江浙沿海桁拖网作业性能的初步研究

本文报道了江浙沿海桁拖网作业性能试验的初步结果。作者将现用的桁拖网分为三个类型、五种结构模式，经过网具模型静水池试验和海上实测，对各类桁拖网的作业性能进行了比较研究。试验的结果表明，在现用的各类桁拖网中，以大网目双囊虾、蟹两用拖网的性能最佳，在相同拖曳阻力条件下，其网口高度最大，而且生产上能兼捕虾蟹。但其网口部位的网目似乎存在偏大的缺点，有漏虾现象。在其他各类中，小网目多囊桁拖网的性能一般优于双囊桁拖网。作者认为，桁拖网捕虾的渔获效率主要取决于网具的水平扩张即桁杆长度。现用桁拖网的桁杆长度已达到渔捞操作所允许的最大长度，但实际作业拖网所用功率一般都在渔船额定功率的10％以下，存在着能耗极不经济的状况，对此，极有研究改进之必要。

PRELIMINARY STUDY ON OPERATING PERFORMANCE OF BEAM TRAWLS IN THE OFFSHORE WATERS OF JIANGSU AND ZHEJIANG PROVINCES

The operating performance of beamtrawls has been carried out in offshore watersof the Jiangsu and zhejiang provinces, during March 1982 through July 1983. Basedon fishing survey, a series tests for four kinds of model beamtrawls have been done instill water tank of Nanjing Forestry College in Apr. 1983. Model nets and actualtrawls were calculated according to the criterion of M.Tauti. The large scale ratio is1/10 and the small scale ratio is 1. Moreover,beamtrawling tests were actually takenin July 7--8, 1983, The power of vessel engine is 120 hp. Towing speed and the warptension are measured. the reading is taken with scale lupe (×10). The tension is lookedup from calculated line. The height of trawl mouth is measured with normal dista-nce meter. The meter is tied on centre of the beam and its gas bag is towed behindfootrope with a short wire. The declination angle of the warp is also measured.The results are concluded as follows:1. Current beamtrawls of Jiansu and Zhejiang are divided into three kinds. Theyare small mesh with twin bags, small mesh with multi--bag all for catching shrimpsand big mesh one with twin bags for catching both shrimps and crabs.2. The common similarities of these beamtrawls is with wider horizontal spread. smaller vertical opening, low towing speed and closely touching to sea bottom. Thesebeamtrawls fully touch to bototm and hence they can effectively catch both shrimpsand crabs, and protect fish resources from harm as well.3. Among these trawls, in the operating performance the big mesh beamtrawlwith twin bags obtains the best result, it can catch not only shrimps but also crabs,and so it is more efficient. But the mesh size is too big that a certain rate of shrimpswill be lost. as to the small mesh beamtrawls that with multi-bag is better than that oftwo bags. The weak point of the small mesh beamtrawls with twin bags is that itshanding ratio is too smaller the net could not spread widely as the trawl was towing.Its belly was attached to sand bottom easily.4. The spent power for operation of all these beamtrawls is less than 1/10. Butall length of the beam almost reaches the limit of operation allowed. In order to getlarge horizontal spread further, it is suggested that to adopt the operation method ofthe veesel with two or three nets. It's necessary to keep the beamtrawl balance intowed period. Finally, this fishing gear and fishing method should be developed offthe shallow waters, and avoid in making any harmness to the resource of fish.