AGE-DETERMINATION AND GROWTH ANALYSIS BAS ED ON SIZE-FREQUENCY HISTOGRAMS OF WHELK NEPTUNEA-ARTHRITICA IN SHIRIUCHI, HOKKAIDO

A biological study was carried out on whelk Neptunea arthritica using samples collected from basket traps during June 1991 to March 1992. Th e shell height distribution of whelk has three modes and bumps. Mean s hell height in each year class and age composition were obtained by an alyzing the shell height distribution. The growth pattern of the mean shell height at age is a sigmoidal pattern. Two types of growth equati ons were estimated and the logistic growth equation gives a better fit for the data. The results indicate the growth rate of whelk is slow i n the early stage of growth, however, the growth then becomes rapid, a nd the shell height at age four approaches 82 mm. In order to manage t his fishery resource, the Shiriuchi Fishing Go-Operative has set a reg ulation that the minimum legal size of harvested whelks is 60 mm in sh ell height. The study showed that this current legal size is so small that most of the female whelks are caught before the first spawning, h ence the legal size should be raised to 75 mm.