Several species of sharks have demonstrated the ability to sense magnetic fields (Kalmijn, 1978; Ryan, 1980; Klimley, 1993; 2002). The Ampullae of Lorenzini organ within sharks is used to detect weak electrical fields at short ranges. The detection range of this organ is effective only within inches, as sharks sense bioelectrical fields in the final stages of prey capture. SharkDefense has found that flux per unit area of certain permanent magnets, particularly Neodymium-Iron-Boride and Barium-Ferrite magnets, corresponds closely with the detection range of the Ampullae of Lorenzini. A permanent magnet with the correct specifications is hypotheiszed to over-stimulate the Ampullae of Lorenzini, and may therefore be used as selective shark repellent.

The fields generated by these permanent magnets (show at left) decreases at the inverse cube of the distance from the magnet to sharks and rays. 

Therefore, at distances of a few meters from the magnet, the field exerted is less than the Earth's magnetic field. Animals which lack that Ampullae of Lorenzini organ do not display aversive behavior in close proximity to the magnetic field, making this technology selective

The ampullae of Lorenzini are small vesicles and pores that form part of a subcutaneous sensory network of sharks. These vesicles and pores are found around the head of the shark and are visible to the naked eye. They appear as dark spots in this photograph of a porbeagle shark head. (Photo: Dr. Steven Campana, Bedford Institute of Oceanography)

OVERVIEW OF MAGNETORECEPTION

There are several theories on the sensory mechanisms responsible for magnetoreception, including magnetite based magnetoreception and indirect magnetoreception by electroreception during electromagnetic induction.

Currently, the most commonly accepted theory on magnetite based magnetoreception involves thousands of small magnetic crystals (magnetite, Fe2O3) linked to the phospholipid bilayer of neurons via glycoproteins (Figure 1).

The glycoproteins act as small stoppers on ion channels when the ambient magnetic field orients the magnetite in such a way as to block the ion channel. When a migratory animal moves through the earth's magnetic field, the magnetite reorients allowing free flow of ions across the phospholipids bilayer generating an action potential which transmits geolocation information to the brain for processing. Magnetite based magnetoreception has been reported for many migratory marine species including yellowfin tuna (Walker et al. 1982), rainbow trout (Diebel et al. 2000), sea turtles (Lohmann et al. 2001) and spiny lobster (Boles and Lohmann 2003).

Elasmobranch fishes (sharks, skates and rays) have demonstrated the ability detect the earth's magnetic field, although the mechanism remains undescribed (Lohmann and Johnsen 2000). Indirect magnetoreception via electromagnetic induction is currently the most widely accepted mechanism, although magnetite based magnetoreception and chemical magnetoreception remain potential explanations.

 Illustration of a potential mechanism for magnetite-based magnetoreception involving changes in magnetite orientation in the presence of magnetic flux. The magnetic torque on the magnetite crystal removes the glycoprotein from the ion channel allowing ion exchange across the nervous membrane and generating an action potential (i.e., nervous signal) carrying the information to the brain for processing.

Elasmobranchs have a unique sensory adaptation that allows them to detect electric fields in the marine environment. This sensory ability is referred to as electroreception and the sensory organ associated with electroreception is the Ampullae of Lorenzini. The ampullae are gel-filled pores homogenously distributed around the nose and mouth. The sensory system is designed to detect weak electric fields generated by mechanical muscle movement (e.g., swimming muscles or a beating heart). In the presents of an electric field, the electric potential at the surface of the animal will vary from the electric potential of the interior of the animal. This potential difference is then detected by the sensory cells that line the ampullae. Once the voltage differential is recognized, the sensory information is transmitted to the brain via afferent neurons (Adair et al. 1998).

ELECTROMAGNETIC INDUCTION

Illustration of electromagnetic induction from a shark swimming through the Earth's magnetic field. Electrosensory organs known as the Ampullae of Lorenzini around the shark's mouth and nose detect the voltage drop induced by electrical current allowing navigation information to he processed by the brain as the fish's head moves back and forth during swimming. (Redrawn from Montgomery and Walker 2001).

When a shark swims through the earth's magnetic field, electromagnetic induction phenomena which generates an electric field as charged particles move through a magnetic field creates an electric field around the shark (Figure 2).

Minute differences in the earth's magnetic field at different locations result in minute differences in the induced electric field which may be detected by the shark's sensitive electroreceptors, especially as the head region moves back and forth during swimming (Lohmann and Johnsen 2000).

The law of electromagnetic induction (Faraday's Law) states that induced electromotive force (EMF) is proportional to the rate of change of the magnetic flux through a coil (an electric current can also be produced within a conductor when the conductor is moved through a magnetic field). This occurs because the force generated by the magnetic lines is applying a force on the free electrons in the conductor, causing the electrons to move. We hypothesize that the shark's body (particularly it's Ampullae of Lorenzini) acts as the conductor moving through the Earth's magnetic field or the permanent magnet's field, registering the induced EMF.

OUR RESEARCH AND TECHNOLOGIES

After being rewarded the grand prize at the 2006 Smart Gear Competition, members of SharkDefense have conducted extensive testing on the effectiveness of Grade C8 Barium Ferrite (BaFe2O4) permanent magnets as an elasmobranch-selective repellent. 

Under the supervision of Dr. Samuel H. Gruber, SharkDefense Technologies LLC has conducted numerous experiments using the facilities at the Bimini Biological Field Station, Bimini, Bahamas. 

Our preliminary experimental analyses began with the development of a magnetic maze.  For this study, we buried permanent magnets just beneath the substrate in a maze-like pattern within a circular pen.  Nurse sharks (Ginglymostoma cirratum) and southern stingrays (Dasyatis americana) had to complete the maze to utilize the remaining portions of the pen.  This experiment demonstrated that the swimming behavior of D. americana and G. cirratum could be manipulated with the use of ceramic magnets.

Secondly, SharkDefense Technologies LLC is conducting a study where the swimming behavior of juvenile lemon sharks (Negaprion brevirostris) is being observed within a circular pen containing a construction-mesh barrier.  This barrier was constructed along the diameter of the circular pen and contained two 0.25 m2 openings on either end of the fence. The magnetic opening (treatment) was surrounded by four C8 Barium Ferrite (BaFe2O4) permanent magnets which measured approximately 400 Gauss at the surface. The control opening was surrounded by four clay bricks of similar size and shape to the magnetic treatment with no measurable magnetic field (figure 2). The sharks were encouraged to swim from one side of the pen to the other by introducing fish juice (blood, fish oil, etc.) into the region of the pen opposite the sharks. Results indicated that N. brevirostris detected and were sensitive to the magnetic flux and avoided the magnetic treatment while swimming through the control a greater number of times. The sharks demonstrated greater avoidance behavior (i.e. accelerations away from, 90 or 180 degree turns) to the region containing permanent magnets when compared to the controls. 

These data suggest that a selective shark exclusion magnetic barrier, in addition to the shark-nets on human populated beaches, may reduce elasmobranch mortality associated with shark-nets.  Shark nets are used to prevent the entrance of sharks to areas where bathers frequent. These nets usually contain mesh holes 50 cm wide which are small enough to entangle the larger sharks, while allowing smaller fish to pass through. The only problem with this development is that not only are shark populations being decimated, but batoids and a variety of marine mammals are being killed within these nets (Dudley & Cliff, 1992).

Alternatively, in the Norwest Hawaiian Islands, the endangered monk seal (Monachus schauinslandi) populations are at predation risk by the Galapagos shark (Carcharhinus galapagensis).  These sharks are prowling the nesting areas of these seals, where they prey upon the adults and pup monk seals as they enter the water in search of food.  This has become an urgent issue and we believe that we could reduce the amount of shark on seal interaction by utilizing this idea of a magnetic fence design.

A selective shark-repellent fence designed by SharkDefense Technologies LLC. Openings are large enough to accomodate seals, turtles, and large fish.

Detail of an opening in the fence. Permanent magnets are secured around the perimeter of the opening to selectively exclude elasmobranchs.

Finally, our Craig O'Connell completed his Master's Thesis on the effectiveness of permanent magnets on the feeding and swimming behavior of elasmobranchs at Coastal Carolina University.  In January 2010, Craig's research was accepted for publication. Continuing work examining the effectiveness of magnets on fishing gear has been explored using rod and reel techniques.  The data obtained from this study demonstrated that elasmobranchs favor hooks that do not contain magnets suggesting that the application of magnets on longlines could have a significant impact on reducing elasmobranch bycatch on longlines.

Detail of a rare earth magnet secured on a longline gangion.