On the possibility of estimating year-class strength by measuring echo-abundance of 0-group fish

Abstract

The possibility of estimating the distribution and abundance of 0-group fish by a combination of echo surveying and fishing experiments with pelagic trawl and purse seine is discussed. The existence of scattering layers, their origin, and the identification of sound scatterers are discussed with special reference to observations made in recent years off northern Norway and in the Barents Sea. The scattering layers observed in late summer and autumn in this area consist mainly of 0-group fish of which herring, cod and haddock are most important. The transport of the larvae from the spawning places in relation to the current systems is described. The observations indicate that during August to October fish fry are abundant in the surface layers of the north and east going currents, and by the end of autumn the fry are aggregated along the frontiers between the cold and warm water masses covering the area from Spitsbergen to Bear Island and further to the east and south over the central and south-eastern parts of the Barents Sea. The relation between echo signals received from scattering agents distributed at a constant depth and the amount of scatterers is discussed, as well as the effect of variations in depth distribution. Special attention is paid to the signal strength received from targets uniformly distributed in a layer or school of wide horizontal distribution. The needs for theoretical as well as empirical studies of the relationship between the amount of echo signals received and the abundance of sound scatterers are stressed. The first results of charting the echo-abundance distribution are presented. For this purpose an electronic echo-integrator was developed to measure exactly the signal voltages received. Some preliminary data on the relationship between echo-abundance and the catch of fish fry with purse seine are also given.