ANALYSIS ON STOCK STRUCTURE OF FREE SWIMMING SKIPJACK TUNA (KATSUWONUS PELAMIS) SCHOOL CAUGHT BY CHINA TUNA PURSE SEINE IN THE WESTERN AND CENTRAL PACIFIC OCEAN

Based on 1343 skipjack tuna from the free 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,biological characteristics such as fork length(FL),weight(W),FL-W relationship,sex ratio,and gonad maturity stages were collected and analyzed by power regression and other statistic methods.The results indicate: 1) The fork lengths of skipjack caught in the fishery were found to range from 243 to 746mm,with the dominant fork lengths of 401 to 600 mm accounting for 88.24% of the total sampled fish.There was significant difference(x2= 843.7,P0.01) in the FL composition between females and males;2) The FL-W relationship could be described as W=6.0983 10-6 FL3.1870(R2=0.9443),and no significant difference was found in such a relationship between body weight and sex(ANCOVA,F=0.549,P0.05).3) The average male-female ratio was 1∶0.95,but sex ratio showed significant changes with fork length from 501 to 550mm(x2=4.00,P0.05) and from 651 to 700 mm(x2=4.17,P0.05).4) Maturity of the skipjack gonad sampled in the fishery were dominant by individuals with maturity stages Ⅳ(28.93%) and Ⅴ(43.67%),and the estimated minimum sizes of female and male skipjack at maturity were 473.15 mm and 440.78 mm,respectively.Both estimates were higher than the sizes observed at sea.The preliminary conclusion of the investigation was that adult fishes with maturity were dominant in the population of skipjack from free swimming in our survey waters.