Age, growth and age composition of the landings of threeline grunt, Parapristipoma trilineatum, in the western Bungo Channel, Japan

Abstract

We examined age and growth by thoroughly observing otolith sections of threeline grunt, Parapristipoma trilineatum, caught by angling, set net and gill net in the western Bungo Channel, Japan, from August 1988 to July 2006. We found that at the otolith margin an opaque zone was formed each year from July to September. Growth of the males and females significantly differed to each other because old males were smaller than females of the same age. However, considering that the difference was slight and that we needed to collect data easily and rapidly, we estimated the growth of the fish by using the data for both sexes altogether to derive von Bertalanffy’s equation: FL_t=320 (1-e^{-0.622(t+0.193)}), where FL_t is the Fork length (mm) at t (years). The estimated maximum ages for male and female were 23 years and 17 years old, respectively. Age composition of fisheries catch was estimated by Age-weight key method. Fish at 1–3 years old constituted 61% of total catch in number. A large proportion of the landings were immature fish under 2 years old.