Determining the entrance position of fish in trawls

The entrance position of a fish is defined as its two-dimensional position in a cross section of a trawl near the centre of the headline. Individual fish entering the trawl: detected by highresolution scanning sonar, were positioned by digitising the co-ordinates of the echo relative to the trawl panels. Entrance probabilities for different species may be computed for grid elements on the cross section, integrating the positional information over an entire trawl haul, and studied as a function of light level, density and fish size. Examples of entrance probabilities for cod and saithe on a few selected trawl hauls are shown