The Obstructive Effect of Lead Net on Passing of Fish

The author conducted fundamental experiments on the obstructive effect of net on fish movement. No. 1 experiment was done with 3 groups of carassius auratas, each group consisting of 20 individuals measuring 5, 10 and 15cm in respective mean body length. For model nets 7 frames were made and fine cremona thread was streched vertically on each frame at intervals of 15, 10, 8, 6, 4 and 2cm except for the 7th one which had no thread as a contrast. The model net was arranged in the center of the tank. (Fig. 1) The fishes were put in a box which was arranged one side of the tank and after 5 minutes they were liberated in the tank by lifting the lid. The number of fish which passed through the frame, avoided the net and remained in the box was counted for 5 minutes. The results are shown in Table 1. The obstructive effect of net differs by the size of fish and thread interval. No. 2 experiment was done with the total of 35 individuals of Carassius auratas in 5 groups each group consisting of 7 individuals. Their average body length, height and breadth were 74mm, 29mm and 14mm respectively. For model nets, 4 fremes were made and fine cremona thread was streched on each frame at intervals of 1.5, 2, 3 and 4cm. Since the purpose of this experiment was to know the obstructive effect of model net when used vertically and horizontally, the same frame was used vertically and horizontally, being arranged in the center of the tank. (Fig. 2) In this experiment the fishes were driven to the model net from one side of the tank by the driving net, a small mesh net, with a speed of 8cm/sec. When the fishes were driven up to a certain position, they passed through the frame. The distance of the driving net from the model net was observed and the result is given in Table 3. A significant difference between vertical net and horizontal net was observed only when the interval was 1.5cm and 2cm