Comparison of hydroacoustic estimates with fish census in shallow Malta Reservoir – which TS/L regression to use in horizontal beam applications?

In shallow waters horizontally aimed transducers are used to maximize usable survey volume due to the physical limitations of the environment. However, horizontal acoustic measurements of fish lengths are problematic because in mobile applications there is no way to determine the orientation of the fish relative to the axis of the acoustic beam. The draining of Malta Reservoir in Poland (average depth 3 m) provided an unique opportunity to check the accuracy of acoustic estimates of fish biomass. Before draining, detailed investigations of the fish population were performed using standard gillnetting and horizontal echosounding (split beam Simrad EY500, 120 kHz). During the draining of the reservoir fish were collected and weighted. Three different methods were used to convert the horizontal measurements of acoustic energy into fish total length and the biomass estimates were compared with the results of the fish census that was performed during the draining of the reservoir. The best agreement between the acoustic estimates and the fish census of 565 kg ha−1 and 576 kg ha−1, respectively, was obtained with Method 1, which uses deconvolution to remove the effect of an assumed random distribution of fish aspect angle and a TS–TL relationship derived from a mixed population of European freshwater species (Frouzova et al., 2005). Deconvolution assumes random distribution of fish orientation, which means this assumption is likely to be valid, at least for Malta Reservoir.