Research On Tuna Fishing Technology,Fishing Grounds And Fishery Biological Characteristics In The South Indian Ocean

The research data in this article comes from the tuna longline fishing vessel"New Century 76",which was conducted in the southern Indian Ocean from July to September 2020.The investigations and researches were mainly conducted on the South Indian Ocean tuna longline fishing technology,fishing grounds,environmental factors and biological characteristics.This article introduces the fishing method of tuna longline fishing gear and proposes improvement measures on this basis.In the study of the fishing ground environment,the data measured by CTD were used,and the GAM model was used to analyze the relationship between the main tuna production and environmental factors.The frequency distribution method was used to analyze the biological characteristics of the main tuna.The study included fork length composition,body weight composition,gonadal maturity,feeding grade and gastric content,and the relationship between fork length and weight was obtained by power function fitting.The results showed:（1）Tuna longline fishing operations accounted for a small percentage of by-catch,the operation method is simple,the freshness of the catch is high,and the economic value is great.It is an environmentally friendly fishing gear.（2）There are 20 hooks between the two floats of the"New Century 76"tuna longline fishing boat,when the catch is dominated by albacore tuna,the number of hooks between the two floats can be adjusted to 16 in order to increase the catch rate.Tuna has the characteristics of vertical movement of diving during the day and floating at night,and has the strongest appetite in the evening and early morning.Through research,the author suggests to adjust the start and end time of the work rope to 10:00-16:00,and the start and end time of the hook to 17:00-05:00.（3）In the choice of fishing hooks,ring hooks should be used.Studies have shown that ring hook albacore has the highest fishing percentage,and bigeye tuna has the highest survival rate.Tuna foraging mainly relies on vision,followed by smell.When the hook is lifted every day,the fishing branch line must be repaired and replaced.Squid is the best bait for tuna longline fishing in the Indian Ocean.Studies have shown that fish bait can reduce the accidental catch rate of blue sharks and sea turtles compared with squid bait.Therefore,in the use of bait,sardines with a heavier fishy smell should be selected,which can ensure a higher fishing rate and reduce bycatch.（4）Oil fish（Lepidocybium flavobrunneum）,opah（Lampris guttatus）,swordfish（Xiphias gladius）have a certain phototaxis,accounting for 39.24%of the fishing percentage of fluorescent artificial lures."New Century 76"hooks 500 fluorescent artificial lures every day.In order to increase the catch rate of by-catch,it can be considered to increase to 800 fluorescent artificial lures.（5）From July to September,the CPUE of albacore and bigeye tuna was at a low level,6.16 fish per thousand hooks and 2.47 fish per thousand hooks respectively,and both showed a trend of first decreasing and then increasing.Albacore tuna CPUE reached its maximum value in September,9.64 fish per thousand hooks;big eye tuna CPUE reached its maximum value in July,4.04 fish per thousand hooks.（6）The GAM model results show that the latitude and 200m water temperature have a significant impact on the albacore tuna catch rate.This survey is a productive survey with no specific sites,and it is at the end of the fishing season that mainly catch albacore tuna,so the relationship between the model factor and the CPUE of bigeye tuna is not significant.（7）The water temperature of 200m has a significant effect on albacore tuna catch rate,and the high catch rate is concentrated in 18.5～21℃.（8）The central fishing grounds of Indian Ocean albacore and bigeye tuna from July to September were distributed at 18°～25°S,66°～78°E.（9）There are 1002 albacore tuna,including 795 males and 207 females.The male albacore tuna’s fork length ranged from 87 to 114cm,with an average of 99.54cm,and the dominant fork length group ranged from 91 to 110cm.The female fork length ranged from 80 to 104cm,with an average of 94.02cm,and the dominant fork length was 91 to 100cm.The body weight of male albacore tuna ranged from 17 to 31kg,with an average body weight of 24.14kg and a dominant body weight of 21 to 30kg.The female body weight ranged from 16 to 28kg,with an average of 21.75kg,and the dominant body weight group was 15 to 25kg.The relationship between fork length（cm）and body weight（kg）of the southern Indian Ocean albacore tuna could be described as follows:whole:W=0.0037FL^1.9085（R²=0.642,n=1002）;male:W=0.0041FL^1.8884（R²=0.0041,n=207）,female:W=0.0012FL^2.1497（R²=0.6065,n=795）.（10）There are 326 bigeye tuna,including 151 males and 175 females.The male big eye tuna’s fork length ranged from 73 to 161 cm,with an average of 104.38cm,and the dominant fork length group was 81 to 110cm.The female fork length ranged from 74 to 157cm,with an average fork length of 107.23cm,and the dominant fork length group ranged from 81 to 110cm.The male bigeye tuna body weight ranged from 9 to 94kg,with an average of 29.56kg,and the dominant body weight group was10 to 30kg.The female body weight ranged from 11 to 84kg,with an average body weight of 31.43kg and a dominant body weight of 10 to 30kg.The relationship between fork length and body weight of southern Indian Ocean bigeye tuna could be described as follows:whole:W=0.00003FL^2.9436（R²=0.9583,n=326）,male:W=0.00002FL^2.994（R²=0.9642,n=151）,female:W=0.00004FL^2.8869（R²=0.9521,n=175）.（11）There are noⅠmaturity stage,the gonad maturity of southern Indian Ocean albacore tuna were mainlyⅢ、Ⅳ、Ⅴ,which accounting for 94.91%of the total sample,only 1 tail ofⅡmaturity stage,Ⅵmaturity stage accounted for 4.99%.There were notⅥmaturity stage individuals in the southern Indian Ocean bigeye tuna samples,the maturity stage were mainlyⅡandⅢ,which is accounting for 62.5%of the total sample,the proportion ofⅠ、Ⅳ、Ⅴis less.（12）The stomach stage of southern Indian albacore tuna is mainly level 1 and 2,accounting for 76.5%.Bigeye tuna’s stomach stage are mainly level 0 and 3,accounting for 46.9%.（13）The stomach contents of albacore and bigeye tuna in the South Indian Ocean are mainly small fish,cephalopods and crustaceans.They accounted for 41.2%、34.9%,36.5%、30.7%,11.7%、8.3%,respectively.