Preliminary Study of the Macrofouling Invertebrate Community on a Wooden Fishing Vessel

Abstract

Sixth codend model nets,including single twine knotted diamond meshcodend,double twine knotted diamond mesh codend,knotless diamond meshcodend and knotted square mesh codend,were made according to the modelrule.Observe the change of net shape and stretching of the mesh in thecirculating water channel under equal current speed and different fishing catch.The drags of the single and double twine knotted diamond mesh codendand knotless diamond mesh codend and the percentage of fishing catch in thecodend showed a linear correlation.The relation between the persentage ofthe volume of fishing catch in knotted square mesh codend and the drag had aexponential relation.In case of empty net,the knotless diamond codend becamecone shape.The single and double twine knotted diamond codends andknotted square mesh codend showed a cylinder shaped spreading.When thefishing catch in the codend was suitable,single and double twine knotted diamondmesh codends had a sphere shape;knotted square mesh codend had a cylindershape;knotless diamond codend had a shape between a sphere and cylinder.Average stretching degree of mesh angles were 20～40 degrees for knotless meshcodend;30～40 degrees for single twine knotted mesh codend;35～45 degreesfor double twine knotted mesh codend.The strecting degree of mesh angle ofknotted square mesh codend remained at 90 degrees.The escaping behavior offish in codend,except the structure of codend,spreading shape and meshstretching degree of mesh angle,it was mainly affected by the mesh size andshape of fish.為瞭解不同結構的袋網，在曳行過程中，袋網各部位之網目展開狀及選擇效果。依據模型法則，製作縮小比為1/6的單、雙線結節菱形網目、無結節菱形網目及方形結節網目等四件模型袋網。在環流水槽內觀察袋網在等流速、不同漁獲量時，其網形的變化與網脚的展開情況。其結果摘要如下：（1）單、雙線結節菱形網目袋網及無結節菱形網目袋網之抗力大小與袋網內漁獲物體積所佔之百分比，呈現高度直線相關。結節方形網目袋網內漁獲物體積所佔之百分比與抗力大小之關係，則呈指數相關。（2）在空網時，無結節菱形網目袋網呈現圓錐形；單、雙線結節菱形網目袋網及結節方形網目袋網則呈圓筒狀。在袋網內漁獲物適當累積時，單、雙線結節菱形網目袋網澎脹成圓形曩狀；結節方形網目袋呈現圓柱狀；無結節菱形網目袋網之形狀則介於圓柱狀與圓形曩狀之間。（3）袋網網目之展開角度，無結節菱形網目袋網介於20°～40°之間；單線結節袋網介於30°～40°之間；雙線結節菱形網目袋網介於35°～45°之間。方形網目袋網則維持在90°。（4）魚類在袋網內之逃脫行為，除與袋網結構、網目展開形狀及網目展開度有關外，主要是受網目大小及魚類體型之影響。