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.
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).
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.