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Two types of depredation mitigation devices (DMD) were designed and tested aboard pelagic longline fishing vessels targeting tunas and swordfish off the Seychelles. The DMDs were designed to reduce the damage and removal of fish from the fishing gear by predators such as toothed whales and sharks. The first DMD was termed "spider" and was made up of a 100 mm diameter plastic disc with sixteen holes in its outer range and a 37 mm diameter central hole.  Eight 1200 mm long hanging legs were created by placing four polyester strands into the outer holes and the entire system is triggered by a biting fish.  The second DMD, "sock", was designed to cover the fish to hide it from predators through either a net made up of fiberglass mosquito netting or of propylene fiber net.  Shark depredation was significantly higher throughout the surveys than toothed whale depredation, but toothed whales caused more damaged to fish through depredation events. The sock DMD did not affect the catch rates of targeted species or depredation events by either sharks or toothed whales.  These results for the spider DMD were not presented. Trigger rates for the two DMDs were up to 87.3% for the spider and 69.2% for the sock.  The protection rate was as high as 80% for spider but only 15.5% for the sock. Entanglement rates of the DMDs with the fishing gear were low for the spider (3.6%) and higher for the sock (17.8%).  In addition, the use of these DMDs significantly slowed down the fishing process, and therefore additional work into the design and application of DMDs needs to be explored.

The Dolphin Wall Net (DWN) was designed by these fishermen to reduce dolphin attacks during fishing operations in Kerala, India. Specifically, humpbacked dolphins (Sousa chinensis) tend to congregate in fishing areas, surrounding the ring seine net and removing fish from the net.  This net design can be used by fishermen instead of crackers, which can cause harm to the dolphins. The authors provided fishermen from four stations in Kerala, India (Kasargode, Kozhikode, Ernakulum and Kollam) questionnaires to fill out. The questionnaires were designed to collect information on ring seine fishing operations, dolphin attacks and the structural details of the DWN when it was used.  The authors presented results on the structure of the DWN used by these fishermen but did not report any information regarding differences in bycatch rates between regular ring net operations and those using DWN.

Incidental captures of the franciscana dolphin (Pontoporia blainvillei) by artesinal coastal fishermen of Buenos Aires, Argentina were investigated. Data was collected on carcasses of 54 incidentally captured dolphins between 2003 and 2009. Results indicated that the majority (69%) were animals four years old and younger, 74% of entangled animals were sexually immature and 85% were physically immature. Entanglements were not associated with any disease or physiological disorder. No bycatch mitigation techniques were tested for this paper.

A modified single-layer codend configuration developed from previous studies was evaluated using BRDs with guide net angles of 30° and 45°. For target species, catch loss rates based on the number and weight of individuals were 9% and 13%, respectively for herring (Culpea pallasii) with the 30° guide net and 16% and 18% with the 45° guide net. For Korean pomfret (Pampus echinogaster), catch loss rates were 24% and 23% at 30° and 4% at 45°. Catch loss rates of target mackerel (Scomber jamponicus) with the 30° guide net reached 56%, whereas hairtail (Trichiurus lepturus) had loss rates of 60% and 65% with the 45° guide net. These results suggest that further modification of BRD is necessary. However, no bycatch of marine mammals occurred during the experimental operations, despite dolphins being observed nearby.

Harbour porpoise (Phocoena phocoena) bycatch was significantly lower (p<0.01) in high-density Iron Oxide (IO) gill nets compared to control nets. There was no significant difference in the acoustic target strength of the IO and control nets, and both nets behaved similarly in flume tank tests. The authors conclude the reduction of harbour porpoise bycatch in the IO nets was a result of the mechanical properties of the nets.

The study tested the vocalization response of harbor porpoises (Phocoena phocoena) to two different acoustic pingers, the AQUAmark 100 (20-160 kHz) and the AQUAmark 300 (10 kHz). Trials in Scotland and Denmark found that the AQUAmark 100 resulted in a significant reduction of porpoise vocalizations at 0,200, and 400 meters distance from the pingers. The vocalization behavior reveal no signs of habituation. Studies of the AQUAmark 300 resulted in a significant reduction of vocalizations at 0m and no significant reduction at 300m. Additionally, at one station, habituation effects were found.

The study evaluated the reactions of a six-year old harbor porpoise (Phocoena phocoena) to a banana pinger (Fishtek Marine Ltd.) in a controlled experiment at the SEAMARCO Research Institute in the Netherlands. The banana pinger sounds were comprised of signal blocks (duration ~0.5 s), each containing four frequency components (8–125 kHz; peak 70 kHz). Sounds were played back at mean received levels of 83, 89, 95, 101, and 107 dB re 1 μPa. Behavioral responses, including increased respiration rate, jumping, and swimming faster and farther away from the source, occurred from 95 dB re 1 μPa. At sea, distances at which harbor porpoises are deterred by pingers will depend on environmental parameters influencing sound propagation. 

Two devices were designed to prevent odontocetes from depredating caught fish and putting themselves at-risk of becoming bycatch. One device used physical deterrence by shrouding the fish with a barrier and the other used psychological deterrence by utilizing prior negative experiences of temporary entanglement in fishing gear. Both devices fit on a branchline at a distance from the hook and descend towards a caught fish using a line tension trigger mechanism. All interactions occurred on control branchlines that were not fitted with a deterrent device, suggesting the potential of this technology to deter depredating odontocetes. The impact on fish catch rates, size, and survival was negligible. 

Odontocete (i.e. toothed whale) interaction with longline fisheries is a global phenomenon that threatens the status of some populations and the economic viability of longline fisheries. This review paper summarizes the trend and geographical extent of interactions, the potential impact on odontocetes and fisheries, and describes potential acoustic and physical mitigation solutions. 

Threatened common bycatch species in the Baltic Sea and North Sea include harbor porpoise (Phocoena phocoena), common eider (Somateria mollissima) and velvet scoter (Melanitta fusca), as well as the non-threatened grey seal (Halichoerus gryphus) and harbor seal (Phoca vitulina). Interactions primarily occur in static net gear, such as gillnets and trammel nets, mainly used by coastal small-scale fisheries. Although technical bycatch mitigation methods are available for most fishing gears interacting with cetaceans and seals, measures focused on mitigating seabird interactions with gillnets are limited.