Diel dynamics of acoustic measurements of Baltic fish

Although acoustic methods play an important role in monitoring vertical and horizontal fish distribution, for example for fish stock assessment, the f ish community as an acoustic target is not precisely specified and is seldo m related to behavioural reactions. This paper describes a new type of appr oach to enhance the possibilities of analysing the time-depth diel variabil ity of the acoustic parameter, a,, which characterizes the volume density o f fish echoes, and is closely related to the backscattering cross-section o f fish. Multi-channel integration of echoes was applied to estimate the average die l models of s(v)-dependence on the time of day in separate depth layers. Th e final characteristics were calculated by computer as a vectoral field of gradients, expressing quasicontinuous diel transformation of the time-depth distribution of s(v) values. Examples based on data collected during RV "B altica" 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 w as found; diel phases of fish behaviour were characterized. Similarity amon g the cruises indicated a highly repeatable pattern of fish behaviour. The differences were mostly correlated to water temperature distribution. Analy sis of the results indicates the necessity of further research on fish targ et strength, as a group behavioural phenomenon, within separate periods of the diel cycle to define biomass conversion factors more accurately. The me thod presented could significantly enrich detail and lead to more systemati c physiological studies on fish behaviour. (C) 2000 International Council f or the Exploration of the Sea.