商用周波交流による電氣スクリーンの性能について
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概要
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To obtain necessary data for designing an electric fish screen more efficient than those tested previously, effect of electric current on fish has been studied along with field tests on the performance of an experimental screen. 1) Effect of electric current of fish:-Electric potential gradient to which fish would react was measured when the current ran in parallel or came at right angles with the body of fish. Critical potential gradient to paralyze a fish was also investigated. The results of the experiments are given in Table 1 and Figure 3. 2) Field test on the performance of an experimental screen:-From a practical point of view a vertical grid type of electric screen was employed to supply an alternating current of 50 or 60 cycles respectively. The screen, as illustrated in Figure 1, consisted of a series of vertical grids of copper wire hung from the feeders spanned across a stream leading to a fish farm. It was arranged in accordance with Tautis theories (1932) so that distribution of the electric potential in the fields between the consecutive grids or electrodes was intensified just as the fish swam ahead by adjusting distance between the poles or using different voltage of the current. Two sets of the screen were installed across the water way, dividing it into three sections A, B and C, as illustrated in Figure 4. A net made of wire or thread to be placed at the ends of the section will serve to keep the fish inside. Keep a similar net between the sections until the fish get calm down in the section B, and when counting the fish fled into sections A and C. Release a certain number of fish in section B, then start the operation to observe effect of the electric current upon behaviors of the fish, taking the net partitions out of the water. Because of difficulty in counting the number of fish driven back by the screen, the screening efficiency was expressd in a ratio computed as N-Nf-Ne/N where N represents the number of fish, Nf the number of fish fled through the sereen, and Ne the fish electrocuted. Table 2 indicates the results thus obtained with detail of the experimental cenditions. Since the potential gradient was distributed as shown in Figure 2, the fish, when they swam in group, would turn back just before reaching the first grid or the second. Even when happened to pass through the first, they would soon come back and remain swimming about in a safe area of B where little or no electric potential-gradient existed. However, when a fish swam independently from the group, it often rushed into the fields inside of the grids to be benumbed, or in the worst, electrocuted. Small sized fish or youngs, especially those reared in a hatchery, were found difficult to be screened away by the electric current.
- 公益社団法人 日本水産学会の論文