Relationship between the temporal-spatial distribution of fish in bigeye tuna fishing grounds and the thermocline characteristics in the tropical Indian Ocean

Abstract

This study plotted spatial overlay maps to analyze the relationship between bigeye tuna(Thunnus obesus) distribution in fishing grounds in the tropical Indian Ocean and thermocline characteristics,calculated from monthly temperature profile data from Argo buoys and the monthly CPUE(catch per unit effort) from bigeye tuna longlines.Seasonal variability was observed in the depth of the upper boundary temperature and in the lower boundary depth.Our analysis suggested that the distribution of bigeye tuna was related to these seasonal variations of the thermocline.The range of the upper boundary depth for high CPUE during the summer monsoon was 30–50 m,which was lower than in the winter(50–70 m).A higher CPUE was found from northern Madagascar to Soma-lia where the lower boundary depth was at 170–200 m.Where the lower boundary depth was more than 300 m,the value of CPUE was low.The range of upper boundary temperatures in the higher CPUE fishing grounds was 27–30 m during the winter monsoon and 24–30 m the summer monsoon.Frequency analysis and the empirical cumulative distribution function(ECDF) were used to compute the optimum range of thermocline characteristics.The optimum depth range of the upper boundary temperature was 26–29 m and the for the lower boundary tem-perature,it was 13–15 m.The optimum range of the upper boundary depth was 30–60 m and the lower boundary depth was 140–170 m.The conclusions were confirmed using Kolmogorov-Smirnov test.The suitable distribution interval and seasonal change characterize of each thermocline in the central fishing ground of bigeye tuna were preliminary obtained in this paper.The results could provide reference to longline production operation in sea and resource management.