In situ TS detections using two generations of echo-sounder, EK60 and EK80: the continuity of fishery acoustic data in lakes

Used for several decades and recognized today as a reliable method, fishery acoustics is commonly used in scientific studies and monitoring surveys. In Europe, during the last decades, the most frequently used scientific echo-sounder has been the EK60 from the Simrad company (Simrad Kongsberg Maritime AS, Horten, Norway). A new echo-sounder model, the EK80, has been recently developed, and it is therefore necessary to check whether the results obtained by the two generations of echo-sounder (EK60 and EK80) give the same results. This is of main importance in the context of time series. Recent works have already addressed this comparison, focusing on SA (nautical area backscattering coefficient) but the systems' reliability to accurately measuring target strength (TS, in dB) has only been tested using calibration spheres. Our work aims to test the hypothesis that, at a given frequency, the TS recorded simultaneously in situ by an EK60 and an EK80, in CW mode, were not statistically different. Data were recorded in two peri-alpine lakes using the two systems sampling sequentially a similar volume. Using statistical tests, acoustics metrics were compared. For TS, statistically significant differences were found between the two generations of echo-sounder. However, these differences were not large enough to affect the fish density used for fishery management or researches on the fish population. The continuity of the time series acquired with the EK60 is ensured when one switches to the EK80 echo-sounder for lake ecosystem research and monitoring.