Preliminary Study On The Biological Characteristics And Stock Status Of Bigeye Tuna (Thunnus Obesus) In The Indian Ocean

Bigeye tuna (Thunnus obesus) is the key tuna species in the tropical Indian Ocean(IO). It is also the target specie of Chinese longline fleet in the IO and plays an important role in the Chinese distant-water tuna fishery. In order to understand the stock statusof the bigeye tuna (BET) in the IO, this thesis investigated the biological characteristicsand characteristics of the capture depth based on the observed data; and then estimategrowth and mortality based on length composition data. Yield-per-recruit (YPR)analysis was further applied to evaluate the exploitation status of this stock. The keycontents and results are as follow:1. Characteristics of the vertical distribution of the BET in the IOBased on hook No. information collected by scientific observers on the Chinesetuna longline vessels in the IO from October2012to January2013, the distribution ofcapture depth of BET was estimated by using catenary curve method. The depth rangeof the captured BET was from68.5to251.1m, with an average of215.2m and standarddeviation of61.1m. With the increase of depths of hooks, the proportion of BET in totalcatch increased to a maximum at the depth of135-155m and then decreased. Theproportion of male BET in catch was higher than female. The mean fork length of BETtended to increase with depths of hooks, In a relatively shallow water (55-175m), themean length of BET was the smallest; In a relatively deep water (175-255m), the meanlength of BET was the largest. This study provided important information for improvingour understanding of BET habitats in the pelagic tuna fisheries.2. Biological characteristics of BET in the IOBased on data collected by Chinese national observers of tuna fishery in the IOfrom October2012to January2013, biological characteristics was analyzed. The resultsare:(1) The dressed weight (DW) range of BET was4-113kg (s.d=21.1，n=1365) withthe mean value of35.8kg，and the dominant weight was10-50kg (accounting for76.69%);(2) The fork length (FL) range was30-185cm（s.d=22.3，n=1377）with the mean value of124.8cm，and the dominant fork length was100-150cm (accounting for79.38%);(3) The sex ratio of female to male was0.53:1;(4) Gonads maturity at stageⅣand V was dominant with a percentage of30.8%and43.8%respectively;(5)Themean gonosomatic index for female is0.211, and the mean gonosomatic index for maleis0.049.3. Estimation of growth, mortality, and YPR for IO BET stockUsing length composition data of BET in the IO in2011(combination of longlineand purse seine fisheries), the growth and mortality parameters were estimated by usingELEFAN technique with FiSATⅡsoftware. The von Bertalanffy growth parameterswere estimated at L∞=270.9cm FL, k=0.08yr-1, and t0=1.1937year. The totalmortality coefficients for the IO by length converted catch curve method was0.61. Thefishing mortality coefficients and the exploitation rate was0.21and0.34, respectively.The analysis of related parameters and relative yield-per-recruit suggested that thebigeye tuna was not under high fishing pressuren the Indian Ocean.