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.