On Measuring the Dropping Rate of Salmon Gill Nets by Means of Underwater Television Techniques

It is very important to measure the dropping rate of salmon gill nets in the high seas from the standpoint of gill net efficiency and resources problems. The measurements of the dropping rate of a salmon gill net of 25 tans (1 tan is 50 meters in length) were carried out four times during the fishing practice of the Hokkaido University training ship, Hokusei Maru, in the Okhotsk Sea, from 1963 to 1967. The measuring method was as follows: The netted fishes in every tan were counted and recorded by an underwater television camera and a video tape recorder mounted on a specially constructed observation boat running at slow speed (1.5-2.0 knot) in parallel with, and about 5 meters apart from, the gill nets. The dropping rate was calculated according to the number of times of the recording operations during the night. The results obtained are summarized as follows: 1) The dropping rate of 25-tan salmon gill nets (1250 meters in length) could be measured under approximately natural conditions by means of underwater television techniques. The dropping rates obtained from four experiments were 4%-25%, and did not change no matter how numerous all netted fishes were. 2) The effects of the state of the sea on the dropping rate are not very clear. 3) The dropping rates of 15-tan salmon gill nets (mesh size 115mm) were always larger than those of 10 tans (mesh size 121mm) in all four experiments.