Akademik Veri Yönetim Sistemi
Birleşmiş Milletler Kalkınma Programı – UNDP Destekli Proje, 2023 - 2025
This project aimed to assess the effectiveness of gear-based mitigation strategies to reduce bycatch of vulnerable species, particularly elasmobranchs and sea turtles, in Mediterranean trawl fisheries. The study, conducted in GSA 24 (Northern Levant Sea, Türkiye), combined sea trials, post-capture survival experiments, stakeholder engagement, and onboard observation programs to provide a comprehensive analysis of bycatch mitigation and its socioeconomic implications. To investigate the potential of excluder grids in mitigating bycatch, extensive sea trials were carried out using commercial trawlers operating in both deep and shallow waters. A total of 40 trawl tows (20 control and 20 grid gear) were conducted in two separate trials. The tested gear configurations included two different excluder devices: a flexible grid with 50 mm bar spacing and a rigid grid with 95 mm spacing, both mounted within the trawl extension. During these trials, 517 individuals of vulnerable species were recorded, with a total biomass of 2,581.4 kg and an overall Catch-Per-Unit-Effort (CPUE) of 0,34 individuals per hour. These results confirm the regular presence of vulnerable species in trawl catches and emphasize the critical role of gear modifications in mitigating their impact. Statistical comparisons conducted using the Wilcoxon Signed-Rank Test indicated a significant reduction in bycatch rates for several elasmobranch species, including Etmopterus spinax and Galeus melastomus, in the test hauls compared to the control hauls. These findings underscore the species-specific effectiveness of the excluder devices, with the most notable reductions observed in small-bodied deep-sea sharks. The test gear demonstrated a clear reduction in the bycatch of elasmobranchs and sea turtles compared to traditional codends. Two Chelonia mydas individuals were caught in the control gear during mitigation trials. However, the catch performance of target species (Aristaeomorpha foliacea and Aristeus antennatus) was lower in the grid gear, resulting in an estimated profit loss of €2.307,50 over 20 hauls. When extrapolated to a typical 100-day fishing season, this corresponds to a projected loss of approximately €23.075,00. Despite the losses in target species revenue and the resulting challenge this presents for the acceptance of the mitigation tool by fishermen, the results demonstrate that grid-based mitigation tools show promising potential as a trade-off between conservation and fishery sustainability. 2 In parallel, onboard monitoring activities and structured interviews were conducted to assess bycatch occurrence and stakeholder perspectives on mitigation measures. A total of 75 onboard commercial trawl observations and 120 structured interviews were completed across Mersin and Adana. Data collection followed GFCM protocols, covering biological data, discard practices, marine mammal interactions, and gear characteristics. No dolphin, whale, seabird, or turtle bycatch was recorded during general onboard monitoring. However, 65% of fishers perceived an increase in marine mammal interactions over the past five years, and 48% reported gear damage caused by non-mammal species, including rays, sea turtles, sharks, and puffer fish. Notably, 100% of respondents supported the idea of a dolphin observer program, reflecting strong community interest in sustainable practices. Workshops and stakeholder meetings helped reinforce awareness and acceptability of bycatch reduction technologies within the fleet. Finally, the project evaluated short- and long-term post-capture survival rates of vulnerable species through experimental trials. Short-term survival assessments using a 1-ton tank system revealed that survival rates exceeded 99% for species such as Gymnura altavela, Rhinobatos rhinobatos, and Rhinoptera marginata. In contrast, long-term survival monitoring in submerged sea cages over 24 hours revealed delayed mortality, particularly in Rhinoptera marginata, suggesting that short-term vitality assessments may overestimate true survival potential. The findings support the feasibility of releasing live elasmobranchs post-capture, provided handling practices are optimized. Cage design also played a critical role, with knotted netting reducing entanglement in ray species. Overall, the project provided strong field-based evidence on the effectiveness and limitations of bycatch reduction devices (BRDs) in reducing bycatch in Mediterranean trawl fisheries. The high short-term survival rates observed in elasmobranch species caught in shallow waters indicate the potential to enhance survival through best practices and gear modifications. However, experiments conducted in deep waters showed that vulnerable species have poor survival following capture, emphasizing the importance of physical exclusion using BRDs in deep-water fisheries. Although BRDs may result in some revenue loss, they offer a viable solution for 3 balancing ecological conservation with commercial sustainability. When long-term ecological benefits—such as reduced mortality of threatened species and healthier ecosystems—are taken into account, the economic losses, although initially not well accepted by fishers, can still be considered justifiable. The results strongly support further refinement of BRDs and the integration of post-release survival data into fisheries management policies.