Stimulating release of undersized fish through a square mesh panel in the Basque otter trawl fishery

Discards of regulated species in the Basque mixed trawl fishery are a challenge. In 2006, a square mesh panel (SMP) was introduced in the fishery to increase the release efficiency of undersized fish. However, studies have shown that the selectivity in this fishery is based on codend selectivity and the release through the SMP is inefficient due to low contact between fish and the SMP. In order to improve contact, we tested four different gear configurations that use different stimulators to lead fish to the panel: without stimulation, with stimulation based on ropes, with stimulation based on ropes and floats, and with stimulation based on LED lights. The experiment was carried out on three of the potential choke species for the fishery: hake (Merluccius merluccius), horse mackerel (Rachurus trachurus), and blue whiting (Micromesistius poutassou). The results showed that stimulators did not significantly improve the release efficiency of hake and horse mackerel through the panel. For blue whiting, stimulation with floats had a significant positive effect on release efficiency, whereas LED light-based stimulation had the opposite effect. In general, the contribution of the SMP to the overall release efficiency of the selective system (SMP + codend) was low. Underwater recordings confirmed that the stimulators generally were not able to lead fish towards the SMP.