Impact of fish distribution and species composition on the relationship between acoustic and swept-area estimates of fish density

Abstract

Bottom-trawl catches were compared with echo-integrator records obtained from a hull-mounted transducer on the towing vessel to examine how far from the bottom fish were available to the trawl. Swept-area density estimates obtained using a length-dependent effective fishing width were converted to equivalent echo-integrator values before being compared with the observed integrator values at different heights above the bottom. For large haddock (Melanogrammus aeglefinus (L.)), saithe (Pollachius virens (L.)), cod (Gadus morhua L.), redfish (Sebastes marinus (L.), and S. viviparus Krøyer) the best correlation between catch and acoustic data was obtained when applying the cumulative acoustic value (from the bottom upward) at 30 to 100 m above the seabed, while for Norway pout (Trisopterus esmarkii Nilsson) the best correlation was obtained at about the height of the headline of the trawl (4 m). Attempts to compensate the acoustic values for losses in the acoustic dead zone did not improve the correlations.