Acoustic Echo Strength Of Three Tropical Tuna Species

Tuna fishing is an important part of the pelagic fishery.There are currently seven major commercial tuna species.They are Skipjack(Kasuwonus Pelamis),Yellowfin tuna(Thunnus albacares),Bigeye tuna(Thunnus obesus),Albacore tuna(Thunnus alalunga),Atlantic bluefin tuna(Thunnus thynnus),Pacific bluefin tuna(Thunnus orientalis)and Southern bluefin tuna(Thunnus maccoyii).At present,acoustic survey is the most effective and intuitive method for tuna fishery survey and research.In order to understand the factors affecting the acoustic echo strength of single tuna,5 samples from three tropical tuna species(captured from the waters off Kiribati in the Western and Central Pacific)were collected in this study.The 3D models of the bodies,skeletons and swim bladders of 1 Skipjack,2 Yellowfin tuna and 2 Bigeye tuna were reconstructed using computed tomography(CT)images combined with 3D reconstruction software.The strength of single target was tested in an anechoic pool in a domestic research institute.The length,width and depth of the anechoic pool were19m,7m and 6m,respectively.The water temperature of the experimental pool was20.5℃,and there was no interference from fish or other noises.The transducers used in the experiment transmitted at frequencies of 50 kHz,70 kHz,90 kHz,120 kHz and200 kHz.The target strength of three tropical tuna species(5 fishes)were tested in different positions(First position:head facing transducer,belly down,back up;Second position:head facing the transducer,belly facing the left,back facing the right;Third position:head upward,body side with the lateral line facing the transducer,back facing the left,belly facing the right)and different incident angles(for each position,at the beginning,belly facing the transducer and denoted as 0°,turn the fish clockwise to90°,then turn the fish anticlockwise to 90°after replace,totally in 180°,the clockwise and anticlockwise rotation angle step was 10°).Finally,Kirchhoff model was used to estimate the target strength of second position under the frequencies of 50 kHz,70 kHz,90 kHz,120 kHz and 200 kHz,and compared with the measured results to analyze the accuracy of Kirchhoff model.The results show that:(1)The skeleton and muscle volumes of Skipjack accounted for 10.08%and 89.91%of the body volume,and the surface area of Skipjack accounted for 33.13%and 66.87%of the body surface area,respectively.For the fish with swim bladder(Yellowfin tuna and Bigeye tuna),because the swim bladder is in the enterocoelia of fish,the swim bladder volume(surface area)is not included in the whole fish body volume(surface area).The skeleton and muscle volume of Yellowfin tuna accounted for 7.83%and92.17%of the body volume,the swim bladder volume accounted for about 0.89%of the body volume,the surface area of skeleton and muscle accounted for 29.74%and71.03%of the body surface area,and the surface area of swim bladder was about 0.74%of the body surface area.The skeleton and muscle volumes of Bigeye tuna accounted for 6.22%and 93.78%of the fish body volume,respectively.The swim bladder volume of Bigeye tuna was about 4.09%of the fish body volume.The surface area of skeleton and muscle of Bigeye tuna accounted for 28.75%and 71.25%of the fish body surface area,and the surface area of swim bladder was about 2.2%of the body surface area.(2)The results of target intensity change after posture change,body length change,frequency change and species change of five tunas were analyzed by experimental measurement:A.Posture change:When tunas rotated from the head facing the transducer to the fish lateral line to transducer under three positions,the target strength gradually increased;During this period,the projection area of fish to the transducer were changed,when the projection area increased,the target strength increased too except some of the position,but some of the posture of echo intensity change law is not obvious;B.Body length change:Body length was one of the factors affecting the target strength of tuna,and the smaller the body length was,the smaller the target strength value was;C.Frequency change:Generally under the laboratory condition,the target intensity of single tuna decreased with the increase of frequency;D.Species change:As the fish target strength of swim bladder accounted for 90%or more of the whole fish strength,in the experiments of three tropical tunas,the target strength fluctuation amplitude of the fish with the swim bladder(Yellowfin tuna,Bigeye tuna)was larger along with the change of the independent variable,but the target strength fluctuation amplitude of the fish with no swim bladder(Skipjack)was not obvious along with the change of the independent variable.(3)After the adjustment of model parameters(the main parameters were input into the model as follows:sound speed in water was 1500 m/s,water density was 1000kg/m~3,Skipjack body density was 1.11 g/cm~3,Yellowfin tuna body density was 1.07g/cm~3,Bigeye tuna body density was 1.07 g/cm~3,the sound speed in the swim bladder was 1500 m/s,the density of the swim bladder was 1.24 g/cm~3),the measured target strengths of Skipjack,Yellowfin tuna and Bigeye tuna were almost the same as the results from Kirchhoff model,and the measured values fitted well with the calculated values when the incident angles were between-80°and 80°.This paper suggests that:(1)Using computed tomography(CT)to study the skeleton of teleost fish was more quickly and conveniently.Especially for the 3D reconstruction for fish skeleton and swim bladder,the accuracy of reconstruction was higher,the image could clearly reflect the fish skeleton and swim bladder structure and the relative position in the fish body,the basic data of the body structure would be available for the study of single tuna target strength,and were benefit to the further study of the influence from the skeleton,swim bladder,muscle volume,density and the relative position to the target strength.(2)Whether there is a swim bladder can be judged by observing whether the target strength is affected by the incident angle of the fish.If the target strength is greatly influenced by the incident angle of fish,this fish is the fish with a swim bladder,whereas for no swim bladder fish.(3)As the body length of Yellowfin tuna and Bigeye tuna decreases,the target strength value also decreases.(4)Kirchhoff model is applicable to the target strength calculation of Skipjack,Yellowfin tuna and Bigeye tuna.