A stationary acoustic system for monitoring undisturbed and vessel affected fish behaviour

Thorough knowledge of the dynamics of natural fish behaviour and distribution within a survey and among surveys is fundamental for reliability of time series of abundance indices. Further, vessel/trawl affected avoidance may greatly affect density estimates from trawl and acoustic surveys. Means for monitoring behaviour of fish during surveys or in special experiments have been limited, time consuming and expensive. This paper describes an acoustic buoy system for monitoring undisturbed distribution and behaviour of fish as well as avoidance reactions of fish in relation to passing surveying or trawling vessel. Operation and results from the first trials are described. The buoy could easily be operated in connection with a standard trawl haul, prolonging the haul time with approximately 30 minutes for launching, retrieval and extra cruising distance. Information on fish distribution and behaviour , i.e. vertical and horizontal movements, is stored in the buoy and can easily be downloaded for later processing and analysis. Observations on cod demonstrated avoidance away from the vessel of variable intensity