Abstract
The writers have already published (1967) a study on the efficiency of the vertical fish finder used in the Northern Pacific salmon fishing grounds in regard to the sigle fish image constituted by a single fish image of Alaska pollack, but not by that of salmon and trout. So we carried out experiments for the purpose of developing a salmon fish finder. The performance of fish finder for salmon, having a frequency of 28 KHZ, is as follows: 1. The results of various experiments by the present writers as well as by others, prove that the swimming layer of salmon at daytime, and also at nighttime, is not more than 50m in depth. So the detection layer need not necessarily cover deeper than 50m in depth. 2. The reason why on salmon image appear with the use of the vertical fish finder may perhaps be due to the fact that the salmon run away as the fishing boats approach, thus slipping out of the sound beam of the fish finder. The direction of the sound beams of our new salmon fish finder, manufactured for trial purposes, can be transmitted towards the ship side downward at 30° oblique. 3. In order to separate the image of salmon from that of Alaska pollack, we attached a special circuit (fish discriminating circuit) to let the smaller reflection level of Alaska pollack cut off because of its smaller size compared with that of salmon, and also a logarithm amplifier to supplement a propagation attenuation of ultrasonic waves by distance. The results of such experiments proved a close correlation between the number of a single fish image as recorded by the net setting and the number of salmons caught by the gill net next morning. The recorded single image of fish is reasonably identified to be that of salmon.
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