- Magnetic shark repellent
Magnetic shark repellents utilize permanent magnets, which exploit the sensitivity of the Ampullae of Lorenzini in sharks and rays (electrosense). This organ is not found on bony fishes (teleosts), therefore, this type of shark repellent is selective to sharks and rays. Permanent magnets do not require power input, making them ideal for use in fisheries and as bycatch reduction devices.
During November 2004, Sharkdefense researcher Eric Stroud accidentally dropped a magnet onto a rubber mat near a captive tank at the Oak Ridge Shark Laboratory. He noticed that juvenile nurse sharks (G. cirratum) near the tank wall swam away. While the initial event may have been due to vibrations, it led him to test the effects of the magnet on the captive sharks. Placing the magnet within the tank, Eric observed that nurse sharks avoided the region around the magnet. Follow-on tests in 2005 with Michael Herrmann at the laboratory used an acrylic Y-Maze and showed preference towards non-magnetic exits and strong conditioning. During February 2005, Patrick Rice and Eric Stroud conducted tonic immobility trials at the Bimini Biological Field Station, Bahamas, which confirmed that juvenile lemon sharks (N. brevirostris) and juvenile nurse sharks (G. cirratum) roused when permanent magnets were presented within 50 cm of the sharks nares. Interestingly, mobility was not terminated when strong electromagnets were placed near the sharks.
In May 2006, SharkDefense won the WWF SmartGear award for its proposal to selectively reduce sharks bycatch in commercial fisheries using permanent magnets. Michael Herrmann accepted the award in Brussels on behalf of SharkDefense. Much excitement was generated in the research and conservation communities as a result of the media coverage of this announcement. SharkDefense is the inventor and is currently the patent application holder for this technology and actively collaborates with other researchers to investigate species-specific effects and gear design.
On January 1, 2009, a peer-reviewed publication described experiments in Australia showing the efficacy of using magnets to deter sharks. The citation is: Rigg, D.P., Peverell, S.C., Hearndon, M., Seymour, J.E. Do elasmobranch reactions to magnetic fields in water show promise for bycatch mitigation? Marine & Freshwater Research, 2009,60, 942-948.
On January 12, 2010, Craig O'Connell from SharkDefense also published a peer-reviewed paper on the efficacy of magnetic shark repellents. The literature citation is: O’Connell, C.P., Rice, P.H., Stroud, E.M., Abel, D.C., Simuro, N.C. The Effects of Permanent Magnets on the Southern Stingray (Dasyatis americana) and the Nurse Shark (Ginglymostoma cirratum). Marine and Freshwater Behavior and Physiology, April 2010. A second, confirming paper is currently in peer review.
Several species of sharks have demonstrated the ability to sense magnetic fields (Kalmijn, 1978; Ryan, 1980; Klimley, 1993; 2002). The Ampullae of Lorenzini organ within sharks is used to detect weak electrical fields at short ranges. The detection range of this organ is effective only within inches, as sharks sense bioelectrical fields in the final stages of prey capture. The flux per unit area of certain permanent magnets, particularly Neodymium-Iron-Boride and Barium-Ferrite magnets, corresponds closely with the detection range of the Ampullae of Lorenzini. The fields generated by these permanent magnets (ferrite and rare-earth types) decrease at the inverse cube of the distance from the magnet to sharks and rays. Therefore, at distances of a few meters from the magnet, the field exerted is less than the Earth's magnetic field. Animals which lack that Ampullae of Lorenzini organ do not display aversive behavior in close proximity to the magnetic field, making this technology selective. A permanent magnet with the correct specifications is hypothesized to over-stimulate the Ampullae of Lorenzini, and may therefore be used as selective shark repellent.
When a shark swims through the Earth's magnetic field, electromagnetic induction – phenomena which generates an electric field as charged particles move through a magnetic field – creates an electric field around the shark. Minute differences in the Earth's magnetic field at different locations result in minute differences in the induced electric field which may be detected by the shark's sensitive electroreceptors, especially as the head region moves back and forth during swimming (Lohmann and Johnsen 2000).
The law of electromagnetic induction (Faraday's law of induction) states that induced electromotive force (EMF) is proportional to the rate of change of the magnetic flux through a coil (an electric current can also be produced within a conductor when the conductor is moved through a magnetic field). This occurs because the force generated by the magnetic lines is applying a force on the free electrons in the conductor, causing the electrons to move. We hypothesize that the shark's body (particularly it's Ampullae of Lorenzini) acts as the conductor moving through the Earth's magnetic field or the permanent magnet's field, registering the induced EMF.
Craig O’Connell, a partner with Sharkdefense, completed his Master’s Thesis at Coastal Carolina University with regards to magnetic shark repellents as selective shark bycatch reduction devices. His studies involved Southern Stingrays (D. americana), adults nurse sharks (G. cirratum), blacktip sharks (C. limbatus), adult lemon sharks (N. brevirostris), and Atlantic Sharpnose sharks (R. terranovae) and showed a reduction of shark catch on demersal longlines and rod-and-reel studies. Craig is currently a PhD student at U Mass Dartmouth, continuing shark magnetoreception studies.
In 2008, the Department of Primary Industries and Fisheries (DPI&F) and James Cook University, Australia, reported success with permanent magnets in captive studies with grey reef sharks, hammerheads, sharp-nosed sharks, blacktip sharks, sawfish and the critically endangered spear tooth shark. AAP Press Release is here.
During July 2008, the MythBusters aired a segment on Discovery Channel Shark Week showing the repulsive effects of C8 ferrite magnets on captive juvenile sharks. A subsequent test at Tiger Beach, Nassau used competitively-feeding adult sharks, and the repulsive effects were not observed – However, it was noted that different magnets were used at Tiger Beach than in the initial tests. It is hypothesized that overstimulation of olfaction overrides the electrosense in sharks, especially when underwater visibility is good and feeding is socially facilitated.
During July 2007, Les Stroud aired a segment on Discovery Channel Sharkweek which concluded that a great hammerhead was not repelled by a rare earth magnet. It was noted that the magnet was placed quite far away from the shark, neglecting that the range of the ampullary organ is less than 60 cm. This test also contradicts recent positive findings (2008) by the Department of Primary Industries and Fisheries (DPI&F) and James Cook University, Australia, with hammerheads.
Kalmijn A.J., 1971 The Electric Sense of Sharks and Rays. Journal of Experimental Biology 55,371-383
Kalmijn A.J., 1982 Electric and magnetic field detection in elasmobranch fishes. Science, Vol. 218, Issue 4575, 916-918
Klimley, A. P. 1993. Highly directional swimming by scalloped hammerhead sharks, Sphyrna lewini, and subsurface irradiance, temperature, bathymetry, and geomagnetic field. Marine Biology. 117, 1-22.
Klimley, A. P., S. C. Beavers, T. H. Curtis, and S. J. Jorgensen. 2002. Movements and swimming behavior of three species of sharks in La Jolla Canyon, California. Environmental Biology of Fishes. 63, 117-135.
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