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In the Indian Ocean tuna gillnet fisheries, cetacean bycatch is very high, but it is understudied and poorly understood. This paper investigates the issue and makes specific recommendations to strengthen knowledge and management: improve catch and bycatch reporting, increase research and analyses of cetaceans in the region in order to assess bycatch, revise the conservation measure adopted to address the bycatch of cetaceans in IOTC fisheries (Resolution 23/06) to include additional reporting for gillnets operating in EEZs, and expand the use of mitigation methods, such as setting gillnets 2m below the surface.

The authors evaluated the effectiveness of DDD®03H Dolphin Dissuasive Devices, or DDD pingers (an acoustic deterrent device) to reduce common dolphin (Delphinus delphis) bycatch in a demersal pair trawler in the Bay of Biscay. Results showed that the DDD pingers reduced common dolphin bycatch by more than 90%, and that common dolphin bycatch in this trawl fishery is related to fishing zone and depth, but not necessarily factors such as time of day and haul duration.

The authors performed onboard surveys (line transects and passive acoustic monitoring) and interviewed gillnet fishers to assess abundance and bycatch of the endemic Burmeister’s porpoises (Phocoena spinipinnis) in northern Peru in 2023. The population abundance in the study area was estimated at 1,696 spatially distributed individuals, with the highest densities found within 25km of shore and within shallow waters ≤50 m. Fishers' descriptions of bycatch aligned with survey results, and most fishers reported experiencing porpoise bycatch the previous year. The annual bycatch rate was estimated to be 420 porpoises across the entire artisanal gillnet fishery of Salaverry, which far exceeds sustainable limits and highlights the urgent need for bycatch mitigation measures.

Three types of acoustic deterrent devices were tested in the Icelandic cod gillnet fishery:  Fishtek Banana pingers, Porpoise ALert devices (PAL), and wideband PAL (wPAL) with a modified pinger signal (PAL device emitting a wideband pinger signal with a frequency range of 20 – 160 kHz and signal duration of 0.3 s (every 4–30 s) with the same source level at 145 db and maximum depth (320 m)). Banana pingers and PALs did not demonstrate any reduction of bycatch of marine mammals, while the wPAL-equipped nets had no bycatch during the trial. Fish catch and overall commercial value was significantly reduced in gillnets equipped with banana pingers and PALs.  

The study examined individual and cumulative bycatch during the first five years of a fisher engagement program in the tuna drift gillnet fishery in Pakistan, where fishers co-developed a sub-surface net setting strategy while also receiving bycatch awareness and safe release training. Captain identity and year were the strongest predictors of sea turtle and dolphin bycatch rates.  

The PearlNet aims to mitigate odontocete bycatch in bottom-set gillnets by enhancing the acoustic visibility of nets through the incorporation of small, acrylic glass spheres. This study found that PearlNet performed similarly to standard nets in terms of catch efficiency and handling time of gear. Performance of the gear in reducing marine mammal bycatch was not assessed during this study, however, no mammal bycatch was reported. 

Between 2019 and 2022, trials using three different pinger models (Future Oceans 10 kHz, Future Oceans 70 kHz, and PAL Wideband) were conducted in Bulgaria during standard turbot fishing operations. Marine mammal bycatch was observed in  61% of all hauls. No significant difference in bycatch rates between active and control nets was observed in trials involving Future Oceans 10 kHz (17 trials) and 70 kHz (6 trials) pingers. However, a significant reduction (74%) in cetacean bycatch was recorded during the trials conducted with PAL Wideband pingers. 

The study tested the use of glass and plastic drink bottles as low-cost bycatch mitigation devices (mechanical pingers and acoustic reflectors, respectively) in gillnet fisheries across three regions/fisheries: surface-set driftnet fisheries in Zanzibar and Peru and bottom-set trammel net fisheries in Brazil. In Brazil, plastic bottles reduced dolphin bycatch while increasing fish catch, although further trials are needed. In Peru, glass bottles had no effect on dolphin, porpoise, or turtle bycatch but altered target elasmobranch and teleost catches; plastic bottles similarly had no effect on dolphin bycatch but increased shark catch. In Zanzibar, no dolphin bycatch was recorded in any sets, though plastic bottles increased tuna catch. 

The study tested the use of glass and plastic drink bottles as low-cost bycatch mitigation devices (mechanical pingers and acoustic reflectors, respectively) in gillnet fisheries across three regions/fisheries: surface-set driftnet fisheries in Zanzibar and Peru and bottom-set trammel net fisheries in Brazil. In Brazil, plastic bottles reduced dolphin bycatch while increasing fish catch, although further trials are needed. In Peru, glass bottles had no effect on dolphin, porpoise, or turtle bycatch but altered target elasmobranch and teleost catches; plastic bottles similarly had no effect on dolphin bycatch but increased shark catch. In Zanzibar, no dolphin bycatch was recorded in any sets, though plastic bottles increased tuna catch. 

The study tested the use of glass and plastic drink bottles as low-cost bycatch mitigation devices (mechanical pingers and acoustic reflectors, respectively) in gillnet fisheries across three regions/fisheries: surface-set driftnet fisheries in Zanzibar and Peru and bottom-set trammel net fisheries in Brazil. In Brazil, plastic bottles reduced dolphin bycatch while increasing fish catch, although further trials are needed. In Peru, glass bottles had no effect on dolphin, porpoise, or turtle bycatch but altered target elasmobranch and teleost catches; plastic bottles similarly had no effect on dolphin bycatch but increased shark catch. In Zanzibar, no dolphin bycatch was recorded in any sets, though plastic bottles increased tuna catch.