Changes of group construction accompanying with growth and maturity in blue-barred parrotfish (Scarus ghobban), and influences of the fishing targeting the immature group to the stock

Abstract

Reproductive biology and growth characteristics of the Blue-barred Parrotfish (Scarus ghobban) were revealed for the purpose of the stock evaluation. Based on both of these findings and body size data in commercial catches accumulated during 19 years, age components in the catch and cohort size were estimated via virtual population analysis for the northern area of the Okinawa Islands population. The population was composed of two separate groups, one of which was a all female group whose members’ ovaries were immature year-round, and the other was a male and female mixed group, of which the majority of females were mature year-round. Fish were mainly up to 3 years of age in the all female group and older than 2 years in the sexually mixed group. Although the majority of females in the sexually mixed group matured year-round, a portion of the rest of the immature females showed evidence of prior maturation, suggesting that each female in the group experienced periods of spawning and resting repeatedly without a seasonal pattern.The fishery at the studied area has mostly targeted the all female group to be about 85% of total numbers; thus, the fishing pressure to mature females was revealed to be very small. However, the VPA highlighted that the most individuals up to 3 years of age belonging to the all female group died either by fishing or naturally. Thus, a few individuals attained sexual maturity in each cohort. Extremely high fishing pressure against the mature age groups shortly added during the study period, thereafter spawning stock biomass (SSB) and %SPR rapidly decreased, the latter to the level of about 20%, beyonding the biological reference point. Cohort sizes during the years 1994–2012 have fluctuated in magnitude twice, and those of latter half period, coincide to both SSB and %SPR decreases, were smaller than those of earlier half but were stable without any trend of decreases.