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A summary study of acoustic methods used to reduce or eliminate marine mammal entanglements was conducted. The authors found little evidence on the effectiveness of the use of sound to reduce or eliminate marine mammal incidental capture. Substantial decreases in cetacean bycatch, due to the use of passive methods such as increasing fishing net reflectivity, were not evident through comparisons of catch rates in commercial fisheries or from observational deterrence studies. Net alarms have been shown to greatly reduce whale entrapment in Canadian fish traps but similar success has not been shown in reducing small cetacean bycatch in many gillnet fisheries. Few controlled studies on the use of sound to reduce or eliminate marine mammal interactions have been conducted. There is a need to additional basic research on echolocation behavior and behavioral interactions between marine mammals and fisheries.

 Preliminary field studies were carried out on a potentially cetacean-friendly acoustic deterrent system for seals (played using underwater loudspeakers). The “smart seal deterrent signal” produces a startle response in seals rather than an aversion to a very loud noise, and is transmitted at a sound level which is not harmful to seals, even at very close distances. There was some evidence that the acoustic deterrent was effective at deterring gray seals (Halichoerus grypus) from fishing operations, although there was a low seal interaction rate regardless of whether the device was on or not. 

This study analyzes the impacts of annual temporal fishing closures of gillnet fisheries in Lake Saimaa, Finland on bycatch/recovery of the Saimaa ringed seal (Pusa saimensis) population from 1991-2021. Results show that  fishing was responsible for 26% of observed mortality and 90% of that was from gillnet entanglement, but most bycatch mortalities are likely not observed and therefore underestimated. Longer temporal coverage of annual closures would improve juvenile survival, but the coverage may be regionally adequate. 

This paper reports on an unusual mortality event (UME) of healthy, nourished, male Baltic grey seals (Halichoerus grypus grypus) that occurred in the Greifswald Bay at the German Baltic coast in autumn 2017. Findings suggest that the cause of death was bycatch in a fyke net, despite the grey seals being a legally protected species. As a result, the fisheries authority ultimately established precautionary provisions, and fyke nets in Greifswald Bay are now (since 2020) required to have an entrance opening size of no greater than 75cm in circumference, which seems to have been effective to prevent gray seal bycatch in the area. However, gray seal bycatch in other areas continues, suggesting the need for more widespread adoption of preventative provisions. 

The authors used onboard observers and fishing records to evaluate marine mammal bycatch in trawl fisheries of the San Matías Gulf, Argentina. Results show that of 46 incidentally caught species, the American sea lion (Otaria byronia) was caught most frequently, and that catch was associated with proximity to colonies and the target species identity. Significantly higher bycatch rates were observed in the hake (Merluccius hubbsi) fleet.

The authors evaluated factors influencing marine mammal bycatch in static net fishing operations in the UK between 1996 and 2023. Since 2014, harbour porpoise (Phocoena phocoena)  bycatch has been decreasing while common dolphin (Delphinus delphis) bycatch has been increasing; grey seal (Halichoerus grypus) and harbour seal  (Phoca vitulina) bycatch has been increasing gradually across the time period. Influential factors included seasonal patterns, water depth, and level of fishing effort. 

Seal exclusion devices (SEDs) are effective in reducing pinniped bycatch in the Patagonian squid fishery, however, drowning may still occur during trawl hauling. 

An analysis of records of static net bycatch sampling in the United Kingdom from 1996-2023 found that use of acoustic deterrent devices (ADDs) reduced harbor porpoise (Phocoena phocoena) bycatch per haul (BpH) to 0.23 of the background rate. BpH increased for harbor seals and grey seals (Phoca vitulina and Halichoerus gryphus) when ADDs were used. Current efforts to reduce harbor porpoise bycatch may be increasing seal mortality in the same fisheries. 

Several designs of seal exclusion devices (SEDs) were tested to reduce bycatch of Australian fur seals (Arctocephalus pusillus doriferus) in the winter blue grenadier (Macruronus novaezelandiae) trawl fishery. A 'top-hatch' SED, or one with a top-mounted escape hatch, resulted in the lowest occurrence of seal bycatch than any other SED design or than nets without a SED. The SED prevented entry into the net codend where drownings occurred. SEDs also successfully expelled seals and limited access into the net via the escape hatch. Cameras are needed to verify the results.

This study aimed to quantify the risks to Australian sea lion (Neophoca cinerea) and New Zealand fur seal (Arctocephalus forsteri) populations from bycatch in a trap (rock lobster, Panulirus cygnus) and demersal gillnet fishery off South Australia. The approach was to: (1) estimate the spatial distribution of foraging effort for different sex and age classes within each species; (2) compare these with the spatial distribution of fishing effort in order to develop spatial estimates of seal-fishery interaction probabilities; (3) undertake population viability analyses to identify the levels of bycatch that would place subpopulations of each species into different risk categories; and (4) examine different bycatch scenarios and identify subpopulations, regions and marine fishing areas with the greatest bycatch risk. Results suggest that the risk of subpopulation extinction, even from low levels of bycatch, was high for sea lions, but very low for fur seals and that the two fisheries lend themselves to different mitigation approaches: gear modification in the lobster fishery, and spatial management of fishing effort in the demersal gillnet fishery.