Sources of the variability of acoustic response of clupeoids in the Baltic

Acoustic information, collected in the form of calibrated measurements of integrated echo energies, has been applied at the Sea Fisheries Institute since 1981 to examine the relationships between clupeoids distribution in the Baltic and the relevant environmental factors. Data were collected during different seasons for each elementary distance unit (EDSU) at standardised depth intervals and compared to the values of selected environmental parameters measured in parallel. Acoustic, biological and hydrological data were correlated temporally and spatially, then transferred to a comprehensive database, thereby enabling the 4D analysis of the factors characterising a wide range of fish behaviour. Selected characteristics describe the variability of environmental factors during the short- and long-term life cycles of clupeoids. Apart from the analysis of abiotic factors, specially prepared cross-sections of the herring in different stages of gonad maturity are presented. The aspects of variability have been described for their inclusion in the measuring and modelling herring and sprat target strength, particularly under the specific circumstances of the Baltic Sea.