Evaluating the efficacy of various types of guarding nets in minimizing bycatch in a Mediterranean trammel net fishery

Abstract

Static nets, like trammel nets, may have less environmental impact than towed gear, but they often result in high discard rates. Trammel nets targeting the common cuttlefish Sepia officinalis in Thermaikos Gulf (N. Aegean) were shown to display a discard rate of 40.2 % with the vast majority of discarded catch consisting of invertebrate species, such as muricid gastropods, sea urchins, and spider crabs. In addition to the environmental impact, capturing some of these species can be a substantial difficulty for small-scale fishers, leading to delays in their fishing operations and damage to their gear. This research examined the effectiveness of different types of guarding nets, namely short gillnets positioned between the main trammel net and the footrope, in terms of biomass and species composition in commercial catches and discards. All tested gears significantly reduced the capture rate of crustaceans, notably spider crabs. However, the results for the other groups of organisms varied. The only gear modification that had no significant effect on cuttlefish catches was the addition of a well-rigged guarding net with a smaller mesh (20 mm instead of 40 mm). On the other hand, when the guarding net was loosely attached to the main net or had a greater mesh size, the biomass of cuttlefish and the abundance of demersal teleosts decreased. Our study showed that, despite the added cost, putting a guarding net during the period when species like crabs are at their seasonal height may benefit fishers in avoiding delays in disentanglement and damage to their fishing gear.