Acoustic studies of spatial gradients in the Baltic: Implications for fishdistribution

Acoustic methods play an increasingly important role in studies of fish gro up behaviour in relation to environmental factors. They are also becoming a promising tool in the evolution of new standards in marine ecosystems rese arch. This paper presents two approaches to treating acoustic, biologic, an d hydrographic data, collected during surveys of significantly large spatia l units of the marine ecosystem. Both methods are designed for studying the spatial structure of abiotic and biotic factors. Firstly, a method for est imation of vertical gradients in environmental factors is defined. Correspo nding to the main range of fish occurrence, it characterises fish distribut ions by day and night in the seasons spring, summer, and autumn of the year s 1983-1996. Secondly, the method of matrix macrosounding, which correlates acoustic and hydrographic data, has been improved and employed to study th e horizontal gradients in fish distribution linked with environmental struc tures. A few applications of the method for short-term and long-term studie s are shown and discussed. (C) 1999 International Council for the Explorati on of the Sea.