Estimation of population size and stock assessment of FuJvia mutica (Reeve) in Tokyo Bay

Abstract

Population size and catchability coefficient of the cockle Fulvia mutica (Reeve) in Tokyo Bay were estimated using De Lury's method. Then, the present status of the stock was assessed based on the yield per recruitment theory of Beverton and Holt. The population size (N0) and the catchability coefficient (g) were estimated as 39 to 788 ( x 104) in number and 0.58 to 2.85 (× 10~3) per boat-day from 1978 to 1989, respectively. Annual fluctuations of N0 are corresponded with those of catches (C) and CPUE, and it shows that the stock is in a very unstable condition. Exploitation rate (C/N0) is between 0.4 to 0.9 and, in most years, it is over 0.6 and fishing effort is in a high level. Also the exploitation rate apparently increased with decrease of the catches, it has the inclination that the fishing effort gets stronger as the stock abundance is poorer. Isopleth diagram of catch yield per recruit ( Y/R) shows that raising the age at first capture is more effective than increasing fishing effort to achieve the maximum yield per recruit and in keeping the stability of the stock.