Distribution detection of Larimichthys crocea cultured in large net-enclosure aquaculture by Small Unmanned Surface Vehicle

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 advantages in providing the right conditions for growth which are 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 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. In order to obtain the distribution characteristics of L. crocea in the large-scale net-enclosure aquaculture area, a multi-function Small Unmanned Surface Vehicle equipped with the sensors of sonar fish detector, temperature, salinity, pH, illumination and dissolved oxygen has been successfully developed. The fish detector (DE-FR01 200 kHz) 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 interval 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 area, we obtained the findings that, ① in terms of horizontal distribution, L. crocea is mainly distributed in the areas with rich reefs and slow water flow in the large-scale net-enclosure aquaculture area; ② in the vertical distribution direction, L. crocea is mainly distributed in the middle and lower layers of the aquaculture area with weighted relative depth of 0.6~0.9; ③ the overall changes of temperature, salinity, pH and dissolved oxygen are small in the net-enclosure aquaculture area. L. crocea is often distributed in areas with relatively weak illumination intensity of (5 921±2 702), (50 799±50 988), (5 990±542), (3 494±695), (6 836±4 761) and (15 395±5 531) lx. In this study, the distribution characteristics of L. crocea in net-enclosure aquaculture area and the influence of marine environment on its distribution were studied systematically for the first time. The research results may provide the basis for the site selection of operation platform, feeding management and catching design of L. crocea.