Attach an electropositive deterrent to fishing gear

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages




  • Behaviour change (4 studies): Three of four replicated studies (one paired and controlled, one randomized and controlled, one randomized, and one controlled) in the Atlantic Ocean, Tasman Sea, and in laboratory conditions, found that the presence of potentially deterrent materials attached near the bait reduced the frequency of feeding attempts and bait consumption of spiny dogfish, great hammerhead and draughtboard sharks compared to the absence of deterrent materials. The other study found that a potentially deterrent material did not reduce bait consumption by bonnethead and young lemon sharks compared to non-deterrents. One of the studies also found that the bait consumption behaviour of commercially valuable Pacific halibut was unaffected by deterrent materials.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, randomized, controlled study in 2007 of an area of fished seabed in in the Gulf of Alaska, USA (Kaimmer & Stoner 2008) found that longline hooks with electropositive metal attached reduced the catches of unwanted spiny dogfish Squalus acanthias, longnose skate Raja rhina and undersized commercially targeted Pacific halibut Hippoglossus stenolepis, compared to standard hooks, and in most cases compared to hooks with non-electropositive metal attached. Catch rates were lower on electropositive hooks than standard and steel-fitted hooks for spiny dogfish (positive: 17, standard: 21, steel: 19 ind./50 hooks) and longnose skate (positive: 13, standard: 24, steel: 23 ind./50 hooks). Undersized (<82 cm) halibut catch rates were lower on electropositive hooks (1 ind./50 hooks) than standard hooks (2 ind./50 hooks) but similar to steel-equipped hooks (1 ind./50 hooks). Overall catch rates of halibut (all sizes) were similar between all hook types (3 ind./50 hooks). In September and October 2007, thirty-six longline sets were deployed in Kachemak Bay at 16–58 m depths of three hook types: circle hooks equipped with electropositive cerium mischmetal (a nonmagnetic metal alloy), standard circle hooks, and circle hooks equipped with inert steel that mimicked the mischmetal. All hooks were baited with 110–150 g of chum salmon Oncorhynchus keta. Hooks were fixed to a groundline in randomized blocks of 150 for each type (450 hooks/set) using 31 cm nylon lines attached to the groundline every 5.5 m. Gear was hauled after a minimum soak time of 90 min (average 192 min).

    Study and other actions tested
  2. A replicated, paired, controlled study in 2006 in a laboratory in the USA (Stoner & Kaimmer 2008) found that the bait attacking behaviour of spiny dogfish Squalus acanthias (an unwanted catch species), but not commercially valuable Pacific halibut Hippoglossus stenolepis, was reduced in the presence of one of two potentially deterrent materials (mischmetal but not magnets), compared to inert metals. Overall, in the presence of mishmetal, both the times taken for dogfish to first bite the bait (mischmetal: 2–19 min, inert: 1–2 min) and the times taken to remove the bait (mischmetal: 4–30 min, inert: 2–4 min) were higher compared to the inert metals, irrespective of the period of food deprivation. Using magnets as the deterrent, overall bait attack and removal times by dogfish were not significantly different compared to the inert metals (magnet: 1–5 min, inert: 1–2 min). However, dogfish showed strong behavioural avoidance responses (flinch, disorientation) to both the deterrent materials, but not to the inert metals. For halibut, there were no responses to the either the deterrent materials or the inert metals, and bait removal times were similar (deterrents: <1–30 min, inert: <1–30 min). In 2006, two types of potential deterrents (non-magnetic mischmetal alloy and magnets) and an inert stainless steel/aluminium material were tested separately on 12 dogfish and 16 halibut held in indoor pools. Each material was suspended on a short section of twine above pieces of squid Loligo opalescens bait (no hooks). Deterrent/baits were presented simultaneously in pairs with the inert/bait material and behavioural responses recorded by video. Trials were done on dogfish (5 trials) and halibut (4 trials) deprived of food for periods of up to 4 d (16–20 trials/species/food deprivation period).

    Study and other actions tested
  3. A replicated, controlled study in 2007 in an area of pelagic water in the Gulf of Maine, USA (Tallack & Mandelman 2009) found that attaching mischmetal (a nonmagnetic metal alloy) to commercial and recreational hook fishing gears did not reduce catches of spiny dogfish Squalus acanthias compared to hooks with no mischmetal. Total numbers of dogfish caught with mischmetal present was not statistically different from when mischmetal was absent for commercial longlines (present: 221 fish; absent: 244 fish) or rod and reel (jig) gear (present: 14 fish; absent: 16 fish). Fishing took place in August/September 2007 over 10 days. Pieces of mischmetal (45 × 45 mm and 5 mm thick) were attached 10 cm above the hook and bait on longlines and jigging gears. A total of 21 longlines (2,080 hooks, half with mischmetal and half without) were set for 1–2 h. Jigging using rod and reel took place during the longline deployments. Three rod and reels were configured with two hooks, one with mischmetal attached and one without. Seventy-three jig lines were set (146 baited hooks, half with mischmetal, half without). Captured fish were recorded by hook and bait type.

    Study and other actions tested
  4. A replicated, controlled study in 2011 in an area of pelagic water in the northwest Atlantic Ocean off Nova Scotia, Canada (Godin et al. 2013) found that longline hooks with electropositive metal attached did not reduce the unwanted catches of sharks (Selachii) overall or of blue shark Prionace glauca compared to standard hooks. Total shark catches (all species) were not statistically different between hooks with electropositive weights (33 ind./1,000 hooks), standard hooks (36 ind./1,000 hooks), or hooks with inert lead weights (44 ind./1,000 hooks). Blue shark catches were also similar across hook types (electropositive: 31, standard: 33, inert: 40 ind./1,000 hooks). Catches of other unwanted sharks (mako Isurus oxyrinchus, porbeagle Lamna nasus) and other, commercially valuable species (bluefin tuna Thunnus thynnus, albacore Thunnus alalunga, anglerfish Lophiiformes spp.) were generally low across all hook types (data not tested statistically). Target catches of swordfish Xiphias gladius were lower on hooks with electropositive weights (12 fish/1,000 hooks) and lead weighted hooks (10 fish/1,000 hooks) compared to standard hooks (23 fish/1,000 hooks). In September and October 2011, a total of 6,300 hooks were set during 70 experimental gear sets (900 hooks/set) in a longline swordfish Xiphias gladius fishery. Each set used three hook types: standard hooks, hooks with electropositive metal weights (neodymium and praseodymium), and hooks with inert lead weights, with 300 hooks/hook type.

    Study and other actions tested
  5. A replicated, randomized study in 2010–2011 in a laboratory in Florida, USA (McCutcheon & Kaijura 2013) found that a potentially deterrent metal (neodymium) attached to bait did not reduce the incidence of bait capture by bonnethead shark Sphyrna tiburuo or young lemon shark Negaprion brevirostris, compared to three non-deterrent materials. Across trials, the percentages of bites taken to remove bait from the neodymium was similar to all three other materials for both bonnetheads (neodymium: 27–32%, acrylic: 23–25%, lead: 22–23%, and stainless steel: 23–27%) and young lemon sharks (neodymium: 35%, acrylic: 23%, lead: 23%, and stainless steel: 21%). A total of 12 bonnethead sharks and 13 immature lemon sharks caught by gillnet and hook and line fishing in September 2010–August 2011, were maintained in an aquarium. After one week to acclimatise, sharks were starved for 48 h. Four equal-sized (2.5 × 2.5 × 0.6 cm) pieces of neodymium (the test material), acrylic, lead, and stainless steel were put in a shallow (0.9 m depth × 4.6 m diameter) tank fixed to a 1m2 acrylic plate. Position of each material was randomized each trial. Sharks were introduced to the tank and the baited (shrimp for bonnethead, mullet and herring for lemon sharks) plates they bit were recorded. Bonnethead sharks were tested individually and in groups of 2–4 and lemon sharks were tested only in groups of 2–4 (see original paper for full methods).

    Study and other actions tested
  6. A systematic review in 2015 of 17 relevant studies of 44 in global pelagic waters (Favaro & Côté 2015) of devices to reduce unwanted catch, found that using electropositive and magnetic materials, or a combination of both, on hooks in longline fisheries did not reduce the amount of unwanted sharks and rays (Elasmobranchii) caught overall, compared to traditional J hooks alone. Data were reported as percentage catch reductions relative to the standard. Numbers of sharks and rays caught on hooks with electropositive and/or magnetic materials were not significantly different overall than those caught on J hooks alone (electropositive: 18% less, magnets: 32% less, combined: 29% less). One study found a reduction in catches of juvenile scalloped hammerhead Sphyrna lewini of 57% on hooks with electropositive materials, but authors note the result was inconsistent with that for adults. The systematic review summarized the effects of various actions to reduce unwanted catch, including using electropositive materials, magnetic materials, or a combination of both in longline fisheries from 10, five and one global studies respectively.

    Study and other actions tested
  7. A replicated, randomized, controlled study in 2010–2013 in one area of sandy seabed in the Atlantic Ocean off South Bimini, Bahamas (O'Connell et al. 2015) found that great hammerhead sharks Sphyrna mokarran avoided bait bags with permanent magnet deterrents attached and fed less frequently compared to bait with no deterrents or bait set with non-magnetic deterrents. All data were reported on a log scale. In two of two experiments, sharks demonstrated greater avoidance behaviour to bait bags with magnet deterrents attached compared to bait bags with no deterrents or nets with non-magnetic deterrents attached. Feeding rates were lower on bait bags with magnet deterrents than bait with no deterrents or non-magnetic deterrents, between which feeding rates were similar. Sharks also exhibited higher rates of avoidance behaviour around ropes set with magnets and non-magnetic deterrents than ropes with no deterrents, around which avoidance behaviour was similar. Two experiments were undertaken between January 2010-March 2013 at depths of 3–8 m. In the first, mesh bags baited with great barracuda Sphyraena barracuda were set on 1 m2 plastic apparatus with either a magnetic deterrent, a visually identical non-magnetic deterrent, or no deterrent (90 trials of 30 min). The deterrents were randomly ordered. In the second experiment, three 6 m surface ropes were set with either no vertical ropes, vertical ropes (1.5 m apart) mounted with the magnetic deterrent or ropes mounted with the non-magnetic deterrent (42 × 30 min trials). Shark behaviour was monitored from a vessel observation platform.

    Study and other actions tested
  8. A replicated, controlled study in 2013–2014 in three areas of sandy seabed in the South Pacific Ocean off the coast of New South Wales, Australia (Richards et al. 2018) found that traps fitted with magnets caught fewer unwanted sharks and rays (Elasmobranchii), and more of the commercially targeted species snapper Pagrus auratus, compared to conventional traps or traps fitted with non-magnetic material. Catch rates of sharks and rays were lower in traps with magnets (0.2 ind./trap) compared to standard traps (0.3 ind./trap) and traps with non-magnetic material (0.3 ind./trap). Target catches of snapper were highest in traps with magnets (magnets: 1.1 kg/trap, standard: 0.8 kg/trap, non-magnetic: 0.98 kg/trap). In addition, the presence of sharks and rays in traps reduced target snapper catches by 38%. Between December 2013 and August 2014, a total of 1,015 traps of three different designs were set in three areas of sandy seabed at 5–102 m depth. The fish traps had a wooden frame 180 x 120 x 80 cm covered in 50 mm wire mesh with a 100 x 60 mm escape panel at the rear. Each trap design had three funnel entrances (290 x 540 mm outer and 60 x 270 mm inner) which had either four magnets (75 x 13 x 16 mm) attached to each funnel, four non-magnetic bars of the same size attached to each funnel, or no change to the standard trap.

    Study and other actions tested
  9. A replicated, controlled study in 2014 in shallow inshore waters in the Tasman Sea off Tasmania, Australia (Westlake et al. 2018) found that the presence of magnets reduced feeding attempts on bait by draughtboard sharks Cephaloscyllium laticeps, compared to bait without magnets. When magnets were present, sharks made 25 and 19 feeding attempts (for two and four magnets respectively), and 53 attempts were made with no magnets. The bait was approached but no feeding was attempted 174 times with two magnets present, 144 times with four magnets present and 123 times with no magnets present. A total of 12 trials were carried out on separate occasions between August–December 2014, each with the following three treatments placed 20 metres apart: two or four magnetic resin blocks attached to rods 0.2 m either side of jack mackerel Trachurus declivis bait, and two non-magnetic blocks on rods either side of the bait. Each device was placed on the seabed and recorded by video for 90 minutes.

    Study and other actions tested
Please cite as:

Taylor, N., Clarke, L.J., Alliji, K., Barrett, C., McIntyre, R., Smith, R.K., and Sutherland, W.J. (2021) Marine Fish Conservation: Global Evidence for the Effects of Selected Interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.

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Marine Fish Conservation

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Marine Fish Conservation
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