Studies on fishing mechanism of semi-trammel net

1988年11月和1989年11月在东京湾进行了半厚网对砂眼的选择性和捕捞效果试验。结果表明，在3.6、5.1和6.8cm的网孔尺寸下，最佳网孔长度分别为13.5、18.8和25.5cm。垂向松弛度的变化对选择性曲线的形状没有明显影响，Matuda提出的刺网浮线高度的近似计算公式被用于估计重刺网和半重刺网的浮线高度。将估算值与水槽试验中得到的实际值进行了比较。在室外水池中，用已知种群大小和叉长分布的实验鱼（虹鳟鱼）进行了半厚网和刺网的捕捞试验。光照强度对半厚刺网渔获量的影响大于刺网，网目尺寸对刺网渔获量的影响大于半厚刺网。在所捕鱼类符合体长分布的情况下，刺网的捕捞效率高于半厚刺网。这一现象与在东京湾进行的捕捞试验结果十分吻合。

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.