Selectivity and escape percentages during three phases of the towing process for codends fitted with different selection systems

We investigated the selectivity of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) escaping from three different selection systems (a conventional diamond-mesh codend, a codend fitted with escape windows, and a codend fitted with a sorting grid) using the covered codend technique. By using a MultiSampler attached to the codend cover to partition the collection of escaping fish, we were able to determine the partitioned selectivity of each of these devices at different phases during the towing process: at depth, during the haul back, and at the surface.The percentages of escaping fish at depth from the diamond-mesh codend were significantly lower than those estimated for the escape windows and sorting grid codends: almost 62% of cod and 79% of haddock escaping from the diamond-mesh codend did so at depth, whereas approximately 75% of cod and 93% of haddock escaped from the other two systems at depth. The partitioned selectivity parameters, l50 and SR, were estimated and modelled for each of the three phases of the towing process and used to compare the selective properties of the three selection systems. A comparison was also made with a traditional whole haul selectivity analysis. Finally, bulk catch was added to the model as an explanatory variable and its effect upon selectivity was measured in both, partitioned and whole haul estimates.