Electropositive metals (EPMs) are a new class of

shark repellent A shark repellent is any method of driving sharks away from an area. Shark repellents are a category of animal repellents. Shark repellent technologies include magnetic shark repellent, electropositive shark repellents, electrical repellents, an ...

materials that produce a measurable

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

when immersed in an

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

such as

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

. The voltages produced are as high as 1.75 VDC in seawater. It is hypothesized that this voltage overwhelms the ampullary organ in sharks, producing a repellent action. Since bony fish lack the ampullary organ, the repellent is selective to sharks and rays. The process is electrochemical, so no external power input is required. As chemical work is done, the metal is lost in the form of corrosion. Depending on the alloy or metal utilized and its thickness, the electropositive repellent effect lasts up to 48 hours. The reaction of the electropositive metal in seawater produces hydrogen gas bubbles and an insoluble nontoxic hydroxide as a precipitate which settles downward in the water column.

History

SharkDefense SharkDefense is a research organization and think tank focused on shark bycatch reduction. Its core research involves chemical and electrochemical shark repellent technologies. SharkDefense's repellent experiments have been featured extensively in t ...

made the discovery of electrochemical shark repellent effects on May 1, 2006 at

South Bimini Bimini is the westernmost district of the Bahamas and comprises a chain of islands located about due east of Miami. Bimini is the closest point in the Bahamas to the mainland United States and approximately west-northwest of Nassau. The populat ...

Bahamas The Bahamas (), officially the Commonwealth of The Bahamas, is an island country within the Lucayan Archipelago of the West Indies in the Atlantic Ocean, North Atlantic. It takes up 97% of the Lucayan Archipelago's land area and is home to ...

at the Bimini Biological Field Station. An electropositive metal, which was a component of a permanent magnet, was chosen as an experimental control for a

tonic immobility Apparent death, colloquially known as playing dead, feigning death, or playing possum, is a behavior in which animals take on the appearance of being dead. It is an immobile state most often triggered by a predatory attack and can be found in a ...

experiment by Eric Stroud using a juvenile

lemon shark The lemon shark (''Negaprion brevirostris'') is a species of shark from the family Carcharhinidae and is classified as a Vulnerable species by the International Union for the Conservation of Nature. Lemon sharks can grow to in length. They are o ...

(''Negaprion brevirostris''). It was anticipated that this metal would produce no effect, since it was not

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

. However, a violent rousing response was observed when the metal was brought within 50 cm of the shark's nose. The experiment was repeated with three other juvenile lemon sharks and two other juvenile

nurse shark The nurse shark (''Ginglymostoma cirratum'') is an elasmobranch fish in the family Ginglymostomatidae. The conservation status of the nurse shark is globally assessed as Vulnerable in the IUCN List of Threatened Species. They are considered t ...

s (''Ginglymostoma cirratum''), and care was taken to eliminate all stray metal objects in the testing site. Patrick Rice, Michael Herrmann, and Eric Stroud were present at this first trial.

Mike Rowe Michael Gregory Rowe (born March 18, 1962) is an American television host and narrator. He is known for his work on the Discovery Channel series ''Dirty Jobs'' and the series ''Somebody's Gotta Do It'' originally developed for CNN. He hosted a s ...

, from

Discovery Channel Discovery Channel (known as The Discovery Channel from 1985 to 1995, and often referred to as simply Discovery) is an American cable channel owned by Warner Bros. Discovery, a publicly traded company run by CEO David Zaslav. , Discovery Channe ...

’s

Dirty Jobs ''Dirty Jobs'' is an American television series that originally aired on the Discovery Channel in which host Mike Rowe is shown performing difficult, strange, disgusting, or messy occupational duties alongside the job's current employees. The sh ...

series, subsequently witnessed and participated in a test using an electropositive metal within 24 hours after the discovery. In the next three months, a variety of

transition metals In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

lanthanides The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yttr ...

post-transition metal The metallic elements in the periodic table located between the transition metals and the chemically weak nonmetallic metalloids have received many names in the literature, such as ''post-transition metals'', ''poor metals'', ''other metals'', ...

metalloids A metalloid is a type of chemical element which has a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals. There is no standard definition of a metalloid and no complete agreement on which elements are ...

, and non-metal samples were screened for rousing activity using the tonic immobility bioassay in juvenile lemon sharks and juvenile nurse sharks. All behaviors were scored from 0 to 4 depending on the response. It was determined that Group I, II, III, and Lanthanide metals all produced rousing responses, but the average score generally increased with

electropositivity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...

. Further testing using

salt bridge In electrochemistry, a salt bridge or ion bridge is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell (voltaic cell), a type of electrochemical cell. It maintains electrical neutrality within the int ...

electrochemical cells were conducted during 2006 and 2007 at the Oak Ridge Shark Lab. Using seawater as the electrolyte and a shark fin clipping as the cathode, voltages measured closely correlated with the standard reduction potential of the metal under test. SharkDefense now hypothesizes that a net positive charge from the cations produced by the electropositive metals accumulate on the electronegative skin of the shark. The net increase of the charge on the shark's skin is perceived by the

ampullae of Lorenzini Ampullae of Lorenzini (singular ''Ampulla'') are electroreceptors, sense organs able to detect electric fields. They form a network of mucus-filled pores in the skin of cartilaginous fish (sharks, rays, and chimaeras) and of basal actinopterygia ...

, and above 1.2 eV potential, aversion is produced. Electropositive metals are

reducing agents In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...

and liberate

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

gas in seawater via

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

, producing a half-cell voltage of about −0.86 eV. Simultaneously, an insoluble metal hydroxide precipitate is produced, which is inert for shark repellent activity. As such, metal is lost to

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

in the process of generating cations. SharkDefense conducted corrosion loss studies in 2008 at South Bimini, Bahamas, and found that a 70 gram piece of a custom electropositive alloy retained more than 50% of its original weight after 70 hours of immersion. Losses due to corrosion are heavily a function of temperature, therefore, the cold seawater at fishing depths serves to reduce the corrosion rate.

Research and testing

Stoner and Kaimmer (2008) reported success using cerium

mischmetal Mischmetal (from german: Mischmetall – "mixed metal") is an alloy of rare-earth elements. It is also called cerium mischmetal, or rare-earth mischmetal. A typical composition includes approximately 55% cerium, 25% lanthanum, and 15 ...

and Pacific

spiny dogfish The spiny dogfish (''Squalus acanthias''), spurdog, mud shark, or piked dogfish is one of the best known species of the Squalidae (dogfish) family (biology), family of sharks, which is part of the Squaliformes order. While these common names ma ...

(''Squalus acanthias'', a type of shark) in captivity, both with tonic immobility and feeding preference tests. Lead metal was used as a control. Encouraged by the results, a longline study was conducted off

Homer, Alaska Homer ( Dena'ina: ''Tuggeght'') is a city in Kenai Peninsula Borough in the U.S. state of Alaska. It is southwest of Anchorage. According to the 2020 Census, the population is 5,522, up from 5,003 in 2010. Long known as the "Halibut Fishing Ca ...

in late 2007 with the cooperation of the International Pacific Halibut Commission. Again, lead was used as a control. This study found a 17% reduction in Pacific spiny dogfish catch, and a 48% reduction in clearnose skate catch. However, Tallack et al. reported that cerium mischmetal was entirely ineffective against Atlantic spiny dogfish in the

Gulf of Maine The Gulf of Maine is a large gulf of the Atlantic Ocean on the east coast of North America. It is bounded by Cape Cod at the eastern tip of Massachusetts in the southwest and by Cape Sable Island at the southern tip of Nova Scotia in the northeast ...

. Mandelman et al. reported that the repellent effect disappeared after starvation using captive Atlantic spiny dogfish, and that a species-specific variation in response to the mischmetals exist between captive Atlantic spiny dogfish and dusky smoothhounds (''Mustelis canis''). Stroud (SharkDefense, 2006) and Fisher (VIMS) observed captive

cownose ray The cownose ray (''Rhinoptera bonasus'') is a species of Batoidea found throughout a large part of the western Atlantic and Caribbean, from New England, United States to southern Brazil (the East Atlantic populations are now generally considered ...

s (''Rhinoptera bonasus'') changing swim elevation and ignoring blue crab baits in cages that contained neodymium-praseodymium mischmetal. The position of the treatment cages were alternated, and all cages were placed in the swim path of the rays. Brill et al. (2008) reported that captive juvenile

sandbar shark The sandbar shark (''Carcharhinus plumbeus'') also known as the brown shark or thickskin shark, is a species of requiem shark, and part of the family Carcharhinidae, native to the Atlantic Ocean and the Indo-Pacific. It is distinguishable by its ...

s (''Carcharhinus plumbeus'') maintained a 50–60 cm clearance in their swimming patterns when a piece of neodymium-praseodymium mischmetal was placed in the tank. Wang, Swimmer, and Laughton (2007) reported aversive responses to neodymium-praseodymium mischmetals placed near baits offered to adult Galapagos (''C. galapagensis'') and Sandbar sharks on bamboo poles in Hawaii. In July 2008 Richard Brill of NMFS/VIMS and SharkDefense both conducted more at-sea trials with electropositive metals in an effort to reduce shark bycatch in commercial fisheries. As of August 2, 2008, Brill reported nearly a 3:1 reduction in sandbar shark catch when plastic decoys were compared to metals. A high statistical significance was obtained, as reported in th
Virginian-Pilot
by Joanne Kimberlin. SharkDefense later developed a simple on-hook treatment and a bait attachment which were being tested on Atlantic longlining vessels in 2008. Favaro and Cole (2013) determined through meta-analysis that electropositive metals did not reduce elasmobranch by-catch in commercial long-line fisheries, which raises concerns on the effectiveness of this approach as a shark deterrent or repellent to protect water users.

Selectivity

As expected, teleosts (bony fish) are not repelled by the electropositive metal's cation liberation in seawater. This is because teleosts lack the

. Teleost response was confirmed using captive

Cobia The cobia (''Rachycentron canadum'') is a species of carangiform marine fish, the only extant representative of the genus ''Rachycentron'' and the family Rachycentridae. Other common names include black kingfish, black salmon, ling, lemonfis ...

(''Rachycentron canadum'') and

Pacific halibut The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

(''Hippoglossus stenolepis''). In July 2008

swordfish Swordfish (''Xiphias gladius''), also known as broadbills in some countries, are large, highly migratory predatory fish characterized by a long, flat, pointed bill. They are a popular sport fish of the billfish category, though elusive. Swordfis ...

(''Xiphias gladius'') catch was reported on experimental hooks treated with electropositive metal.

Limitations

As with all shark repellents, 100% effectiveness will not be achieved with electropositive metals. The metals are particularly effective when the shark is relying on its electrosense. It is likely that electropositive metals are ineffective for deliberately stimulated (chummed) sharks, competitively feeding sharks, and shark "frenzies". The metals are very useful in the environment of commercial fisheries, and possibly recreational and artisanal fisheries.

History

Research and testing

Selectivity

Limitations

References

Further reading