Behaviour oj jish is one oj most potential sourees oj bias in jisheries aeousties. The paper describes methods and results oj analyses oj unique aeoustie, hydrologie, and biologie data base eolleeted by researeh vessels oj Sea Fisheries Institute in the southern Baltic in the years 1989-1998. On the basis oj numerie models oj measurements, interactions between environmental and behavioural jaetors and fish aeoustie response have been estimated. Significant influence oj environmentally modulated

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... jish behaviour on effective value oj the aeoustie back-scattering cross-section was jound, deseribed, and diseussed. Results indicate a necessity oj jurther research on fish target strength within separate periods oj a diel cycle, to define more accurately biomass eonversion jactors or to eliminate jrom the surveys the periods oj the day, characterized by high instability oj jish acoustic properties. Models oj environmental background oj jish diel eycle ean be also applied in wider ecological studies.