Abundance of small fishes of the west coast of Istra estimated by use of the pelagic trawl and echo sounding
Abstract
The paper comprises five chapters. The first of them deals with the significance of small pelagic fish for the Yugoslav fisheries and with the difficulties in estimation of available quantities. A description of the investigated area is given in the second chapter. Roughly delineated, the area extends between the 13th meridian and the 45th parallel, bordering on the Yugoslav coast. The third chapter contains all the data in connexion with investigations and the results obtained by them. All the values, indispensable for an estimate, are also given. The values concerned are the following: 1) a 1,725 nautical miles exploratory cruise was undertaken, making it possible to investigate an area of 800 square nautical miles; 2) the investigations, performed during three successive years, took place by day only; 3) it took a total of 39 days (126 hours) to perform the operations comprising 89 hauls by trawl; 4) the speed of hauls ranged from 3 to 5,5 knots, and the net mouth was 150 m2; 5) 400 nautical miles were covered while hauling the trawl; 6) the weight of the total catch in the course of the operation amounted to 20,000 kg; 7) 942 fish signals were recorded by means of the ultrasonic detector. The fourth charper deals with the working method consisting in tracking down the fish schools. An estimate of the quantity of fish in the investigated area is made in the fifth chapter and put at 37,000 tons. The latter value was arrived at by considering a) the total catch; b) the quantity of sea water that seeped through the net (111,000,000 m3); c) a half of the water volume in the investigated area — all these enlarged five times. (A half of the water volume was taken in consideration of the fact that fish had been detected and caught almost exclusively in the lower half of the water column, and all was enlarged five times because the experience has showed that only a fifth of fishes present in an area get caught when using pelagic trawls in shallow water).
