Review of Magnetic Shark Deterrents: Hypothetical Mechanisms and Evidence for Selectivity

Abstract

Several papers published since 2006 describe effects of magnetic fields on elasmobranchs and assess their utility in reducing negative interactions between sharks and humans, including bycatch reduction. Most of these repeat a single untested hypothesis regarding physical mechanisms by which elasmobranchs detect magnetic fields and also neglect careful consideration of magnetoreception in teleosts. Several species of teleosts are known to have magnetoreception based in biogenic magnetite, and direct magnetic field detection also has support in several species of elasmobranchs. The overly narrow focus of earlier papers on the unsupported hypothesis that magnetoreception in elasmobranchs is based in the ampullae of Lorenzini creates the impression that all teleosts will be insensitive to magnetic deterrents. However, magnetite based magnetoreception has been demonstrated in several teleosts, and is supported in others. Furthermore, electroreception is present in many teleost species; therefore, the possibility of induction based indirect magnetoreception should be considered.

Passive electroreception in bottlenose dolphins (Tursiops truncatus): implication for micro- and large-scale orientation

“…Because electric and magnetic fields are closely linked in the marine environment ( Newton et al, 2019 ) and given that any electrosensitive organism can potentially sense a magnetic field ( Courtney et al, 2015 ), it is possible that the ability of dolphins to detect weak electric fields bears the potential for orientation to the Earth's magnetic field through induction-based magnetoreception ( Formicki et al, 2019 ; Johnsen and Lohmann, 2005 ; Jungerman and Rosenblum, 1980 ; Lohmann and Johnsen, 2000 ; Molteno and Kennedy, 2009 ; Newton et al, 2019 ). As described for sharks, induction-based magnetoreception means that an animal experiences potential discernible differences induced in its body as it swims through the Earth's magnetic field ( Kalmijn, 1974 , 1977 , 1978 , 1981 , 1982 ; Paulin, 1995 ).…”

“…Bioelectric fields facilitate essential activities such as finding prey ( Kalmijn, 1966 ), locating a mating partner ( Tricas et al, 1995 ; Tricas and Sisneros, 2004 ) and avoiding predators ( Kempster et al, 2013 ; Sisneros et al, 1998 ). In addition, passive electroreception generally has the potential for large-scale orientation based on the perception of geomagnetic fields through electromagnetic induction in marine habitats ( Courtney et al, 2015 ; Jungerman and Rosenblum, 1980 ; Kalmijn, 1981 ).…”

“…After these prior data clarified that the bottlenose dolphin must also be considered an electroreceptive species, we examined in this follow-up study the sensitivity of the system by determining detection thresholds for weak electrical DC and AC fields for two bottlenose dolphins. Because any electrosensitive organism can potentially sense a magnetic field ( Courtney et al, 2015 ), we discuss the potential importance – beyond close-range function – of passive electroreception in dolphins for large-scale orientation.…”

Evidence for Magnetoreception in Red Drum (Sciaenops ocellatus), Black Drum (Pogonias cromis), and Sea Catfish (Ariopsis felis)

“…The hypothesis that magnetoreception in elasmobranchs occurs via electromagnetic induction in the ampullae of Lorenzini gives rise to an expectation that teleosts cannot sense magnetic fields, even though several species of teleosts have been shown to be magnetoreceptive. (Courtney et al, 2014) The adoption of magnetic deterrents to reduce shark bycatch depends upon deterrent selectivity to only deter unwanted species and not reduce the catch of target species. Determining whether or not target teleost species are sensitive to magnets requires experiments in a range of teleost species.…”