The influence of towing speed and fish density on the behaviour of haddock in a trawl cod-end

In this study underwater video footage from cameras placed inside and outside the cod-end was analysed to provide a quantitative description of haddock behaviour inside the cod-end of a commercial whitefish trawl. The behaviour was quantified in terms of orientation, swimming activity, and duration in view of the camera and could be divided into optomotor and erratic swimming behaviour. Optomotor response was the most common behaviour observed, increasing in prevalence with increasing density of fish. Both successful and failed escape attempts were also documented, including location and pre-attempt behaviour. Escapes were concentrated in the areas where meshes were most open, in the rows just ahead of the catch. Over the observed area, between 20 and 35% of the observations of fish approaching and striking the netting resulted in successful escapes. The rate of these netting strikes was generally higher at higher towing speeds and at lower densities.