CATCH YIELD FLUCTUATION AND POPULATION-ST RUCTURE OF FULVIA-MUTICA (REEVE) IN TOKYO BAY

The present work was carried out on the catch yield fluctuation and po pulation structure of the cockle Fulvia mutica in Tokyo Bay. The fishi ng season of the cockle is from April to early August. Most of the cat ches are recorded in May and June. The CPUE is high in the beginning o f the fishing season and shows a sharp decrease throughout the fishing season. It seems that the CPUE is a good index of the population size of Fulvia mutica. The catches are divided into six market size catego ries after landing. The two smallest market size categories account fo r most of the catches, and their percentages in the catch increase dur ing the fishing season. However, shell lengths of the. six market size categories are between about 43 to 70 mm, and the difference in categ ories does not mean that in year class. More than 50 to 70 percent of the total catches are from 50 to 60 mm in shell length, which correspo nds to just one year in age. It is considered that the population of t he cockle is dominated by one year class. Simplicity of age compositio ns in the population was supposed to be one of the important factors c ausing the drastic fluctuation in the catches in Tokyo Bay.