Abstract: Fish target strength (TS) is the key quantity in the acoustic assessment of fish abundance and biomass. Since TS is the function not only of fish length, but also depends on species, physiological state, behavior, and environmental parameters, calculation of fish real length on the basis of acoustical data is not a trivial task. Commonly fish size distribution is estimated from TS distribution using Love’s formula. In the present paper fish target strengths for typical European freshwater species were determined experimentally and based on these results a mathematical model has been constructed. The model accounts for the TS/L relationship for a given species and for fish behavior. From the single fish measurements in cages three types of fish behavior were distinguished and incorporated into the model. Fish size distributions resulting from the model were compared with those obtained using directly Love’s formula and from the net catches.