Studies on fishing mechanism of semi-trammel net

1988年10-11月和在东京湾进行了捕捞试验，以测定半拖网对鱼的选择性和捕捞效率。结果表明，计算了小孔网目尺寸的选择性，绘制的选择性曲线向右显示，3.6、5.1和6.8 cm网目的最佳长度分别为13.5、18.8和25.5 cm。垂直松弛的变化对选择性曲线的形状没有任何实质性的影响。用Matuda预测刺网浮线高度的近似公式来估算索网和半索网的浮线高度。将估计值与水槽试验的实际值进行了比较。在室外水箱中进行了半拖网和刺网的捕捞试验，已知其种群数量和叉长分布。光强对刺网渔获量的影响大于刺网，而网目尺寸对刺网渔获量的影响大于半刺网。在渔获物符合体长分布的情况下，刺网的捕捞效率高于半网式。这一现象与东京湾的捕捞试验结果十分一致。

Fishing experiments were conducted in Tokyo Bay in October-November, 1988 and 1989 to determine the selectivity and catching efficiency of semi-trammel net to gizzard shad. Results were showed that the selectiveties of the mesh sizes of small-meshed net were calculated and the plotted selectivity curves were showed to the right with optimum lengths of 13.5,18.8 and 25.5 cm for 3.6,5.1 and 6.8cm mesh size, respectively. Variations in vertical slack did not show any substantial change on the shape of the selectivity curves.Matuda's approximate formula for predicting the floatline height of gillnet was utilized in estimating the floatline height of trammel and semi-trammel net. The estimated values were compared with the actual values obtained from a flume tank experiments. The resulting estimating values were found to be in close agreement with the actual values.The fishing experiments for semi-trammel net and gillnet were conducted in out door water tank by using experimental fish (rainbow trout), of which the population size and the fork length distribution were known. The effect of light intensity on catch by semi-trammel net was greater than that by gillnet, while the effect of mesh size on catch by gillnet was greater than that by semi- trammel net. On case where the fish caught were suited into the body length distribution, the catching efficiency of gillnet was higher than that of semi- trammel net. This phenomena was in close agreement with the results obtained in the fishing experiments in Tokyo Bay.