PRESEMO - a predictive model of codend selectivity - a tool for fishery managers

The codend selectivity simulation model PRESEMO is a predictive model based on an understanding of the physical, biological, and behavioural mechanisms that underpin codend selection. In this paper, PRESEMO is used to predict the selectivity of a large range of codends of varying design. In particular, the selectivity of codends with mesh sizes in the range 80-160 mm, number of meshes around in the range 60-140, and netting twine thickness in the range 3-6 mm are predicted and, where possible, the predictions are validated with experimental data. Using the simulated data, the codend selectivity parameters are expressed in terms of the gear design parameters and in terms of both catch size and gear design parameters. The potential use of these results in a management context and for the development of more selective gears is highlighted by plotting iso-/(50) and iso-sr curves used to identify gear design parameters that give equal estimates of the 50% retention length and the selection range, respectively. It is emphasized that this approach can be extended to consider the influence of other design parameters and, if sufficient relevant quantitative information exists, biological and behavioural parameters. As such, the model presented here will provide a better understanding of the selection process, permit a more targeted approach to codend selectivity experiments, and assist fishery managers to assess the impact of proposed technical measures that are introduced to reduce the catch of undersized fish and unwanted bycatch.