Technology for Sustainable Fishing in Set Nets : A Simple Modification

Set nets (trammel nets and gillnets) vary in selectivity due to their several gear characteristics, thus they are used to catch a variety of commercially valuable species by vessels of the Portuguese coastal and small-scale (SSF) multi-gear fleets. The variability in the selectivity of set nets often results in impacts, both on benthic populations and their habitats, primarily catching corals, sponges and kelps. Previous studies focused on attaching a “greca” or guarding net to the bottom of trammel nets and while there was a decrease in non-commercial by-catch, there was often also a decrease in the catch rates of commercial species, causing this modification to be poorly received and ultimately not adopted by the local commercial fishing communities. In this study, a modification was applied to both trammel nets and gillnets which raises the set nets off the bottom through a system of ropes (‘aranha’, according to the local fishers) in either a vertical pattern or diagonal, depending on the preference of the local fishers that were chosen to assist with the experimental trials. Specifically, the modification was tested in the cuttlefish (Sepia officinalis) trammel net fishery in the south of Portugal and the hake (Merluccius merluccius) gillnet fishery in the northwest of Portugal, with a preference to fish near rocky bottom area, where by-catch of invertebrates, including sensitive habitat-forming species is high. The results showed a reduction in the by-catch of corals and sponges in the modified nets, with no significant impact on the commercial capture of the target species in the hake fisheries of the northwest. However, while there was a reduction in by-catch of the habitat-forming species in the south, there was also a reduction in catch rates of the commercial species, including the main target species, the cuttlefish. During both experiments, a video analysis was conducted in order to account for all individuals of the habitat-forming species caught in high volumes that were overlooked during real time data collection onboard due to regular hauling procedures by commercial fishers. After the experimental trials were completed, net damage assessments were carried out which showed that in the rockier areas of the northwest, the damage in the standard nets was three times greater than that in the modified nets. However, in the softer sediment of kelp beds in the south, the damage was similar and minimal. The financial impact of the use of the modified nets compared to the standard net will be discussed as well as how quantifying the number of habitat-forming species as individuals is difficult to assess, and finally the future of the ongoing project and how the modification is applicable for the sustainability in other fisheries on a global scale.