Considerations of stock structure of yellowfin tuna (Thunnus albacares) in the Indian Ocean based on fishery data

Abstract

ABSTRACTYellowfin stock structure in the Indian Ocean was studied by using industrial tuna longline fishery data. Three types of test variables were used to detect stock structure, i.e., CPUE, age‐specific CPUE, and coefficient of variation for size. Time‐series data of test variables were compiled for six sub‐areas that were arranged by dividing the whole region systematically along longitude lines every 20 degrees. Then time‐series data were smoothed by moving averages, and regressed by simple models. Patterns of time‐series trends were graphically and statistically compared to classify homogeneous sub‐area groups. Two assumptions were (a) that homogeneous stocks exist longitudinally and overlap in adjacent waters, and (b) that test variables within homogeneous sub‐area groups are equally affected, and hence patterns of the time‐series trends are similar. After graphical screening for significant sub‐area groups, analysis of covariance was applied to test homogeneity of regression parameters representing patterns of the time‐series trends. By classifying homogeneous sub‐area groups, stock structures were determined at the P <0.05 and P <0.50 levels. The P<0.50 level was recognized as a useful criterion for ‘weak’ test variables since masked or vague structures at the P <0.05 level were likely cleared at this level in many cases. Results of this study and past stock structure studies were reviewed and compared. It was concluded that there are two major and two minor stocks of yellowfin tuna. The two major stocks (the western and the eastern) are located at 40o‐90oE and 70o‐130oE respectively. The minor stocks are the far western and the far eastern stocks (the latter possibly being a part of the Pacific stock), which are located westward of 40oE and eastward of 110oE respectively. Neighboring stocks are intermingled in adjacent waters.