Quantifying the amount of fish unavailable to a bottom trawl by use of an upward looking transducer

Abstract

When surveying demersal fish with a wide and variable vertical distribution, it is desirable to combine information from a bottom trawl survey with the information from a hydroacoustic survey into an absolute abundance estimate. To do this requires an estimate of either the amount of fish lost in the bottom acoustic deadzone or the amount of fish unavailable to the bottom trawl. In the latter case, this .quantity is not easily estimated using an hull-mounted transducer due to vertical movement between the moment a fish passes the acoustic beam and the moment it reaches the trawl. For such situations, we have examined an alternative procedure based on' an upward-looking, trawl-mounted acoustics system designed to directly measure the amount of fish passing above a 5;awl. This paper describes a pilot experiment for such measurements. To avoid disturbances of both fish behaviour and trawl geometry dueto a cable connection between the vessel and the trawl, the echo sounder, the data logging system (a portable PC) and the power supply were put in an underwater housing and mounted on the trawl together with the transducer. An underwater camera showed that the attached equipment did not have any influence on the trawl geometry. The equipment is described and some preliminary results are shown.