Investigation On Feeding Ecology And Habitat Characteristics Of Bigeye Tuna(thunnus Obesus)in The Northern Pacific Ocean

Bigeye tuna(Thunnus obesus)belongs to the family Scomberdae.It is a highly migratory tropical pelagic fish,which is widely distributed in the middle and low latitude of Pacific Ocean,Indian Ocean and Atlantic Ocean.Thunnus obesus was one of the main species caught by tuna longline fishery in North Pacific Ocean,and it is of great significance to explore the feeding ecology and habitat characteristics in pelagic tuna longline fishery for China.Based on the data obtained by the tuna longline fishery scientific observer during the experimental fishery project in the North Pacific Ocean,the biological characteristics of Thunnus obesus were analyzed,and the feeding ecology of Thunnus obesus was analyzed by using the stomach samples obtained,and the habitat environment of bigeye tuna was analyzed by combining the environmental data measured by the CTD.The main research contents and results are concluded as follows: 1?Biological characteristicsThunnus obesus is an important target species of pelagic tuna fishery in Pacific Ocean for China.According to the biological measurement data collected during the North Pacific Tuna experimental fishery project from September 2018 to February 2019,the distribution of fork length,the relationship between fork length(FL)and body mass(Ww),gonad maturity and feeding grade of 1754 ind tuna were studied by statistical and regression methods.The results show that:(1)The dominant FL of Thunnus obesus was 100-110 cm,accounting for 26.00%,and the average FL was 113.58cm;the dominant Ww was 10-20 kg,accounting for 48.97%,and the average Ww was 15.83 kg.(2)According to the power function regression,the relationship between the FL and the Ww was: Ww= 5.1540×10-5FL2.7553(R2 = 0.9158,male and female),Ww = 6.8882×10-5FL2.6932(R2 = 0.8908,male),Ww= 6.5929×10-5FL2.7046(R2 = 0.9205,female).(3)The gland maturity of male and female of Thunnus obesus was mainly in stage II,accounting for 63.30% and 68.53% respectively;the proportion of male individuals was slightly higher than that of female individuals,the sex ratio of male and female individuals was 1.14: 1.The male of Thunnus obesus with FL of 70-90 cm accounts for 51.11%,the male of individual with FL of 90-110 cm accounts for 55.77%,the male of individual with FL of 110-130 cm accounts for 51.48%,and the male of individual with FL of 130-150 cm accounts for 47.17%;Sex accounted for 47.17%,and 43.75% of the individuals with fork length more than 150 cm were males.(4)The feeding grades of Thunnus obesus are mainly 0,1 and 2,accounting for 24.13%,46.41% and 21.56% respectively,accounting for 92.10% of the total;the feeding grades of male are 1 > 0 > 2 > 3 > 4,and that of female are 1 > 2 > 0 > 3 > 4.2? Feeding ecologyThe present study analyzes the stomach contents of 146 individuals collected from September to December 2018 by the Chinese Longline Fisheries Observer Programme in the subtropical waters of the north Pacific(30°N?37°N,150°W?164°W).The results show that a total of 36 species preys belonged to 28 families were identified.Especially,Alepisaurus ferox,Eucleoteuthis luminosa,Lestidium prolixum and Diaphus were dominant preys,and the percentages of relative importance index(IRI%)of which were 25.65%,14.48%,7.56% and 6.77%,respectively.The highest feeding intensity of bigeye tuna occurred in the range of 100?110 cm FL and at the stage III of gonadal maturity,while the lowest was observed at the stage V.Moreover,the higher feeding intensity was found in 150?250 m water layer than other water layers.3? HabitatUsing the single factor habitat index,we analyzed the relationship between vertical distribution and the concerned environmental factors of Thunnus obesus of Chinese longline fishery in subtropical waters of Northern Pacific.The results indicated that the optimal water layer of Thunnus obesus range at 152.3?199.6 m.Analysis of single factor habitat index showed that the optimal sea surface temperature(SST)of Thunnus obesus was 20?22? in the north Pacific Ocean.The optimum water temperature,salinity and chlorophyll concentration range were13?14 ?? 35.8?35.9 and 0.15?0.20?g/l in the vertical direction,respectively.The depth range 54.0?169.5m was thermocline,and the temperature range was 13.23?19.23?.The catch of hooks in the thermocline accounted for 64.88% of the total,and the average catch of hooks in the thermocline was 2.76 ind/(1000hooks),while that outside the thermocline was 2.01 ind/(1000hooks).The hooks with high catch were mainly distributed near the lower and upper part of the next thermocline,the upper part of the next thermocline accounted for 58.48% of the total catch,and the lower part of the next thermocline accounted for 23.18% of the total catch.