| Larimichthys crocea is one of the four traditional marine economic fishes in China.It is rich in protein,vitamins and unsaturated fatty acids.It is a high-quality marine aquatic product and the main cultured fish species in China.The main aquaculture methods of L.crocea are nearshore net cage,offshore net cage and net enclosure in shallow sea.Among them,the large-scale net-enclosure aquaculture is a newly developed high-end marine aquaculture mode in recent years.Several net-enclosure facilities have been constructed in Shandong province,Fujian province and other places in China for its advantages in providing the right conditions for growth which closer to the original environment for fishes.However,it is difficult to precisely determine the positions of the fishes and to feed and harvest L.crocea for its large aquaculture spaces.Therefore,it is of great significance in application to study the distribution characteristics of L.crocea in large-scale net-enclosure aquaculture culture.In this study,the relationship between the hardness value of the fish school obtained by the DE-FR01 single-beam fish finder and the number of fish and the depth of the fish was first studied through field experiments,the relationship among them is obtained and verify the rationality of the fitting formula through the known number of L.crocea in the cage.Secondly,on this basis,in order to obtain the distribution characteristics of L.crocea in the large-scale net-enclosure aquaculture culture area,a multi-function small unmanned vehicle equipped with the sensors of sonar fish detector,temperature,salinity,pH,illumination and dissolved oxygen has been successfully developed.Among them,the fish detector(DE-FR01 200 k Hz)is consisted of the single beam energy converter which can integrate transmitting and receiving ultrasonic signals.The working principle is that the acoustic signal transmitted by the energy converter propagates in water,when encountering the medium different from water,part of the signal will be reflected back.According to the time between of signal transmission and receiving,it can be used to measure the distance between the target and the energy converter,then analyze and process the echo signal strength(hardness),and through 4G-Network communicates with shore-based system.Finally,the depth and hardness of the target are displayed in the software control terminal.Several field trials have been carried out in Taohua Island,Zhoushan City,Zhejiang Province.By analyzing the detected fish distribution position,water temperature,salinity and illumination of the marine environment in net-enclosure aquaculture culture area,the following conclusions are obtained:(1)The data of hardness value,depth position and quantity of fish body obtained from the experiment are analyzed by fitting,and the relationship between the three is obtained:(?).(2)In the horizontal direction of the large-scale net-enclosure aquaculture culture area of Taohua Island,the L.crocea with a gain coefficient of 36 and a body length of about 22 cm is mainly distributed in the notch of the northwestern area of the net-enclosure aquaculture culture area and in the coastal reef area,that is,the L.crocea are mostly distributed in areas with abundant reefs and relatively slow currents.(3)In the vertical direction of the Taohua Island culture area at a depth of about 10 m,the L.crocea with a gain coefficient of 36 and a body length of about 22 cm is mainly distributed in the position where the absolute water depth is 5～7 m and the weighted relative water depth is 0.4～0.8.That is,L.crocea are mostly distributed in the middle and lower layers of the aquaculture area.(4)Marine environmental conditions such as temperature,salinity,pH,and dissolved oxygen in the aquaculture area change little,and L.crocea are often distributed in areas with relatively weak illumination intensity. |
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