IMPACT OF FISH DISTRIBUTION AND SPECIES COMPOSITION ON THE RELATIONSHIP BETWEEN ACOUSTIC AND SWEPT-AREA ESTIMATES OF FISH DENSITY

Bottom-trawl catches were compared with echo-integrator records obtain ed from a hull-mounted transducer on the towing vessel to examine how far from the bottom fish were available to the trawl. Swept-area densi ty estimates obtained using a length-dependent effective fishing width were converted to equivalent echo-integrator values before being comp ared with the observed integrator values at different heights above th e bottom. For large haddock (Melanogrammus aeglefinus (L.)), saithe (P ollachius virens (L.)), cod (Gadus morhua L.), redfish (Sebastes marin us (L.), and S. viviparus Kroyer) the best correlation between catch a nd acoustic data was obtained when applying the cumulative acoustic va lue (from the bottom upward) at 30 to 100 m above the seabed, while fo r Norway pout (Trisopterus esmarkii Nilsson) the best correlation was obtained at about the height of the headline of the trawl (4 m). Attem pts to compensate the acoustic values for losses in the acoustic dead zone did not improve the correlations. (C) 1996 International Council for the Exploration of the Sea.