Environmental effect on acoustic measurements of Baltic fish

Behaviour of fish is one of most potential sources of bias in fisheries acoustics. The paper describes methods and results of analyses of unique acoustic, hydrologic, and biologic data base collected by research vessels of Sea Fisheries Institute in the southern Baltic in the years 1989-1998. On the basis of numeric models of measurements, interactions between environmental and behavioural factors and fish acoustic response have been estimated. Significant influence of environmentally modulated fish behaviour on effective value of the acoustic back-scattering cross-section was found, described, and discussed. Results indicate a necessity of further research on fish target strength within separate periods of a diel cycle, to define more accurately biomass conversion factors or to eliminate from the surveys the periods of the day, characterized by high instability of fish acoustic properties. Models of environmental background of fish diel cycle can be also applied in wider ecological studies.