| For recent years, acoustical technology has been one of important methods of acoustical surveys and monitoring.it is more effective, easier and do not harm fish stocks, comparing with other methods in both acoustical surveys and monitoring. The target strength of fish(TS) is an important parameter in both fish cage monitoring and acoustical surveys.Yellow croaker, Pseudosciaena polyactis and pomfret are important commercial fishes in china east sea, where Yellow croaker is an important aquaculture object, Pseudosciaena polyactis and pomfret are important fishing targets so the study on the target strength of that is the basics of rational exploitation and scientific breeding.In this article, we studied several commercial fishes in China East Sea with both theoretical and experimental methods,and used ellipsoid model to compute target strength of Yellow croaker, Pseudosciaena polyactis and pomfret. We developed an adjustable scattering angle experimental instrument for target strength measurement, and applied this instrument to study the relations among target strength, tit-angle and length of Yellow croaker, Pseudosciaena polyactis, and pomfret, coming up with an empirical equation of target strength changing with weight and length of fish body. Comparing with experimental and theoretical results of TS changing with tilt-angle, we established a modified ellipsoid model. Then we used this model established a fish cage monitoring model and discussed the method of Pseudosciaena polyactis’ resources investigation, the main results are showed as below:We used adjustable scattering angle experimental instrument to measure Yellow croaker, Pseudosciaena polyactis, pomfret, Pagrosomus major. After studying, we found that Yellow croaker, Pseudosciaena polyactis and pomfret’s target strength depend linearly on logarithm of its body length and weight where the equation is:TS=23.5log L-75.8, TS=23.5log L-73.1, TS=14.1log L-83.4,TS=8.8Log G-64.4, TS=8.7Log G-60.3, TS=4.9log G-76. Yellow croaker, Pseudosciaena polyactis, and pomfret with different specifications’ target strength has roughly same trend changing with tilt-angle. The ellipsoid modeling results has big difference between the experimental results, but after modified, the ellipsoid model has very good reaction on the target strength changing with tilt-angle and has a certain reference value on target strength studying. All the results above shows that, the tilt-angle and body length of fish are important parameters that reflect TS, we should concern enough of that during both acoustical surveys and fish cage monitoring; The establishing of yellow croaker’s fish cage monitoring model and discussing of Pseudosciaena polyactis’ resources investigation has an important practical significance in both yellow croaker’s fishery industry and Pseudosciaena polyactis’ protection. |
