Abstract
To estimate the natural and fishing mortality of whelk, Neptunea arthritica, a multiple tagging experiment was carried out in Shiriuchi, Hokkaido between August 1991 and March 1994. Whelks collected by trapping surveys were used in tagging experiments. All recaptured whelks were released back again so that a statistical model based on binomial distribution could be developed. Maximum likelihood estimator of the natural mortality coefficient and that of exploitation rate were determined numerically. Analysis of the recovery data show: (i) no size-dependent mortality was observed in whelks larger than 40 mm in shell height; (ii) the daily natural mortality coefficient was 1.438×10-3, thus annual natural mortality rate was 41%; and (iii) the exploitation rate of commercial fishing in 1994 was approximately 77.3%. At least 53% of the total mortality was attributable to fishing in the exploited area.
