Stock assessment of grey mullet Mugil cephalus in Taiwan by cohort analysis.

Grey mullet Mugil cephalus (LINNAEUS) is one of the most important fish migrating to Taiwan in winter. Many studies and results concerning the biology of this species have been undertaken, but only two of these have dealt with its population dynamics. Since grey mullet is an intensively fished stock and the age composition is available in this study, the cohort analysis was applied to assess the stock abundance. The age composition and catch from 1977 to 1987 were used to calculate the number in the stock. First, the sensitivities of the estimated stock to terminal F and M values were examined. The differences were rather small with even considerably different values being used, and the following values were adopted for the calculation: F=0.99 for female, F=1.36 for male, and M=0.33. The mean stock size and recruitment of the years 1977 to 1986 were calculated to be 1.40×106 and 0.70×106 individuals for female and 2.16×106 and 1.34×106 individuals for male fish. Both the stock size and recruitment showed decreasing trends. As the age composition is shifting towards younger ages and the catch is decreasing sharply in recent years, the stock of grey mullet should be taken into serious consideration.