桁杆张网渔具菱形和方形网目网囊的选择性研究

使用套网法作为选择性试验方法,Logistic曲线作为选择性曲线模型,并运用极大似然估计法进行曲线参数估算,对桁杆张网渔具不同网目尺寸(25mm、30mm、35mm、40mm以及45mm网目大小)、不同网目形状(菱形网目和方形网目)的网囊选择性进行分析。结果显示:各菱形网目网囊对黄鲫和银鲳的50%选择体长(或叉长)比相应方形网目的大(分别为3.79%和31.25%),而对小黄鱼的50%选择体长比相应方形网目小(7.92%);各菱形网目对黄鲫、小黄鱼和银鲳的选择范围都比相应方形网目大(分别为20.76%、5.36%以及45.99%)。这与传统认为方形网目选择性优于菱形网目的观点存在差异。通过讨论,认为黄鲫和银鲳体形较扁,游泳能力较弱是造成这些差异的主要原因。因此,在进行网囊结构改革时,不能盲目遵循方形网目优于菱形网目的传统观点。

Study of the mesh selectivity of diamond and square mesh codend of two-stick swingnet

The selectivity of square and diamond mesh codends with different mesh sizes (25mm, 30mm, 35mm, 40mm and 45mm) of two-stick swingnet was analyzed through covernet method selective experiments, in which Logistic curve was regarded as the selection curve and the maximum likelihood method as estimation method of the parameters of selectivity model. The result demonstrated that the 50% retention body length (or fork length) for Setipinna taty and Pampus argenteus of all diamond mesh codends were larger than those of all square mesh codends (3.79% and 32.15%, respectively), while that for Pseudosciana polyactis was smaller than that of square mesh codends (7.92%). Additionally, the selection ranges for the three species mentioned above of diamond mesh codends were larger than those of square mesh codends (20.76%, 5.36% and 45.99%, respectively). There exists some difference between the experimental result and the traditional opinion. In the discussion, we consider that the relatively flat body shape and the low swimming ability of Setipinna taty and Pampus argenteus mainly contribute to the difference. As a result, the traditional viewpoint that square mesh is superior to diamond mesh should not be adhered blindly to when we want to reform the construction of codend.