体長と体周の関係から求められる刺網網目選択性曲線

抄録

It is assumed here that body girth for each length of fish is distributed according to a normal distribution with a common standard deviation, and that all fish are held anywhere between the end of the opercle and the point of maximum girth by the mesh encircling the body. When a fish population encounters the gill-net, some fish will pass through the mesh because their maximum girth is smaller than the perimeter of the mesh. Some fish will alter their swimming course because they do not enter far enough to be held fast and the others will be retained. The probability that fish of length l do not pass through the mesh can be described by the following quation:
P1=∫∞Gmc-(p+ql)/σGm1/√2πe-x2/2dx.
And the probability that fish which enter the mesh can be described by the following equation:
P2=∫Goc-(v+wl)/óGm-∞1/√2πe-x2/2dx.
The probability of fish which will be retained may be described by the area under both the cumulative distribution curves.
This theoretical mesh selectivity curve was applied to the herring gill-net. The lengthgirth relationships of herring were:
Gm=0.659l-3.106, Go=0.487l-1.089.
The values of σGo were approximately common for various lengths, but the values of σGm showed increasing trends according to the increase of length. The averages were σGo=3.57mm and σGm=6.56mm. Goc and Gmc were estimated from the relationship between the mesh-perimeter and girth at the net-mark. Goc=2.15∅, Gmc=2.22∅. From the above values, the mesh selectivity curves for each mesh-size were calculated, and the curves were compared with the length distribution of the herring-catch in the discussion.