Comparative biological characteristics of skipjack tuna Katsuwonus pelamis between free school catch and log school catch

Free swimming school and log school are the two main targets of tuna purse seine fishery at present.On the basis of 2000 biological measurements of skipjack tuna Katsuwonus pelamis collected from purse seiner Jinhui No 6 of Shanghai Pelagic Fishing Corp Ltd,when the authors conducted the research project on technology and fishing situation predict of tuna purse seine fishery in the Western and Central Pacific Ocean between Oct.2006 and Feb 2007,biological differences were evaluated in skipjack tuna between catches from the free swimming school and the log school,including fork length(LF),body weight(W),sex ratio,gonadal development stages,and feeding intensity by linear regression methods.The results showed that fork length of skipjack from free swimming school ranged from 331 to 738 mm with the dominant fork length of 425-575 mm;and that from log school varied from 264 to 719 mm with the dominant fork length of 325-475 mm.The dominant fork length in free swimming school was significantly larger than that from the log school(P0.01),and the incidence of individual fishing over 450 mm in fork length in the catch from free swimming school was higher than that in the catch from log school.There was no significant difference in fork length distribution between the male and female in the catch from free swimming school(P0.05),while the fork length of male skipjack from the log school was significantly higher than the that of the female one.Relationship between fork length and body weight of the skipjack from the free swimming school and the log school was described by functions W1 =8.319×10-6L3.15F(R2=0.946)and W2=4.505×10-6L3.24F(R2=0.977),respectively.Sex ratio(female to male) of skipjack catch was 1∶1 and 2∶1 from the free swimming school and the log school,respectively.Gonadal development was dominated by phase Ⅳ(30.49%)and Ⅴ(44.92%)in the catch of free swimming school,and by phaseⅠ(21.00%) and Ⅱ(56.00%)in the catch of log school.The feeding intensities of skipjack in the catch of free swimming school were mainly at the level of empty stomach(37.39%),class I(28.61%) and class II(35.14%).Almost 99.50% of the fish in the catch of log school was found empty stomach.