Sharks
are apex predators that occupy a variety of different niches in the ocean. They
have an amazing sensory system, consisting of vision, hearing, lateral line,
chemoreception, and electroreception. However, they make up a large amount of
bycatch in the fisheries industry. These
accidental catches normally result in death.
Sharks can get caught in almost any type of fishing gear, like
longlines, gillnets, and trawls. Therefore, it is important to find mechanisms
to reduce the bycatch of these apex predators. Currently, there are net size
limits and excluder devices that attempt to reduce the bycatch on almost every
fishing gear (Jordan et al. 2013). Therefore, this is an analysis of the sensory
mechanisms, such as chemical, mechanical, visual, and electrical, in their
attempt to reduce the bycatch of sharks and the successfulness of each
mechanism.
 |
| Sharks caught as bycatch are generally discarded at se and are rarely recorded in commercial
fishery landings statistics. Photo source |
Sharks,
like many fish, are sensitive to chemicals involved in taste and smell, using
both the olfactory and lateral line system (Jordan et al. 2013). Many chemicals
have been tested to repel sharks, like rotenone, metals, chlorine, irritants,
and ink, but none have been successful. Previously, scientists thought that
rotten shark flesh containing copper and acetate, along with copper acetate and
dye, were effective, but recent studies show that copper acetate is actually
ineffective. However, dyes have proven promising. Studies show that toxins, like
paradaxin proteins, tend to either inhibit feeding or trigger retreat
responses. Unfortunately, these chemicals are difficult to produce in high
concentrations to actually work efficiently (Hart and Collin 2015). Necromones
have also shown potential to repel sharks, but it seems to be species specific.
The most effective chemical shark repellant is sodium dodecyl sulfate (SDS) and
lithium dodecyl sulfate (LDS) at concentrations of 83 to 175 ppm. It is the
most effective because the sharks do not habituate to the chemical, but it is
not extremely practical due to the high concentration needed (Hart and Collin
2015). Unfortunately, there are numerous challenges with chemical mechanisms,
like dispersion rates and how to isolate the chemicals that are non-toxic but
effective at low concentrations (Jordan et al. 2013; Collin and Hart 2015).
Mechanical
mechanisms include those involved with hearing and water flow. Sharks hear using their mechanosensory system
and skin receptors. They are sensitive
to sounds ranging from 20 to 1000 Hz, but are specifically attracted to low-frequency,
irregular sound pulses between 25 and 50 Hz (Jordan et al. 2013). However, sudden, high-intensity sounds at 10 m
with rapid increases in loudness and medium frequency pure tones tend to repel
sharks, but habituation does occur (Jordan et al. 2013; Collins and Hart 2015). Infrasound can repel sharks as well, but it
is fairly expensive and large (Hart and Collin 2015). Acoustic pingers have
proven useful, yet hearing damage is a major possibility (Jordan et al. 2013). The
only auditory mechanical device being used today is the Sharkstopper, which is
used for both personal protection and to repel sharks from fishing gear. This
portrays pulsing sounds ranging from 30-500 Hz or 200-1500 Hz, but sharks do
habituate to this too so it must be used for short periods of time (Hart and
Collin 2015). Water flow is detected using mechanosensory systems as well. A sharks lateral line can detect frequencies
less than 200 Hz. Unfortunately, this
sensory system is not well understood. However, in the past, water jets on
trawls have proven effective at repelling sharks (Jordan et al. 2013). In the
end, mechanical mechanisms of sounds do not seem to be very practical in
repelling sharks.
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| Sensory-based deterrents attached to fishing gear. (A) Illuminated gill net (photograph credit Jesse Senko). (B) Beam trawl fitted with electric pulse generator, electrodes, and raised groundrope (Hovercran shrimp pulse trawl, photograph credit ILVO, Belgium). (C) Longline gear with lanthanide metal secured near hook (photograph credit Kieran Smith). Source: Jordan et al. (2013). |
Vision
is a dominant sensory system in sharks.
Recent studies have shown that sharks are attracted to specific colors
and have large visual fields. Sharks are
sensitive to light wavelengths ranging from 480 nm to 561 nm (Jordan et al.
2013). Visual repellants can be the most effective. The Shark Screen, one of
the most efficient visual repellants, is a large impermeable bag with inflating
devices to keep it afloat and open at the top. It hides the swimmer visually,
does not emit bodily chemicals, and does not portray body movements.
Interestingly, sharks are less attracted to low reflectance blue and black
colors and more attracted to high reflectance white and silver colors. Therefore,
cryptic or camouflaged patterns may also repel sharks. Another effective visual
repellant is a barrier of vertical kelp-like pipes combined with magnets.
Bubble curtains are currently being tested and look fairly promising, but are most
likely species specific (Hart and Collin 2015).
Flicker frequencies, which changing light speeds, can also be used to
deter sharks. Sharks tend to be sensitive to flicker frequencies between 16 and
25 Hz, and using flicker frequencies around 30 Hz should not attract sharks. Increasing
the visibility of the fishing gear has been proven to successfully repel
sharks. Ultimately, visual mechanisms seem to be very helpful in reducing the
bycatch of sharks (Jordan et al. 2013).
All
sharks have Ampullae of Lorenzini, which is their electrosensory system that is
extremely sensitive to electrical pulses. For instance, research shows that
sharks can detect below 1 nV/cm from 40 cm away. Sharks can detect many different types of
electrical stimuli, like currents, geomagnetism, cables, and bioelectric
fields. Magnets, metals, and powered
electrical devices can produce a strong enough electrical signal to
over-stimulate the sharks system and repel them (Jordan et al. 2013). Active
electrical repellants use a power source, like the Shark Shield, which consists
of a battery with electrodes, but this tends to be very species specific. The
SharkPOD (Protective Oceanic Device) uses an electrical waveform generator with
electrodes that is widely used today. Anti-shark electrical cables have been
attempted, but they end up being expensive with hard upkeep. Passive electrical
repellents include electropositive metals and permanent magnets.
Electropositive metals excite the shark’s electroreceptors, but hungry sharks
tended to ignore these devices. Permanent magnets can either be ceramic or of
the rare-earth type, but both have inconsistent findings. A device called SMART
(Selective Magnetic and Repellent-Treated) hooks use electricity and magnets to
successfully repel sharks (Hart and Collin 2015). However, these electrical deterrents have been
found to work best in coastal, benthic areas (Jordan et al. 2013). Many things can affect these repellants, like
type, sensitivity, shark biomass, and hunger level (Hart and Collin 2015). Unfortunately,
there are economic and logistical challenges that need to be overcome before
these tactics become fully feasible (Jordan et al. 2013). Metals are expensive,
potentially toxic, and must be reapplied often (Hart and Collin 2015).
Clearly
more studies need to be done to determine which sensory mechanism is the most
successful and efficient. From reading these articles, visual mechanisms seem
to be the best at preventing shark bycatch. However, if more research is done,
both chemical and electrical mechanisms could be very helpful when improved
(Jordan et al. 2013). The most effective solution is to combine multiple
repellent devices that affect different shark sensory systems, like using both
visual and auditory stimuli (Hart and Collin 2015).
References
Hart, N. S. and
S. P. Collin. 2015. Shark senses and shark repellents. Integrative Zoology
10:38-64.
Jordan, L. K. et
al. 2013. Linking sensory biology and fisheries bycatch reduction in
elasmobranch fishes: a review with new directions for research. Conservation
Physiology 1.
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