Mitigating bycatch in trammel net fisheries using species-specific gear modifications

Small-scale fisheries (SSF) use static gears like hanging nets, which are thought to interact with marine ecosystems more benignly than towed gears. Despite this, trammel nets, one of the most regularly used fishing gears in the Mediterranean SSF, generate large amounts of discards, which can account for 25% or more of the captured biomass. Discarded organisms may include endangered or threatened species such as elasmobranchs, as well as non-commercial invertebrates that damage fishing gear or cause disentanglement delays. We tested various trammel-net gear modifications, including (a) the use of a guarding net attached to the footrope, (b) increasing the size of the rigging twine between the footrope and the netting panel, and (c) decreasing the mesh size of the outer panels. The last two modifications were successful in lowering captures of the marbled electric ray Torpedo marmorata, which is commonly discarded in the study area. Both sorts of modifications are relatively simple and inexpensive to implement. The current study shows that prior evaluation of the discards profile of distinct métiers is essential to accomplish species-specific gear modifications and underlines the importance of collaboration among scientists, fishers and gear manufacturers.

小型渔业（Small-scale Fisheries, SSF）多采用悬挂网等定置渔具，这类渔具被认为相较于拖曳渔具对海洋生态系统的扰动更为温和。尽管如此，作为地中海小型渔业中最常使用的渔具之一，三重刺网（trammel nets）仍会产生大量丢弃渔获，其生物量占总捕获生物量的比例可达25%甚至更高。被丢弃的生物个体可能包含濒危或受威胁物种，如板鳃类（elasmobranchs），以及会损坏渔具或导致解网延误的非经济无脊椎动物。本研究测试了多款三重刺网的渔具改良方案，具体包括：（a）在底纲加装防护网；（b）增大底纲与网片之间的绳缆绳径；（c）减小外网板的网目尺寸。其中后两项改良方案可有效降低云纹电鳐（Torpedo marmorata）的捕获量——该物种在本研究区域内常被作为丢弃渔获处理。上述两类改良方案均操作简便且实施成本低廉。本研究证实，预先评估不同渔法类型（métiers）的丢弃物特征，是实现物种针对性渔具改良的必要前提；同时也凸显了科研人员、渔民与渔具制造商三方协作的重要价值。