An Echo Survey on the Distribution of Fish Schools that Pass into the Trawl Net Mouth

Abstract

Some field experiments on board a two-boat trawler were made with an under-water echo-sounder of 200 kHz in the East China Sea in November, 1973.
Throuthout the experiment, the three transducers were tightly fixed onto the three respective positions, i. e., the center and both ends of the front head rope of the square part of the trawl net. Thus, three series of echograms from fish schools, which pass through under the respective transducers, were drawn separately on an echo chart during trawling at a speed of 4 knots.
The interrelation between the number of fish traces on the respective echo charts and the catch per haul were considered acoustically and statistically as follows:
1. The amount of fish school that passes into the trawl net along each sides near the wing net at the net mouth, might be about 26% more than that center part.
2. The highest correlation coefficient of catch per haul among respective number of fish, estimated from three series of echo charts per haul, was found to be 0.72 for that from the center transducer.
3. The author suggests that it may be possible for the transducer in such a system to be attached onto the center of the head rope to predict the amount of trawl catch.