Observation of fish behaviour, density and distribution around a surveying vessel by means of a deployable echo sounder system

Detailed acoustic obsewations of fish behaviour, distribution and density in the trawl area are necessary in order to establish quantitative relationships between trawl samples and fish density. Acoustic observation close to the bottom may reduce the near bottom acoustic dead zone. A transducer deployed close to the bottom may resolve deep targets and avoid effects of the bottom acoustic dead zone associated with deep recordings. Ship avoidance is also a well known problem related to acoustic fish abundance surveying. This paper describes a system consisting of a Simrad EY500 echo sounder built into an aluminium underwater bottle and a portable PC. Two different split-beam transducers can be connected to the bottle. The system is remotely controlled and can be used down to 400 rn depth, either powered from a battery bottle or from an external 2201110 V AC supply. All logged data are stored on an internal disk or sent via Ethernet through fibre optical cables to a shipboard device, depending on the application. The system is used for different purposes. Mounted on the trawl headline, with the transducer pointing upwards, it is used to monitor the escape of fish over the trawl headline. The system can also be mounted on a towed, remotely controlled vehicle (FOCUS 400), to constitute a deeptowed acoustic system to improve precision. of acoustic surveys on bathypelagic and deep dwelling demersal fish, or to record fish in the near bottom acoustic dead zone. The vehiclemounted system is also used to observe fish behaviour relative to trawl and vessel, e. g. abundance variation in areas around the trawl and between the trawl and vessel to facilitate establishing of the catchability of survey trawls. Deployed as a free floating buoy it is also used to quantitatively determine fish avoidance to a surveying vessel