Life history variables, annual change in sex ratios with age, and total mortality observed on commercial catch on Pacific steephead parrotfish, Chlorurus microrhinos in waters off the Okinawa Island, southwestern Japan

Abstract

In order to evaluate the impact of fishing on the Pacific steephead parrotfish (Chlorurus microrhinos) resource around the north of Okinawa Island, life history variables such as growth, ages of both sexual maturation and sexual transition, and the spawning season for the species were examined. Body size data of fish caught by commercial fishermen have been gathered for 24 years. Growth was estimated by the number of opaque bands observed in the thin sectioned otolith. The body sizes of males were much larger than those of females in the same age group, but the maximum age of females was older (17 years of age) than that of males (13 years of age). Sexual maturation in females starts from three years of age and all female individuals have fully matured by five years of age. Sexual transition from female to male was confirmed at ages from two to six, accompanying the changes of pectoral fin color from brown to blue on the inside. Smaller body size individuals captured by gill net dominated about 50% of the catch in numbers during the earlier third of the survey period; however, gill net influence gradually decreased and smaller body size individuals dominated about 30% in the last third of the period. Body size data distinguished between the initial phase (IP) and terminal phase (TP) by the pectoral fin colors indicated that the ratio of IP and TP in length intervals changed every year. Age components in catches were estimated using life history variables and these body size data. IP and TP ratio with age in each cohort showed the change in ages between three and eight/nine; the ratio varied significantly between each cohort at five and six years of age in particular, accompanying serial changes between the cohorts. The values of total mortality coefficient (Z) estimated from age components in catch in the survey years decreased from about 0.5 to about 0.4 with the decreased influence of gill net. Z occasionally decreased to about 0.1 with a relatively increased number of older aged individuals in the catch due to the emergence of a large cohort. These phenomena strongly suggest that the traditional stock evaluation using Z,F, and M seems to be inadequate for the species.