Prediction of hydroacoustic target strength of vendace (Coregonus albula) from concurrent trawl catches

Despite frequent applications of hydroacoustic methods to estimate freshwater fish densities and behaviour, species-specific regressions to predict fish total length (TL) from target strength (TS) and vice versa are surprisingly rare in literature. Here, night trawl catches and trace track analyses of concurrent hydroacoustic surveys of a vendace stock in a deep lake during three sampling occasions over 2 years were combined. Frequency distributions of TL and TS revealed three modal lengths on each occasion and were used to calculate a linear regression (TS (dB) = 25.5 × log10 TL (cm) − 70.9) which predicted higher TS values than the standard formula by [Love, R.H., 1971. Dorsal aspect of an individual fish. J. Acoust. Soc. Am. 49, 816–823] for vendace smaller than about 12 cm TL, but lower TS values for larger fish. In an alternative approach, an iterative estimate of the intercept of the TS–TL equation based on the main mode per frequency distribution was conducted and this also substantially differed from the general formula of Love. By correlating the cumulative TS distributions from the echo tracking and those calculated from the equations based on either three or one mode per sampling and the species-unspecific formula of Love, the three modes approach and the Love formula performed comparably well, whereas the one-mode approach did not reliably predict the TS distribution of the tracks. A re-calculation of a previous hydroacoustic survey from the same lake revealed that the newly derived vendace-specific formula predicted a lake-wide vendace biomass about 13.8% lower than one based on the formula of Love.