Cod-end selectivity, fish escape behaviour and fish morphology in the Mediterranean sea trials of UE project PREMECS-II

A predictive model was developed within this EU project carried out by IFREMER, MARINLAB, DIFRES and ISMAR laboratories. This deterministic model is based on the understanding of the mechanical and biological processes which control the selectivity of the cod-end of the trawls. The influence of the characteristics of the twines that constitute the trawl, the interactions between the vessel, fishing operations, state of the sea, rig of the trawl and design of the cod-end, the morphology and behaviour of fish escapement were taken into account in the final model. Tests on selectivity and submarine visualization of the operations were completed by the research vessel DALLAPORTA. Some physical parameters of the vessel (warp tension, number of propeller, fuel consumption) and trawl gear (spacing between panels, horizontal and vertical openings) were recorded in the course of fishing. With an aim of minimizing the environmental parameters, all hauls were achieved during the same periods of time, of depth and state of sea. The selectivity of the cod-end was measured by the technique of the double cod-end, the latter being mounted on hoops. Experiments more particularly dedicated to the analysis of the behaviour of escapement and its quantification were performed using mini TV cameras fixed inside and outside of the cod-end to quantify the escapement. The morphology of the principal species was measured starting from samples trawled in April-May during the post breeding phase and September after the summer feeding period. The results of this study more particularly underline the influence of the number of meshes of the circumference of the cod-end, the twine diameter and even more important, its stiffness. As many parameters are liable to alter the effects of an increase in mesh size. This study also led to the development of software PRESEMO to simulate the various modes of escapement for fish populations of given morphological characteristics and distribution.