Inland Waters (2014) 4, pp. 435-444 © International Society of Limnology 2014 DOI: 10.5268/IW-4.4.733 435 Article Comparing hydroacoustic fsh stock estimates in the pelagic zone of temperate deep lakes using three sound frequencies (70, 120, 200 kHz) Jean Guillard, 1 * Anne Lebourges-Daussy, 2 Helge Balk, 3 Michel Colon, 1 Adam Jóźwik, 4 and Małgorzata Godlewska 5, 6 1 INRA – Université de Savoie, UMR CARRTEL, Thonon les Bains, France, 2 UMR LEMAR (UBO/IRD/CNRS/Ifremer), IRD France Nord Bretagne, Plouzané, France 3 University of Oslo, Department of Physics, Oslo, Norway 4 Institute of Biocybernetics and Biomedical Engineering, Polish Academy of Sciences, Warszawa, Poland 5 Stanislaw Sakowicz Inland Fisheries Institute, Olsztyn, Poland 6 Polish Academy of Sciences, European Regional Centre for Ecohydrology under the auspices of UNESCO, Łódź, Poland * Corresponding author: jean.guillard@thonon.inra.fr Received 3 February 2014; accepted 13 June 2014; published 8 October 2014 Abstract Several decades of research have led to the acceptance of hydroacoustics as a reliable measurement method to monitor fsh population in lakes, but full standardisation and intercalibration are still lacking. The aim of this study was to investigate the effect of sound frequency on acoustic parameters, such as volume backscattering strength, target strength, and the estimation of fsh abundance. Data were recorded in situ using 3 frequencies (70, 120, 200 kHz) si - multaneously in 2 different lakes. The results among the frequencies were compared and statistically tested. Data from the 70 and 120 kHz frequencies yielded similar results, but the 200 kHz echosounder estimates in temperate lakes seemed different, especially in cases of high fsh abundance, which is typical of eutrophic ecosystems. This work indicates that the abundance estimates of fsh populations in temperate lakes based on 200 kHz frequency may differ from results obtained using lower frequencies, and that further study is needed. Key words: hydroacoustics, multifrequency, standardisation, target strength, WFD Introduction Lakes are impacted by many anthropogenic uses of water resources (Hermoso and Clavero 2013) that alter their ecological function. There is a need to monitor water volumes using bioindicators. Fish, due to the variety of trophic positions of fsh communities and their longevity compared with other components of the aquatic biocoenosis, have been recognised as particularly appropriate bioindicators (Karr 1981, Argillier et al. 2012). Hydroacoustics have been increasingly used in recent years and provide a wide range of information on aquatic ecosystems, from distribution and abundance of fsh populations to bottom characteristics such as bathymetry and sediment classifcations, but also on other biotic communities such as zooplankton and macrophytes (Trenkel et al. 2011). Several decades of research at sea and in freshwaters have led to the acceptance of hydroa- coustics as a reliable measurement method, both in marine and lake ecosystems (Simmonds and MacLennan 2005, Rudstam et al. 2012). Instrumentation has matured to be used routinely in scientifc studies and monitoring surveys in various freshwater ecosystems (Godlewska et al. 2004, Winfeld et al. 2007, Djemali et al. 2009, Sotton et al. 2011), but full standardisation and intercalibration are still lacking. The Study Group on Fisheries Acoustics in the Great Lakes has guided research toward measurement standardisation (Rudstam et al. 2009) and has developed a standard operating procedure (Parker-Stetter et al. 2009). In Europe, the Comité Européen de Normalisation (CEN) standard, Water Quality – Guidance on the Estimation of Fish Abundance with Mobile Hydroacoustic Methods (CEN 2009), has recently been accepted (Hateley et al. 2013). The effect of collection parameters on results must