A comparison of biological characteristics of yellowfin tuna (Thunnus albacares) in the Western and Central Indian Ocean

Based on the yellowfin tuna (Thunnus albacares) biological data collected from two surveys conducted on board two Chinese tuna longliners in the Western and Central Indian Ocean from Sep. 2008 to Jan. 2009 (the 1 st survey) and from Oct. 2013 to Apr. 2014 (the 2 nd survey) respectively, this paper analyzed the biological characteristics of yellowfin tuna by statistic methods. The goal of this study is to determine if there was an impact of piracy activity on the yellowfin tuna resource in the Western and Central Indian Ocean. Our results indicated: (1) There was significant difference (p = 0.0404<0.05) between the yellowfin tuna fork length distribution of the above two surveys. The average fork length was 136.7±1.43cm , with dominant fork length of 125~145cm from the samples collected in the 1 st survey and was 140.2±1.37 cm, with dominant fork length of 115~165cm from the samples collected in the 2 nd survey. (2) There was significant difference between the gilled and gutted weight of two surveys (p=0.0414<0.05). (3) The sex ratio (male: female) of the yellowfin tuna sampled between two surveys was significant difference (p = 0.00927 < 0.05), the sex ratio was 2.14:1 in the 1 st survey significantly and was higher than that (1:1) of the 2 nd survey. (4) Gonad maturity of the sampled yellowfin tuna in the 1 st survey was dominated by grade IV(40%) and V(41.1%) and that of the 2 nd survey was dominated by grade III (31.2%) and IV(23.9%). There was significant difference (p= 1.186*10 -12 < 0.05) in gonad maturity stages between two surveys. (5) There were significant differences in somatic index (SI) ( p = 2.2*10-16 < 0.05) and gonadosomatic index (GSI) (p = 0.0108 < 0.05) between two surveys. (6) The relationship between fork length (L) and somatic wet weight (WG) of yellowfin tuna was expressed by WG1S =1.9807×10 -5 L 2.9292 for the 1 st survey and