Diel dynamics of acoustic measurements of Baltic fish

Abstract

Although acoustic methods play an important role in monitoring vertical and horizontal fish distribution, for example for fish stock assessment, the fish community as an acoustic target is not precisely specified and is seldom related to behavioural reactions. This paper describes a new type of approach to enhance the possibilities of analysing the time-depth diel variability of the acoustic parameter, s v , which characterizes the volume density of fish echoes, and is closely related to the backscattering cross-section of fish.Multi-channel integration of echoes was applied to estimate the average diel models of s v -dependence on the time of day in separate depth layers. The final characteristics were calculated by computer as a vectoral field of gradients, expressing quasi-continuous diel transformation of the time-depth distribution of s v values. Examples based on data collected during RV “Baltica” research cruises carried out in October in 1994-1996 are given and discussed. A high level of variability in s v values with time and depth was found; diel phases of fish behaviour were characterized. Similarity among the cruises indicated a highly repeatable pattern of fish behaviour. The differences were mostly correlated to water temperature distribution. Analysis of the results indicates the necessity of further research on fish target strength, as a group behavioural phenomenon, within separate periods of the diel cycle to define biomass conversion factors more accurately. The method presented could significantly enrich detail and lead to more systematic physiological studies on fish behaviour.