ACOUSTIC METHODS APPLIED TO FISH ENVIRONMENTAL-STUDIES IN THE BALTIC SEA

Acoustic methods can play an important role in studies of fish group b ehaviour in relation to environmental factors. This paper presents new approaches to treating acoustic, biological and hydrological data col lected during surveys of pelagic fish resources in the Baltic Sea. It must be underlined that the Baltic ecosystem is characterized by low s alinities (2-20 PSU) and a two-layer structure. A method of macrosound ing, giving for single transects a graphic visualization of current sp atial gradients in fish distribution in macro-scale was enriched by ad ding the overall community structure (e.g., temperature, salinity, oxy gen, etc.), expressed by appropriate isolines. A more developed versio n, called 'matrix macrosounding', designed for description of cross-se ctions, characteristic for the Baltic ecosystem, structured in the for m of elementary rectangles (matrix elements), is introduced and exampl es of its application for short-term or long-term ecological studies a re given. Basic diel fish environmental preferences were estimated by correlating acoustic and hydrological data collected over the period 1 981-1996 in the southern Baltic. Preliminary models of day life-cycle of fish in relation to main environmental factors were then formulated , and applied to fish behavioural studies. Examples of fish behaviour models, determined for the autumn, are shown. (C) 1998 Elsevier Scienc e B.V.