Acoustic methods applied to fish environmental studies in the Baltic Sea

Acoustic methods can play an important role in studies of fish group behaviour in relation to environmental factors. This paper presents new approaches to treating acoustic, biological and hydrological data collected during surveys of pelagic fish resources in the Baltic Sea. It must be underlined that the Baltic ecosystem is characterized by low salinities (2-20 PSU) and a two-layer structure. A method of macrosounding, giving for single transects a graphic visualization of current spatial gradients in fish distribution in macro-scale was enriched by adding the overall community structure (e.g., temperature, salinity, oxygen, etc.), expressed by appropriate isolines. A more developed version, called 'matrix macrosounding', designed for description of cross-sections, characteristic for the Baltic ecosystem, structured in the form of elementary rectangles (matrix elements), is introduced and examples of its application for short-term or long-term ecological studies are given. Basic diel fish environmental preferences were estimated by correlating acoustic and hydrological data collected over the period 1981-1996 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 Science B.V.