Modify fishing pots and traps to exclude mammals
Overall effectiveness category Likely to be beneficial
Number of studies: 6
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Background information and definitions
Marine and freshwater mammals may predate on catches in fishing pots and traps, which can result in mammals being trapped or entangled. Modifying traps to exclude mammals may reduce this risk. Losses to fishers and damage to gear may also be reduced, thereby reducing human-wildlife conflict at wild fisheries. Methods to exclude mammals may include changing the pot/trap configuration, securing pot/trap doors, using strengthened materials, or adding a physical device (sometimes referred to as a ‘Depredation Mitigation Device’) to prevent entry, such as a metal grid or rods.
Supporting evidence from individual studies
A controlled study in 1998 in an estuary in the Indian River Lagoon, Florida, USA (Noke & Odell 2002) found that securing crab pot doors with a V-shaped bungee cord strung through the wire mesh of the pot reduced the number of common bottlenose dolphin Tursiops truncatus interactions compared to conventional methods, but using a V-shaped bungee cord attached to three steel rings did not. Securing the door with a V-shaped bungee cord strung through the wire mesh on each side resulted in fewer dolphin interactions with the pot (1 in total) than conventional methods of securing the door (29 in total). The difference was not significant when a V-shaped bungee cord attached to three steel rings was used (total 38 interactions). Twenty wire crab pots (51 x 51 x 51 cm) were deployed by a blue crab Callznectes sapidus fishery with each of three methods of securing the bait-well door with 5-mm elastic bungee cords: V-shaped cord strung through wire mesh; V-shaped cord attached to three steel rings; conventional method (diagonal cord attached to two steel rings). The 60 pots were baited with herring and checked at 48 h intervals in July–October 1998. Fishers recorded signs of dolphin interactions (broken bungee cords, doors forced open, missing bait).Study and other actions tested
A controlled study in 2001 of a pelagic area in the Gulf of Finland, Finland (Lehtonen & Suuronen 2004) found that installing a wire grid and strengthened netting in trap-nets resulted in higher catches of undamaged salmon Salmo salar but not whitefish Coregonus lavaretus, likely due to reduced grey seal Halichoerus grypus predation. Average catch rates of undamaged salmon were greater in fish bags of modified trap-nets (6 kg/haul) than in conventional trap-nets (4 kg/haul). The difference was not significant for catch rates of undamaged whitefish (modified trap-nets: 33 kg/haul; conventional trap nets: 29 kg/haul). Five modified and five conventional salmon trap-nets were hauled 241 and 242 times respectively in June–August 2001. Four modified and four conventional whitefish trap-nets were hauled 173 and 180 times respectively in September–November 2001. Modified trap-nets had a wire grid (1.2 x 0.8 m with 2 mm wires spaced 175 mm apart) in the funnel of the trap and a fish bag made from Dyneema netting. Conventional trap-nets had an open funnel and fish bags made from elastic nylon (salmon trap-nets) or stiff polyethylene (whitefish trap-nets). Fish catches were weighed, and fish damaged by seals or seabirds were recorded, during each haul.Study and other actions tested
A replicated, controlled study in 2003–2004 of five pelagic areas in the Bothnian Sea, Finland (Suuronen et al. 2006) reported that five designs of modified trap-net with wire grids and strengthened netting had lower proportions of salmon Salmo salar damaged by seals than traditional trap-nets. One design of modified trap-net (a ‘pontoon’ trap) had a significantly lower average proportion of seal-damaged salmon/haul (1%) than two types of traditional trap-net (30%), and total catch rates were similar (modified trap-nets: 2.3 fish/haul; traditional trap-nets: 1.9–2.4 fish/haul). Four other designs of modified trap-net had lower proportions of seal-damaged salmon (4–27%) than traditional trap-nets, although statistical significance was either not assessed or not reported. Four commercial fishers each deployed modified trap-nets and two types of traditional trap-net (number of each not reported) at random locations within their fishing grounds. Four designs were tested for four weeks in June 2003 (‘pontoon’ trap, ‘pipe’ trap, ‘protection-net’ trap, ‘large-mesh’ trap). Two designs were tested for 19 days in June 2004 (‘pontoon’ trap, ‘folded-hoop’ trap). Modified trap-net designs had a wire-grid within the funnel and/or ‘seal-safe’ netting around the fish bag (see original paper for details). Each trap was hauled once/day. Researchers recorded fish catches and seal-damaged fish during each haul in 2003 and 2004.Study and other actions tested
A controlled study (year not stated) in coastal waters of the Indian Ocean, Western Australia (Campbell et al. 2008) reported that installing steel rods on lobster pots resulted in fewer Australian sea lion Neophoca cinerea pups entering the pots, and a smaller gap at the pot opening excluded more sea lion pups. Results are not based on assessments of statistical significance. Fewer sea lion pups successfully entered pots with steel rods fitted (0–45%) than pots without (82%). More sea lion pups were excluded from pots with a smaller gap between the rod and pot opening (60 mm gap: 55% of pups excluded; 40 mm gap: 72%; 20 mm gap: 95%; 0 mm gap: 100%). Daily catch rates of target rock lobster Panulirus cygnus did not differ significantly between pots with or without steel rods (see original paper for data). A lobster pot with a steel rod and a control pot (without a steel rod) were filled with 10–15 lobsters and deployed in shallow water adjacent to a sea lion breeding colony. The height of the steel rod was varied to create four gap sizes at the pot opening (0, 20, 40 or 60 mm). Trials were carried out for each of the four pot treatments until all lobsters were removed or sea lions moved away from the area (dates not reported). Sea lion pups that placed their head into the pot (at risk of drowning) were counted as entering pots.Study and other actions tested
A controlled study in 2012 at a bay and harbour in the North Sea, Scotland, UK (Harris et al. 2014) found that fishing bag-nets with rigid steel bars, along with other modifications to prevent seal access, had greater catches of Atlantic salmon Salmo salar undamaged by seals than conventional bag-nets, but salmon took longer to pass through the modified net and a greater proportion escaped. Catch rates of undamaged salmon were almost twice as high in modified bag-nets than in conventional bag-nets (data reported as a catch rate index). However, salmon in the modified bag-net took longer to pass through the net (average 200 seconds) and a larger proportion swam back out of the net (65%) than in the conventional bag-net (average 44 seconds; 28%). A modified salmon bag-net and a conventional bag-net were deployed 250 m apart at a bay and a harbour. Modifications to the bag-net prevented seals from entering the inner chamber and trapping fish (e.g. rope-framed entrance replaced with rigid steel bars, heavier net material, a reduced mesh size in the net floor, tight corners inside the chamber closed off). Fishers reported fish catches and seal damage for modified and conventional bag-nets during a total of 130 hauls in July–August 2012.Study and other actions tested
A replicated, controlled study in 2009–2010 of three pelagic sites in the Baltic Sea and one seabed site in the North Sea, Sweden (Konigson et al. 2015) found that fishing pots with metal frames installed had fewer trapped grey seals Halichoerus grypus and harbour seals Phoca vitulina than conventional pots. No seals were trapped in pots with metal frames installed, whereas 3–9 seals/site (11 grey seals and 13 harbour seals in total) were trapped in conventional pots without metal frames. Catches of target cod Gadus spp. varied with different designs of metal frame (see original paper for details). In 2009–2010, baited fishing pots (with two chambers and 1–2 entrances) were deployed in 5–12 floating strings of ≤8 pots (three sites), or individually with ≤6 pots secured to the seabed (one site). The pots were deployed for a total of 2–20 months without metal frames installed followed by 3–10 months with frames. Metal frames of five designs were vertically mounted to the narrow end of pot entrances (see original paper for details). Fishers or observers on board fishing vessels checked the pots every 1–28 days in 2009–2010 and recorded the number and species of trapped seals.Study and other actions tested
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This Action forms part of the Action Synopsis:Marine and Freshwater Mammal Conservation
Marine and Freshwater Mammal Conservation - Published 2021
Marine and Freshwater Mammal Synopsis