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

Michalsen, Kathrine; Aglen, Asgeir; Somerton, David; Svellingen, Ingvald; Øvredal, Jan Tore
Working paper
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URI
http://hdl.handle.net/11250/100417
Date
1999
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  • ICES CM documents authored by IMR scientists (1949-2011) [3139]
Original version
This report is not to be quoted without prior consultation with the General Secretary.  
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.
Publisher
ICES
Series
ICES CM documents
1999/J:08

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