.