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Abstract

Jack mackerel (Trachurus japonicus) is one of the most important fish resources in the northeastern Taiwan. Because of the flourishing of Taiwanese seiner fisheries in recent years, the proportion and catch of jack mackerels have increased. However, the studies on their age and growth are outdated, thus data on the catch-at-age composition are lacking. The present study collected jack mackerel otolith samples from May, 2010, to April, 2011, for age determination and combined those data with length and weight data from 2005 to January, 2012, to investigate the catch-at-age of jack mackerel in the northeastern waters off Taiwan. The regression equations for fork length (FL) and body weight (BW) of jack mackerel were significantly different between females and males (p < 0.05). The equations were BW = 3.6 × 10-2 × FL2.6771 for females and BW = 3.2 × 10-2 × FL2.7108 for males. The formation of opaque zones was used as the standard for ring calculation. Ring formation occurred once every year during January and February regardless of sex. The obtained ages and corresponding FL data were introduced into the von Bertalanffy growth function showing that the maximum FL was 40.4 cm with growth coefficient of 0.223 yr-1. The jack mackerel catch in the northeastern waters off Taiwan were the age-0 (39%) and age-1 (38%), with high proportion of immature fish.

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