Stock structure of skipjack tuna (Katsuwonus pelamis) from the log school by purse seine in the Western and Central Pacific Ocean

Skipjack tuna(Katsuwonus pelamis)is one of the main target species of tuna purse seine fishery.The study of its biological characteristics will help the better understanding of its stock structure,and have significance to the reasonable utilization of the resource.Based on the observation data of 975 skipjack tuna from the log school sampled on board purse seine vessels operated in the Western and Central Pacific Ocean waters from October 2006 to February 2007 and from October 2007 to January 2008,the basic biological characteristics such as fork length(FL),mass(BM),FL-BM relationship,sex ratio,and gonad maturity stage were analyzed by power regression and other statistic methods.The results indicated that the fork length distribution of the skipjack tuna did not follow normal distribution(P0.05).The fork length was from 243 to 733 mm,and the dominant one was from 302 to 500 mm,accounting for 80.62% of the total sampled fish.There was significant difference(t=3.106,P0.05)in the FL distribution between females and males.The FL-BM relationship could be described as BM=6.0983×10-6 FL3.1870(R2=0.9618),and no significant difference was found in such a relationship between females and males(ANCOVA,F=2.756,P0.05).There existed significant difference in the female-male ratio between the fork length sets from 351 to 400 mm(χ2=7.784,P0.05)and from 401 to 450 mm(χ2=17.147,P0.05).The average female-male ratio of the skipjack tuna was about 1∶0.83,and also showed significant difference(χ2=5.878,P0.05).The gonad maturity of the skipjack tuna was dominant by the maturity stages of Ⅳ and lower,accounted for 84.11% of the total number of sampled fish,and the estimated sizes at first maturity of female and male skipjack tuna were 520.62 and 527.52 mm,respectively,being higher than the observed minimum maturity sizes.