套网法对张网渔具选择性的影响

套网法是一种比较常用的选择性研究试验方法,但是,使用套网法会对网囊网目选择性产生一定的影响.通过使用5种不同规格的方形网目网囊分别进行试验,比较两种不同的试验方法即套网法和平行作业法对张网渔具网囊网目选择性进行研究.结果显示,使用平行作业法估算的50%选择体长比使用套网法所得的大7.8%,选择范围比套网法的估算值小24.5%,选择性因素比套网法估算值大7.4%,但差异并不显著(P＞0.05).在张网渔具最适网目试验中,如果直接使用套网法的试验结果制定最小网目,那么这一尺寸会偏大,造成部分鱼类过多释放.

Influence of cover net method on stow net selectivity

The cover net method is widely used in testing fishing gear selectivity. However, there exists influence of this method on the mesh selectivity of codend because of the blocking (mask) effect. In this paper, the selective characteristics of the codend by cover net method were compared with those by the parallel haul method based on the square mesh codends with the mesh sizes of 25, 30, 35, 40 and 45mm respectively. The selectivity curves were described by the logistic equation. The selective parameters and the main selective indexes were estimated by the maximum likelihood method in the case of cover net method, and by the SELECT model when using parallel haul method. The standard errors of the parameters were estimated by the Fisher information matrix and the goodness-of-fit of different models was assessed by the AIC (AKAIKE information criterion). The selectivity of different codends was obtained by the analysis of the catch data of anchovy (Setipinna taty). In the selective analysis of parallel haul method, it was found that the equal split model was fitted better than the estimated split model through the comparison of the AIC. Then the main selective indexes estimated from different experimental methods were compared and the result demonstrated that the 50% retention lengths, the selective ranges and the selective factors obtained from the parallel haul method are respectively 7.8 percent larger, 24.5 percent less and 7.4 percent larger than those estimated from the cover net method, but no significant differences (P>0.05) among them were found. If the minimal mesh size of stow nets is determined based on the result from the cover net method, the mesh size will tend to be bigger, resulting in excessive escape of the fish inside the codend.