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Stroud claims that sharks are effectively repelled by the use of rare earth metals in the water. There has even been a patent issued by the same author for rare earth-wrapped fish hooks to reduce the shark by-catch.

Apparently these metals work by generating an electric current that repels sharks via the shark's electric sensing organ.

Is the repellent action of rare earth metals scientifically confirmed? And if so would it be effective even on the largest and aggressive white sharks?

Also if effective why haven't dive equipment manufacturers developed shark repellent devices for sport divers?

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closed as off-topic by rg255, Chris, WYSIWYG, March Ho, The Last Word Jun 2 '15 at 8:34

  • This question does not appear to be about biology within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ @AliceD from Canberra - thanks for the edit and go at it. So looks like good science hasn't sifted all the facts through the sieve quite yet. There are demonstrations of the effect on YouTube & the NOVA episode with David Pogue that are impressive. But looks like no one is investing in the design of rare earth diving gear quite yet ; ) $\endgroup$ – docscience Jun 1 '15 at 3:11
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    $\begingroup$ @AliceD A small correction. Elements cannot degrade (unless it is a nuclear reaction). You should say that lanthanides react with water. $\endgroup$ – WYSIWYG Jun 1 '15 at 5:25
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    $\begingroup$ I'm voting to close this question as off-topic because this question does not ask about the biology of this phenomenon and is therefore much more suited to skeptics SE $\endgroup$ – rg255 Jun 1 '15 at 5:43
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    $\begingroup$ @WYSIWYG regarding "one electrode" not necessarily so. You can consider the shark's body serving as the other electrode with seawater as the electrolyte, but also as current paths. Current grows larger as the shark gets closer to the metal. Like biting down on aluminum foil with amalgam fillings. Just a few milliamps - very uncomfortable. $\endgroup$ – docscience Jun 1 '15 at 13:39
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    $\begingroup$ Please don't kill the question. Give it some time and perhaps someone working in this area will come up with an answer. If you throw it over to physics, chemistry or skeptics it certainly won't receive the same attention some ichthyologist may give it. It is a multidisciplinary question but belongs with biology. Also I'm not asking for experimental 'proof' - that doesn't exist. And I'm not asking for help from the mythbusters. Just looking for strong experimental inference and further detail of what's going on. $\endgroup$ – docscience Jun 1 '15 at 13:48
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Short answer
Electropositive metals such as the Lanthanides generate electrical currents when they come in contact with (sea) water. This electrical activity may repel sharks by over-stimulating their lateral line organ. However, the effectiveness of lanthanides as shark repellent depends on many factors. For example, hungry sharks, or sharks engaged in a feeding frenzy may simply ignore the stimulus altogether.

Background
Rare earth metals are Lanthanides - metals with a low electronegativity. Metals with low electronegativity are called electropositive. Electropositive metals (EPMs) react vigorously with water when immersed and generate electrical potentials that are thought to overstimulate the sharks’ highly sensitive electroreceptors, the ampullae of Lorenzini.

Many electropositive metals fall in groups I and II of the periodic table and explosively react with water. Examples include sodium and lithium. The Lanthanides, on the other hand, contain some less reactive elements.

There is in fact a considerable body of primary literature on the use of EPMs to deter sharks, because the by-catch of sharks is a recognized problem of modern fishery. However, much of this literature shows very mixed results of EPMs, and therefore I have based my (second :-) answer attempt on a recent review article by Hart and Collin (2015), which is available on-line and cites some fifteen primary research articles on the topic:

EPMs are primarily intended as a mechanism to reduce shark bycatch and catch depredation in longline fisheries, without affecting the catch of non-electrosensitive target teleost fishes. As you indicate in the question, EPMs can be formed into shapes that are easily attached to a fishing line or hook.

Early tests in the laboratory with captive sharks showed that several EPMs (including magnesium and rare-earth lanthanide elements such as cerium, lanthanum, neodymium and praseodymium) were effective in repelling various shark species. However, more recently, several studies showed no such repellent effects other species of shark, and it was shown that aversive behavior was highly dependent on various factors such as duration of food deprivation prior to testing. Specifically, hungry sharks ignored the EPM, especially when feeding in groups (feeding frenzies).

The results of field trials with EPMs were also mixed, with some studies showing a reduction in the catch rate of various shark species, while other studies showed no such effect. The authors of the review article summarize a range of factors that may influence the efficacy of EPMs in repelling sharks, including the type of EPM used, the relative sensitivity of the electrosensory system, shark density and competition, hunger level, and differences in feeding ecology. Other impediments to large-scale adoption of EPM technology by commercial fisheries include the relatively high cost of the rare-earth metals (although magnesium may represent a cheaper alternative), potential toxicity to other marine animals, and the fact that they dissolve rapidly in seawater and, therefore, must be replaced frequently. The use of EPMs for personal shark deterrent devices is also hampered by the limited effective range (<85 cm) of the electrical field they generate.

Instead, for use in scuba dive gear a rechargeable battery-powered capacitor-discharge unit connected to two or four electrodes is sufficient as shark repellent. Also, electro-magnetic induction using permanent magnets may be a more suitable alternative (Hart and Collin, 2015).

Regarding the great white: this shark has a lateral line featuring ampullae of Lorenzini, so EPMs may repel it, theoretically.

Reference
- Hart & Collin. Integrative Zoology; 10(1): 38–64

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    $\begingroup$ "Lanthanides are very rare elements, and that is the reason your article speaks of rare earth elements" - actually 'rare' is a misnomer. The lanthanides are highly abundant in the earth's crust. Reference to rare rather comes from the difficulty in separating them from one another since the chemical properties are very similar. $\endgroup$ – docscience Jun 1 '15 at 2:11
  • $\begingroup$ "However, when a magnet is combined with a rare earth element the repellent action is more vigorous" So then why? Is it something chemical plus the magnetic field of the ferro magnet or just because you can make stronger magnetic fields with rare earth materials? $\endgroup$ – docscience Jun 1 '15 at 2:12
  • $\begingroup$ @docscience - The Lanthanide definition is updated in my secnd attempt, and it's about current, not magnetism for EPM, so your second comment is also answered as such. $\endgroup$ – AliceD Jun 1 '15 at 14:30
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    $\begingroup$ Nitpicking: Magnesium does not react explosively with water. From the second group calcium metal would react strongly. $\endgroup$ – Chris Jun 1 '15 at 14:34
  • $\begingroup$ @Chris - I have deletet the Mg example, thanks! Nit pick away! $\endgroup$ – AliceD Jun 1 '15 at 14:35

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