Biorock electrical fields inhibit shark biting

Article by Diana Crow published on April 5th 2018 in the Sierra Club magazine
Original article @

Electric Shark Boogaloo

Is there such a thing as an electric fence, but for sharks?



Marine biologist Marcella Pomárico Uchôa stood at the edge of a small boat in the Bimini region in the Bahamas, watching a floating piece of white PVC pipe, rigged with wires and a bag of minced meat, bob up and down with the waves. It wasn’t long before the sharks arrived.

The sharks weren’t shy about their interest in the minced meat. They charged toward it at full-speed, only to swerve away at the last moment. In contrast, the Bermuda chubs and bar jacks swam right up to the rig and grabbed a snack without hesitation. Something was changing the sharks’ behavior.

The two species Uchôa’s study focused on—bull sharks (Carcharhinus leucas) and Caribbean reef sharks (Carcharhinus perezi)—can sense electric fields in the water. Their electrosensory organs—called the ampullae of lorenzini—are sensitive enough to detect the electric activity in their prey’s nervous systems, allowing sharks to lunge at their prey blind.

As Uchôa and her colleagues reported in the journal Animal Biology last year, the wire and PVC rig emitted a low voltage electric current that seemed to befuddle the two species of shark. Ordinary fish—without an electromagnetic sixth sense—didn’t seem to notice the electricity at all.

As far as the observers on the boat could tell, the sharks weren’t hurt by the electric field. “Sharks just avoid them because it’s confusing,” explains the study’s co-author Thomas Goreau of the Global Coral Reef Alliance, an organization that restores coral reefs by building artificial electric reefs.

This confusion could open up new markets for Goreau’s coral reef restoration business. Back in 1987, Goureau was writing coastal zone management plans for hotels and fisheries in Jamaica when he met an architect and inventor named Wolf Hilbertz. Hilbertz had been developing construction materials for underwater buildings when he found that electrically charged metal attracts dissolved minerals in seawater. Over time, these minerals build up, forming a material similar to concrete–or to the calcium carbonate of coral reefs.

The two began designing synthetic electric reefs—which they called “Biorocks”—meant to slow coastal erosion and provide habitat for coral reef species in areas that had seen massive coral reef damage. About 400 were installed in over a dozen countries including off the coast of Panama, the Saya de Malha bank near the island nation of Seychelles, and Gili Trawangan in Indonesia. Most are close to shorelines and draw from the nearby islands’ power grids, but Goreau and his colleagues have been experimenting with using renewable power sources such as solar panels and wave power generation.

In thirty years, Goreau had never seen a predatory shark hanging out near a Biorock reef. Then, while giving a talk at the University of the Basque Country in Spain, he met Uchôa, who was a marine science grad student at the time. The two began looking into whether Goreau’s experience could be backed up by real-world experiments, and whether Biorocks could function sort of like underwater electric fences, steering sharks away from popular diving areas.

Shark bait experiment in progress. Photo courtesy of Marcella Pomárico Uchôa.

Using sharks’ electromagnetic sense to direct shark traffic away from humans isn’t a new idea. Several electricity-emitting “shark-repelling”products–most of them wearable or attachable to surf boards—are already on the market. Whether these electromagnetic shark deterrents actually work is another question. “It depends on what you mean by working,” says marine biologist Charlie Huveneers of Flinders University in Australia. “If you’re asking whether they would stop or protect people all of the time in 100% of situations, then no, they don’t work. If you’re asking whether they have an effect on the behavior of sharks, then yes, they do work.”

Shark deterrent field tests by academic marine biologists—who are independent of the deterrent-making companies—have found that those effects can vary quite a bit. Sometimes, the sharks seem to hesitate in the presence of an electric field but go in for the kill anyway. Sometimes, they don’t go for the bait but stay within a few meters of the boat. The effects differ between species, and a few people have even been bitten while wearing electromagnetic shark “deterrents”.

Ideally, says says shark biologist Ryan Kempster of the University of Western Australia, the electrical field produced by a shark deterrent should be tailored specifically to the size and species of the shark in question, because every species detects and responds differently to electric fields of varying strengths and frequencies.

“The problem with shark deterrents,” adds says Huveneers, “is that there’s no real regulation in terms of what the deterrents need to be able to do to be called ‘deterrent’. And manufacturers can make a lot of claims about the device that they’re selling without ensuring the veracity of those claims,”

If Biorocks work to keep sharks away from beaches that are popular with divers, such a scenario could be beneficial to sharks, since they are more likely to be hurt or killed by humans than the other way around. But Goreau freely admits that more research is needed. The PVC pipe rig in Uchôa’s experiment emitted an electric field very similar to that of a Biorock reef but not identical. In the majority of the experiments, sharks didn’t swerve from the PVC pipe rig until they were just a few feet away from the reef, which could mean that Biorock placement would have to be strategic to prevent sharks from swimming through areas that the field doesn’t reach to.

Goreau admits that it’s possible that no one has seen large predatory sharks swimming around Biorock reefs simply because there are so few large sharks left worldwide. Rays and nurse sharks, which can also sense electricity, live on and near Biorocks and do not appear to be affected by the Biorocks’ electric fields. It is possible, though, that the electrical field could have some effect on the behavior of sharks, rays, and skates that is not readily apparent. That alone is reason to be cautious, according to Uchôa.

In the meantime, Goreau remains excited. Students monitoring the Biorock reefs in Indonesia have noticed large numbers of young fish swimming around the artificial reefs. Because sharks, rays, and skates are the only fish known to have electrosense, this raises the question of what is bringing them there. “We do get enormous recruitment of larval fish when the power is on, much more so than when the power is off,” says Goreau. “There’s an enormous need to expand this work.”

Fluorescence for Coral Recruitment Research

By Charles Mazel
original article @

Fluorescence is a valuable tool for research on coral recruitment and survivorship. Fluorescence makes it much easier to locate tiny corals both on the reef and in the lab.

On the reef –

With conventional searching techniques it is essentially impossible to locate juveniles until they are at least 5 – 10 mm in diameter. By this time they are between 6 months and a year old. This misses an important early part of their life history and makes it difficult to estimate survivorship in natural conditions.

With the BW-1 and FL-1 dive lights you can use fluorescence to search for coral recruits and juveniles both at night and in the daytime. Detecting small things is all about contrast, and fluorescence brings juvenile corals out in strong contrast. If a coral fluoresces it will generally appear as a bright green spot against a dark background. Researchers can find corals 5 mm in diameter from more than 2 meters away, and corals as small as 1 mm in diameter in routine sweeps of patches of reef. juvenile coral white light
Arrow pointing to juvenile coral, white light (c) Charles Mazel
nightsea juvenile coral <1mm diameter fluorescing
Juvenile coral <1mm diameter fluorescing (c) Charles Mazel

In the lab –

Researchers often deploy settlement tiles on the reef and collect them later to find out what has settled on them. Fluorescence makes it easier to find corals on the settlement tiles. The images below are white-light and fluorescence images of a settlement tile that had been deployed on the reef in Bonaire.

nightsea Settlement tile white light
Settlement tile – white light (c) Charles Mazel
nightsea Settlement tile – fluorescence. Scale in cm.
Settlement tile – fluorescence. Scale in cm. (c) Charles Mazel

It helps to inspect the tiles under a stereo microscope, and it is now easy to add an economical fluorescence capability to your existing stereo microscopes with the Model SFA Stereo Microscope Fluorescence Adapter.

Coral polyp on settlement tile – white light
Coral polyp on settlement tile – white light. (c) Alina Szmant
Coral polyp on settlement tile – fluorescence
Coral polyp on settlement tile – fluorescence. (c) Alina Szmant
nightsea Coral polyp on settlement tile – white light
Coral polyp on settlement tile – white light. (c) Alina Szmant
Coral polyp on settlement tile – fluorescence
Coral polyp on settlement tile – fluorescence. (c) Alina Szmant

Several publications deal specifically with this application of fluorescence.

Baird, A. H., A. Salih, and A. Trevor-Jones, 2006. Fluorescence census techniques for the early detection of coral recruits. Coral Reefs, 25:73-76.

Hart, J. R., 2011. Coral recruitment on a high-latitude reef at Sodwana Bay, South Africa: Research methods and dynamics. MSc thesis, University of Kwa-Zulu, Natal, 81pp.

Korzen, L., A. Israel, and A. Abelson, 2011. Grazing effects of fish versus sea urchins on turf algae and coral recruits: Possible implications for coral reef resilience and restoration. Journal of Marine Biology, Vol. 2011, Article ID 960207 doi:10.1155/2011/960207. [Reprint on-line.]

Piniak, G. A., N. D. Fogarty, C. M. Addison, and J. Kenworthy, 2005. Fluorescence census techniques for coral recruits. Coral Reefs, 24:496-500.

Roth, M. S., and N. Knowlton, 2009. Distribution, abundance, and microhabitat characterization of small juvenile corals at Palmyra Atoll. Mar. Ecol. Prog. Ser., 376:133-142. [Reprint available on-line.]

Salinas-de-León, P., A. Costales-Carrera, S. Zeljkovic, D. J. Smith, and J. J. Bell, 2011. Scleractinian settlement patterns to natural cleared reef substrata and artificial settlement panels on an Indonesian coral reef. Estuarine, Coastal and Shelf Science, 93: 80-85.

Salinas-de-León, P., C. Dryden, D. J. Smith, and J. J. Bell, 2013. Temporal and spatial variability in coral recruitment on two Indonesian coral reefs: consistently lower recruitment to a degraded reef. Marine Biology, 160(1): 97-105.

Schmidt-Roach, S., A. Kunzmann and P. Martinez Abrizu, 2008. In situ observation of coral recruitment using fluorescence census techniques. JEMBE, 367:37-40.

Answers to some common questions:

Can you find ALL recruits with fluorescence?

No. Not all coral recruits fluoresce, and some that do fluoresce do not glow brightly enough to be found easily. Keep in mind that not all adult corals fluoresce either, and even within a species there may be both fluorescent and non-fluorescent morphs.

How far away can you spot a 1 mm recruit?

From our own experience, we have certainly seen many, many small bright dots from a meter or more away without looking very hard.

If you do find a small (1 – 2 mm) fluorescing feature, is it definitely a coral?

Not necessarily. There are many small non-corals that fluoresce, including anemones, corallimorphs, zoanthids, hydroids, etc. Even small mobile invertebrates such as some polychaetes fluoresce. It can be a challenge to know what is a coral and what is not.

How do you identify what you have found?

Identifying to species difficult to impossible. In the field you at least want to be able to distinguish coral from non-coral. A good magnifier is recommended.

Do you have to dive at night?

No. With the NIGHTSEA BW-1 and FL-1 lights you can find coral recruits in the daytime.

Does fluorescence work with settlement tiles?

Yes, you can use any of the NIGHTSEA underwater lights to inspect settlement tiles visually, or use fluorescence photography to systematically document growth on the tiles. Fluorescence also works well when inspecting settlement tiles under a stereomicroscope. NIGHTSEA offers a simple adapter that adds a fluorescence capability to just about any existing stereomicroscope.

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