Survey boat effect on YOY fish schools in a pre-alpine lake: evidence from multibeam sonar and split-beam echosounder data Introduction Hydroacoustics are known to provide a reliable method for estimating fish populations in the marine environ- ment, and also in lakes (e.g., Brandt 1996; Wanzenbo ¨ck et al. 2003; Coll et al. 2007; Guillard et al. 2006b; Mehner et al. 2007), dams (Djemali et al. 2009; Tuser et al. 2009), rivers (Hughes 1998; Kubecka & Duncan 1998; Matveev 2007), estuaries (Guillard et al. 2004) and lagoon channels (Brehmer et al. 2006b). Hydroa- coustic assessment using echosounding with the acoustic beam pointing straight down is the most commonly method (Simmonds & MacLennan 2005). In the last few years, it has been used for behavioural and ecosystem studies in both marine and freshwater environments (e.g., Winfield et al. 2002; Guillard et al. 2006a; Mehner 2006; Schmidt & Gassner 2006; Ce ˆch et al. 2007). Fish behaviour is important for both aquatic ecosystem research and monitoring, because of its impact on the results of fish stock assessments and ecosystem studies (Fre ´on & Misund 1999). Only a few studies have been carried out in freshwater ecosystems (Drastik & Kubecka 2005), although the consequences for stock estimations are similar to those observed in marine environments. The behaviour of fish schools in front of ships is not known in lacustrine ecosystems. The impact of a boat on a school can be described in terms of the change in the morphological parameters of the school (Gerlotto et al. 2004; Brehmer 2004), and thus sometimes in its volume, and which usually occurs just below the surface (0–20 m) (Fre ´on et al. 1992). This can be observed even with a small boat (Brehmer et al. 2003), like those used for acoustic surveys of lakes. During September 2004, a multibeam sonar was deployed in the Lake Annecy to determine the 3-D structures of young-of-the-year (Y-O-Y) fish schools (Guillard et al. 2006a), and an echosounder with the acoustic beam pointing straight down was used simultaneously. In this study, we first investigated Ecology of Freshwater Fish 2010: 19: 373–380 Printed in Malaysia Æ All rights reserved Ó 2010 John Wiley & Sons A/S ECOLOGY OF FRESHWATER FISH Guillard J, Balay P, Colon M, Brehmer P. Survey boat effect on YOY fish schools in a pre-alpine lake: evidence from multibeam sonar and split- beam echosounder data. Ecology of Freshwater Fish 2010: 19: 373–380. Ó 2010 John Wiley & Sons A ⁄ S Abstract – Hydroacoustic methods are widely employed by fish scientists for assessing fish stocks. The method most often used is echosounding, beaming vertically. Nowadays the multibeam sonar, and therefore the 3-D presentation of fish schools, has yielded better knowledge of school morphology. Using the data collected simultaneously by both sonar and echosounding in a lake, we have identified boat-induced behavioural changes in small pelagic fish schools. Using high resolution sonar data, we showed that the fish schools detected under the boat have a significantly larger volume than those alongside the boat. This finding is explained according to behavioural response due to the theoretical characteristics of the boat diagram sound pressure, and the existence of a strong thermocline. Then we compared two descriptors, the height of the fish school and the backscatter energy. We found significant differences, which reveal vertical fish school compression occurring simultaneously with the horizontal and sidelong escape behaviour. J. Guillard 1 , P. Balay 1 , M. Colon 1 , P. Brehmer 2 1 INRA, UMR 042 Carrtel, BP 511, 74 203, Thonon-les-bains, France, 2 IRD, Laboratoire des sciences de l’environnement marin (UMR UBO- CNRS-IRD 195), BP 70, 29 280, Plouzane ´, France Key words: fish school; behaviour; avoidance; multibeam sonar; echosounder; lake P. Brehmer, IRD, Laboratoire des sciences de l’environnement marin (UMR UBOCNRS- IRD 195), BP 70, 29 280, Plouzane ´, France; e-mail: patrice.brehmer@ird.fr Accepted for publication March 9, 2010 doi: 10.1111/j.1600-0633.2010.00419.x 373