Biology of kawakawa Euthynnus affinis in the East China Sea: Growth, reproduction, and stable isotope ratios

Abstract

This study elucidated the age–growth relationship, maturation body size, and stable isotope ratios of kawakawa Euthynnus affinis in the East China Sea. A total of 396 individuals were collected, of which 266 were captured off the west coast of Kyushu and 130 around Okinawa. Ages were determined from otolith analysis, and the estimated von Bertalanffy growth functions were FLt = 1007{1 – exp[–0.107(t + 3.835)]} near Kyushu and FLt = 640{1 – exp[–0.381(t + 1.111)]} around Okinawa, where FLt is fork length (mm) at age t for ages 0.25 to 5. The estimated fork lengths of the fish near Kyushu were larger than those around Okinawa. The smallest fork length of a fish in the spawning stages was about 440 mm. Mature females and males were observed from May to October near Kyushu and from March to July around Okinawa. Females in the spawning stages were observed from July to September near Kyushu and from April to June around Okinawa. The δ15N of fish captured near Kyushu (12.39‰ ± 0.55‰, mean ± standard deviation) was higher than that of those captured around Okinawa (9.94‰ ± 1.39‰). This suggests that the kawakawa caught in each area occupy distinct habitats, although some mixing of individuals between the two habitats might occur.