Estimating codend selectivity and fish escapement from a covernet of an insufficiently small mesh size

This paper describes a method of evaluating virtual codend selectivity with covernet selectivity when the mesh size of the covernet is not small enoug h to retain all the fish that enter the covernet. If the covernet mesh size is not small enough, small fish can escape through the covernet mesh after passing through the codend, and then their body length proportion retained in the codend will be overestimated. We developed a model in which coverne t selectivity can be estimated at the same time as accurate codend selectiv ity. The validity of the method was examined with data from a covered-coden d experiment on the Patagonian grenadier Macruronus magellanicus collected in the southwest Atlantic. The model fits the data well even though the plo ts of proportion retained were U-shaped. For both the codend and covernet s electivity estimated in the model, fish of length close to 100% selectivity had a girth close to mesh perimeter. This supported that both the codend a nd covernet selectivity estimated in the model were reasonable. The analysi s proposed in the present study would be useful to obtain accurate estimate s of codend selectivity by evaluating the effects of covernet selectivity o n the data.