Study On Fisheries Biology For Yellowfin Tuna(thunnus Albacares)in The Western Indian Ocean

Yellowfin tuna(Thunnus albacares) is at the top of the food chain in Marine ecosystem which mainly distributes in the tropical area of the oceans. It’s also one of important commercial species because of its taste, meat quality and nutritional value. Since 1990’s, this tuna has becamean important target species for Chinese distant-water fisheries fleet. Ussually it is very difficult to collect the biological information of high migrated species in open sea like yellowfin tuna. In this study, the focus is on the fisheris biology of yellowfin tuna in the western Indian Ocean according to analyzing the biological data of yellowfin tuna collected by Chinese national observers for tuna fishery from October 2012 to January 2013 and from October 2013 to February 2014. 1223 yellowfin tuna samples were collected by the observers. The mainly biological parameters including dressed weight(DW), fork length(FL), sex ratio, percent of maturity, gonado somatic index(GSI),etc, were analyzed and estimated which supplies for the new information of this tuna so that stock assessment could be conducted better. The results indicated:(1)The dressed weight(DW) range of yellowfin tuna was 7-88kg(SD=17. 56, SE=0. 53) with the mean value of 34.05 kg,and the dominant weight was 10-40kg(occupied 61.69%).DW range of the female was 9-79kg(SD=16. 78, SE=1.15) and the dominant female weight was 10-15 kg and 30-45kg(occupied 47.80%).And the DW range ofthe male was 7-88kg(SD=17.53, SE=1.08),and the dominant male weight was 10-15 kg and 30-45kg(occupied 59.20%).(2)The fork length(FL) range of yellowfin tuna was 65~180cm cm(SD=25.48, IV SE=0.73)with the mean value of 124.35 cm,and the dominant fork length was 100-110 cm and 130-150cm(occupied 65%).The FL range of the female was 75-175 cm(SD=21.82, SE=1.50),and the dominant female length was 85-95 cm and 120-150 cm(occupied 59.38%). The FL range of the male length was 65-180 cm(SD=22.05, SE=1.36), and the dominant male length was 130-165 cm(occupied 60.04%).(3)The discontinuity point for larger and smaller groups was 115 cm,and the percentage of larger female and male were relatively 63.13% and 75.40%, and there was significant different between male and female.(4)The relationship between FL and DW was described as W=3×10-6L3.3413(R2=0.9635)The relationship between FL and DW of the female could be formulatedby W=7×10-6L3.144(R2=0.961),and the male was expressed by W=6×10-6L3.197(R2=0.956).(5)The dominant male length of feeding intensity in the class 0 was 130-160 cm(occupied 80.72%);the dominant male lengthof feeding intensity in the class 1 was130-160 cm(occupied 58.62%);the dominant male length of feeding intensity in the class 2 was 90-100 cmand 120-150 cm(occupied 68.54%);the dominant male lengthof feeding intensity in the class 3was 120-150 cm(occupied 59.48%);the dominant male length of feeding intensity in the class 4 was 120-160 cm(occupied 70.13%).Feeding intensity of yellowfin tuna was mainly in the class 0(occupied 31.92%) or class 4(occupied 29.62%).(6)The total sex ratio of female to male was 0.74:1, the sex ratio of mature female to mature male was 0.69:1.(7)The gonadosomatic index of the female was from 0.39 to 5.92, and at 2 and larger occupied 52.03%; the gonadosomatic index of the male was from 0.44 to 3.37, and at 2 and larger occupied 31.43%.(8)The ovarian weight of the female was 60~1750g,the ovarian weight of the male was 118~1185g.(9)The logistic curve for fork length(FL) at the mature female and male were respectively P=1/[1+e0.08276×(122.991-Lm)] and P=1/[1+e0.03767×(118.618-Lm)].(10)Both the percentage of mature female and male reached the relatively highest values in January and December, and the relatively lowest values.(11)Maturity at II and V of the gonads was dominant up to 49.28% and 22.83% for yellowfin tuna, and the dominant fork length was 90-100cm(occupied 83%) and 100-130cm(occupied 94%), respectively.(12)Based on samples taken by Chinese observers from 2013 to 2014,data were analyzed to the M value of the natural mortality. M was 0.258 ~ 0.826 by calculating eight kinds of model, and the results was closed to the value was estimated in previous studies.