Aggregation Characteristics Of Fish Aggregating Devices(FADs)in Tuna Purse Seine Fisheries In The Western And Central Pacific Ocean

Drifting fish aggregating devices(DFADs,or FADs)has become popular since the late 1980 s due to the behavior of associating the floating objects at surface of the sea in the purse seine tuna fisheries.To date it accounts for more than half of the world’s tuna Seine catch.However,the obvious negative effects caused by large-scale deployment of FADs,such as marine garbage pollution,serious by-catch and ghost fishing,as well as the hidden negative effects on fishery resources and pelagic fish ecosystem balance,have gradually attracted wide attention worldwide in the past 20 years.Based on the data of echo-sounder buoys collected by Chinese tuna purse seine vessels in 2021 in the Central and Western Pacific Ocean,this paper aims at the drifting speed and fish aggregation effect of three newly designed FADs with different underwater lengths.Non-parametric independence test was used to analyze the significance of first arriving day of fish,the vertical distribution of tuna species,and the difference of fish size composition under FADs.In addition,a Generalized Additive Mixed model(GAMMs)was used to analyze the variation of fish aggregation biomass with FADs drifting time,underwater length and drifting speed.(1)The actual drifting speed of the three new FADs is significantly different and increases with the increase of the length of the underwater structure(P<0.05).The drift speed of FADS-60,FADS-80 and FADS-90 is(0.57±0.31)kn,(0.66±0.39)kn and(0.75±0.44)kn respectively.(2)The first arriving day of non-tuna and tuna species was(16.0±8.9)days and(3.8±4.2)days,respectively,indicating that the first arrival day of tuna species was much earlier than that of non-tuna species(P<0.05).The average time for fish arrving FADs for the first time was(8.9±9.0)days,and there was no significant correlation with FADs drift speed(P>0.05).(3)The first arriving day of tuna species at FADs was significantly different from that of the underwater structure of FADs(P<0.05).The first arriving day of tuna species at FADs-60 and FADs-90 was significantly earlier than that of FADS-80,but the length of underwater structure of FADs had little effect on the that of non-tuna species(P>0.05).(4)Drifting speed of FADs has a significant effect on the biomass of tuna species.When drifting speed is low(generally <0.5 kn),the aggregated biomass of the surrounding tuna species was always at a high level,but with the increasing drifting speed of FADs,the aggregated biomass of tuna was continuously decreased.(5)The aggregation process of tuna species under FADs changed dynamically.In general,the aggregated biomass of tuna around FADs kept accumulating with the increase of time,and then gradually decreased after reaching the peak at about the 25 th day,and tended to be stable at about the 50 th day.The results show that the aggregation mode of tuna under FADs has a significant difference in the length of the underwater structure of FADs,and the time of peak of aggregation biomass of FADs-90 is the shortest,and that of FADS-60 is the longest.At the same time,the length of FADs underwater net clothing had different random effects on the aggregation biomass of tuna,among which the random effects of FADs-80 were the highest and that of FADs90 was the lowest,indicating that FADs-80 had the strongest aggregation performance of tuna in this sea trial.(6)The vertical distribution of tuna species under FADs was significantly different from the drift time and the length of the underwater structure.With the increase of the drifting time and the length of FADs underwater net,the biomass of tuna in deep layer was higher than that in shallow layer,indicating that tuna tended to gather in deep water with the increase of FADs drifting days.(7)The underwater length of FADs has a significant effect on the size composition of tuna species.As the underwater length of FADs increases,the proportion of small-sized fish under it gradually decreases,while the proportion of large-sized fish gradually increases,indicating that FADs with longer underwater length are more likely to gather large-sized fish.This paper uses the buoy acoustic data of tuna purse seine vessels of China to study fish aggregation characteristics under FADs for the first time,which fills in the gap of relevant research in the Western and Central Pacific Ocean in China,and has positive significance for improving the fishing selectivity of FADs associated schools in this area.At the same time,the research is helpful to further understand the behavior pattern of the FADs associated schools,and provide scientific basis for the development of efficient and eco-friendly FADs and the development of conservation and management measures for the FADs associated schools.