Abalone resource decline and a recovery attempt in Chiba Prefecture,Japan

摘要

Landings of abalone (H. discus, H. gigantea, and H. madaka) in Chiba Prefecture, Japan exceeded 700 t in 1966 and fluctuated between 600-800 t until 1978. The landings started to decline in 1979 and reduced to less than 100 t in 1997. Although landing of H. discus has been slightly increased since 2000, that of H. gigantea abalone is still decreasing. We analyzed the data of investigations on decline in abalone landings and have come up with "environment theory", "over-fishing theory" and "disease theory." In the environment theory various factors were considered, such as biomass decrease of purple sea urchin; which is used as a shelter by juvenile abalone, sedimentation flux increase, withering of seashore (Isoyake), oceanic dynamic change, predation damage, coastline development, and endocrine disruptors, of which the sedimentation increase was thought to be the most influential factor. In the over-fishing theory, decrease in the number of reproductive adults due to reinforcement of the fishing intensity was considered to reduce the fertilization success and caused the decline in recruitment. The disease theory was found to be not applicable because virus infection of seeds has not been reported in Chiba Prefecture. To solve the problems related to decrease in adult population number, various research approaches are possible: development of techniques to create suitable reproduction habitat, long term observation of reproductive conditions in a perpetual observation area, improvement of seed transportation and release methods, and development of fishing ground construction technology. We have been conducting research on the effects of leaving the fishing ground fallow for 5 y, and we are also investigating the amount of resources in different developmental stages. The present study summarizes the abalone landing decline factors in Chiba Prefecture and describes the present status of our research.