In this paper, experimental results on the equivalent backscattering cross-section and target strength (TS) of individual and small monospecific groups, of encaged but freeswimming fish, are given. Three freshwater species are involved in this study : arctic char (Sa/velil1us a/pil1/1s), roach (RI/ti/us /'lili/us) and perch (Perca jl/lviatilis). The acoustical system is a broad-band sonar system, using a linearly frequency modulated chirp (20 to 140 kHz in 0.5 ms). Il was calibrated using a 60 mm diameter calibration copper sphere. The dependence between the target strength and the weight and frequency (in the range of 50-140 kHz) obtained with least-squares regression methods are presented. Regression coefficients are given with a 3 kHz step width. A positive slope against weight and a slightly negative against frequency of the TS dependence are shown. Il is also shown that these general results change deeply with environmental conditions as illustrated by two experiments on perch. The obtained data demonstrate also that the averaging over a broad frequency range makes measurements more independent of the fluctuations in the target's response and less sensitive to external parameters.