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Bycatch of Australian sea lions in rock lobster and shark gillnet fisheries is a key threat to the species. This report outlines and assesses methods for mitigating bycatch in these fisheries in South Australia. Gear modification was considered in the rock lobster fishery and spatial closures in the shark gillnet fishery. Spikes of different heights were tested to deter sea lions from entering rock lobster pots. Entry success significantly decreased with spikes extending high up into the collar (70 mm + from base) compared to other treatment and control pots. 

An acoustic deterrent device (ADD) was tested at a Scottish salmon double bagnet to deter seals (grey and harbor). The ADD was randomly assigned to be on or off during observations, which were carried out from shore. When the ADD was on, there were significantly fewer seals observed and 1/3 more fish were landed per hour than when the ADD was off. Seal damaged fish were only found when the ADD was off.   

Trials of acoustic deterrent devices (ADD) were carried out on the River North Esk and River Conon in Scotland to test the effectiveness of deterring seals from a specific area of the river and as a barrier to upstream movement of the seals. The ADD was switched on and off alternatively for periods of several days and surveys were carried out to estimate the number of seals present within each river. The ADD had no significant effect on the abundance of seals in the survey area, but it did reduce seal movement upstream significantly. The reduction in movement was constant over the four month period of the trial.  

This study reviews existing data and information from key stakeholders on interactions between the fishing industry and seals. The data suggests that seal interactions with commercial fisheries in Ireland are most significant inshore (<12 nautical miles from shore), involves static-net (passive) fisheries (e.g. gill/tangle nets) and have increased following the driftnet ban in 2006, suggesting displacement of seal interaction from salmon to other fisheries. Pollack, angler fish, cod, and turbot fisheries were the main inshore fisheries impacted by operational damage to gear and catch by seals. The extent to which changes in fishing practices and gear has reduced damage levels is unknown.

The long-term effectiveness of acoustic pingers in reducing marine mammal bycatch was assessed in the California swordfish and thresher shark drift gillnet fishery. Bycatch was observed at sea between 1990 and 2009, with acoustic pingers being present from 1996 to 2009. Bycatch rates of cetaceans decreased by around 50% when pingers were present; the decrease was mostly driven by common dolphins (Delphinus delphis). Beaked whales have not been incidentally caught since 1995. Pinger failure occurred in less than 4% of observed sets. Cetacean bycatch was 10 times higher in sets where more than one pinger failed. There was no evidence of habituation to pingers by cetaceans. Bycatch rates of California sea lions (Zalophus californianus) was almost double in sets with pingers than without, leading the authors to examine the "dinner bell" effect of pingers. Depredation of swordfish by sea lions was not linked to pinger use, instead the best predictors were total swordfish catch, month and area fished, and nighttime light use.   

The authors conducted an extensive literature review to determine if Sea Lion Excluder Devices (SLEDs) allow sea lions to escape from trawl nets and if they subsequently survived. In addtion, the literature was reviewed to determine if head trauma was likely when sea lions come into contact with stainless steel SLED grids. According to the literature, SLEDs are effective in reducing sea lion bycatch in trawl nets and resulted in a reduction of observed sea lion mortalities. In addtion, the literature indicates that increased head trauma, due to contact with the SLED grids, is unlikely. 

This article reviewed bycatch mitigation methods that rely on sensory ecology to reduce interactions with sea birds, sea turtles, pinnipeds and blue-water fish. Panels containing patterns of low spatial frequency and high internal contrast can be used for all of these species. These panels are not likely to reduce catches of target species and are easy and inexpensive to apply. The authors suggest that sound signals on gillnets are not recommended for most bycatch species, excpet for cetaceans. Cetaceans require both visual cues and warning through echolocation.

Depredation by Antarctic fur seals (Arctocephalus gazella), sperm whales (Physeter macrocephalus) and orcas (Orcinus orca) were studied in a Patagonian toothfish (Dissostichus eleginoides) fishery operating around South Georgia Island. While sperm whales come in contact with about one third of longlines (averaged over 1997 and 2012), depredation events from this species were considered insignificant. Orcas interacted with only 5% (averaged over 1997 and 2012) of longlines, but can remove over half the catch during an interaction. Interactions with orcas are spatially limited to migration routes; however, fishing is concentrated in these areas during migrations times. Interactions with fur seals were the most frequent until 2009, averaging just under 9% from 1997-2012. The catch-per-unit-effort of toothfish was higher when no mammals were observed near longlines. 

Experiments were conducted along the Swedish coast to investigate the impact of seal exclusion devices (SEDs) on seal bycatch. Pot entrances were fitted with SEDs of different shapes and sizes. The pots were a two-chambered single entrance floating cod pot. Pots were equipped with a 45 mm escape window. Experimental pots had vertically mounted metal frames that were secured with nylon line. Five different types of SEDs were used. They had different metal frames, different shapes, thickness of material or inner circumference. Bycatch of seals was reduced to zero when SEDs were used and the pots' catchability was not negatively affected. The symmetrical oval shaped entrance and larger rectangle shaped entrance (two smaller openings) increased the pots' catchability.

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.