The emerging field of electrosensory and semiochemical shark ...

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C.P. O’Connell et al. / Ocean & Coastal Management xxx (2012) 1e10
and commercial fishing industries where shark densities are
sufficiently low. Lastly, Jordan et al. (2011) examined the deterrent
capabilities of an EPM stimulus (NeodymiumeNd) on the smooth
dogfish (M. canis). When tested in groups, M. canis preferred to feed
from EPM-associated baits, whereas individually tested M. canis
were significantly deterred by EPM-associated baits. This demonstrates
that the effectiveness of EPMs, or more specifically,
Neodymium on M. canis, may be heavily influenced by conspecific
density. Therefore, future studies should account for animal density
as these studies illustrate that density-induced behavioral changes
toward electrosensory deterrents is plausible.
3.4. Salinity
In regards to EPMs, the conductive medium is an essential
component to the successful transfer of cations from the surface of
the metal to the approaching electronegative shark. The lower the
resistivity, or opposition of electric current, as seen in highly saline
environments (Chave et al., 1991; Eidesmo et al., 2002), will result
in the successful transfer of these cations and prove the metals to
be more effective (Stroud, 2008a). It is essential that future studies
monitor salinity, as deploying metals in open ocean regions (i.e.
peak salinity) may produce drastically different results in
comparison to estuarine or nearshore environments that are
heavily influenced by freshwater input.
It is uncertain how salinity may impact elasmobranch magnetoreception.
In order for elasmobranch magnetoreception to occur,
two key elements are required for active and/or passive electromagnetic
induction: a magnetic field (B) and velocity (v) (Kalmijn,
1982, 1984). For the active mode of electromagnetic induction, the
velocity corresponds to movement of the organism through B h
(horizontal portion of Earth’s geomagnetic field); however, in the
passive mode, the velocity component refers to the velocity of the
ocean currents through B v (vertical portion of Earth’s geomagnetic
field). Although v and B are key elements required for elasmobranch
magnetic field detection, another element is the conductive
medium that the animal is swimming within (Kalmijn, 1974, 1984,
1997). The resistivity of seawater and freshwater drastically differs,
with freshwater being characterized of having a resistivity of 2e
200 U m and seawater 0.3 U m(Pashley et al., 2005; Chave et al.,
1991; Eidesmo et al., 2002). In lower salinity conditions, the electric
potential gradients between the pore (apical) and internal
(basal) structures that are responsible for neural activity, tend to be
minimal thus making the induction mechanism less efficient in
freshwater (Kalmijn, 1974, 1981, 1984). Therefore, salinity may
impact the magnetoreception capabilities of elasmobranchs and
estuarine, brackish water, or even freshwater ecosystems may not
be ideal ecosystems for magnetic deterrents (Kalmijn, 1974, 1984).
3.5. Animal size/canal length
Ontogentic shifts in electroreceptor sensitivity has been
demonstrated with several elasmobranch species (Sisneros et al.,
1998; Sisneros and Tricas, 2002). It is suggested that these shifts
or increases in sensitivity as an animal matures is directly related to
increasing ampullary canal legnth. The ampullae of Lorenzini
function on voltage gradients, with neurotransmitters being
released to primary afferent neurons when a difference exists
between the pore (apical) and internal (basal) potentials (Bennett,
1971; Tricas, 2001). Thus enhanced sensitivity is a function of canal
length since greater voltage gradients exist between the pore
surface and internal ampullary canal as ampullary canals lengthen.
In addition, previous studies pertaining to the Atlantic stingray
(Dasyatis sabina) and clearnose skate (Raja eglanteria) demonstrate
that with maturity there is a gain of electrosensory primary
afferents and, presumably, neural sensitivity (Sisneros et al., 1998;
Sisneros and Tricas, 2002). In R. eglanteria, the neural sensitivity
was eight times greater in adults and five times greater in juveniles
in comparison to embryos (Sisneros et al., 1998). Similarly, in
D. sabina, the neural sensitivity was four times greater in adults and
three times greater in juveniles when being compared to embryos
(Sisneros and Tricas, 2002). Therefore, animal size which is related
to ampullary canal length, and in some cases, an increase in the
quantity of afferent neurons, may influence the success of electrosensory
deterrents, with larger and more mature animals having
enhanced sensitivity and thus a greater potential to be deterred.
4. Commercial application
Besides assessing the effectiveness of these repellents on
interacting species, it is essential to also consider a variety of other
characteristics/issues pertaining to these repellents, such as: cost,
safety, pollution, and handling logistics.
4.1. Cost of repellents
The use of certain EPM and magnets may be prohibitively
expensive when deployed in large quantities, such as for
commercial fishing hooks. Jordan et al. (2011) proposed the use of
neodymium metal due to its field strength and relative cost in
comparison to other lanthanide metals, but it is likely that using
a pure lanthanide metal in a commercial fishery is economically
impractical. While the PEW Institute (Cosandey-Goden and
Morgan, 2011) conclude that “significant cost” precludes EPMs as
an “economically viable option”, the authors fail to consider
abundant and cost-effective reactive metals. Metals with a higher
reduction potential at a fraction of the cost of pure lanthanides have
been shown to be effective (O’Connell et al., submitted for
publication). Magnesium and certain magnesium alloys, at $0.05
US/gram in quantity for 99% þ purity, are much more practical and
easier to fabricate for fishing gear requirements.
4.2. Magnet-associated issues
Besides varying successes with magnetic repellents (Rigg et al.,
2009; O’Connell et al., 2010, 2011a, 2011b), there exists a variety of
issues which one must consider prior to implementation and/or
application. (1) When working in close proximity to neodymiume
ironeboron and bariumeferrite magnets on a research vessel or
platform, these magnets can be prohibitively difficult to work with
and potentially very dangerous and cause bodily harm if a finger or
other body part becomes trapped between two magnets. (2) These
materials have the ability to permanently destroy electronic
equipment (e.g. cameras and computers) and thus should be
handled with care. (3) When applying magnets directly to a fishing
hook, the presence of this large visual stimulus (e.g. the magnet)
may potentially interfere with target catch unless hooks are
magnetized thus removing the visual component. (4) If applying
magnets directly to fishing hooks or for beach net-associated
applications, it is inevitable that some magnets will be lost
causing pollution. (5) Lastly, it is currently not understood how
exposing organisms to strong magnetic fields may impact their
electroreception of navigational capabilities and thus prior to
commercial application this topic should be extensively studied.
4.3. EPM-associated issues
EPMs have shown promise in both laboratory (Stoner and
Kaimmer, 2008; Brill et al., 2009; Jordan et al., 2011) and field
experimentation (Kaimmer and Stoner, 2008; Wang et al., 2008).
Please cite this article in press as: O’Connell, C.P., et al., The emerging field of electrosensory and semiochemical shark repellents: Mechanisms of
detection, overview of past studies, and future directions, Ocean & Coastal Management (2012), http://dx.doi.org/10.1016/
j.ocecoaman.2012.11.005