Fishes are divided into bony fishes and cartilaginous fishes, whose most prominent subspecies are the elasmobranches, which are starring famous species like ray or shark. Elasmobranches are quite ancient species which exhibit a slow growth rate combined with late maturity and low fecundity (biologists call that K-selected). This selection may be based on the predatory nature of elasmobranches, which would otherwise, paired with their longevity, their effective immune system and their lack of natural enemies lead to an imbalance of the ecosystem. Nowadays, however, their K-selection causes a severe problem for elasmobranches, since many of them end in beach nets or as unasked-for ‘bycatch’, leading to a fast decline of their numbers.
Consequently there exist three motivations to develope effective elasmobranch repellents: The first is species preservation, the second is an improved protection of beachgoers from sharks while the third motivation originates from the fishing industry and combines protection of gear and staff with the desire to catch only economically valuable fish, which does not include elasmobranches.
It is known for almost 400 years know, that elasmobranches possess a unique sensory organ called the ampullae of Lorenzini. However it took almost 350 years to get to an idea what this organ actually senses. The ampullae are a system of small pores located at the front of the snout. These pores are filled with a conductive jelly, thereby enabling the shark to sense electric and magnetic fields. Elasmobranches may benefit from these ampullae twofold: They may be used to navigate in the earth’s geomagnetic field and to better pinpoint nearby prey by sensing the bioelectric field generated by muscle and gill movement. Bioelectric fields may be perceived because they generate a voltage gradient over the ampullaes pores, which again results in a neuronal response.The detection of magnetic fields, however is thought to work via electromagnetic induction, thereby being indirect.
The ampullae of Lorenzini of a tiger shark, identifiable as numerous black dots at the predator’s snout.
That elasmobranches are repelled by magnets is simply attributed to the much higher field strength of permanent magnets when compared to the earth’s magnetic field. This strong field may cause an overstimulation of the ampullae of Lorenzini, resulting in a aversion reaction which can be compared with our reaction when we put our hand onto something really hot. Equally incomplete is the knowledge about the repelling effect of electropositive metals. They may again work via overstimulation, since their reduction generates a voltage that is orders of magnitude greater than the bioelectric field. Still there are several ways how this voltage could affect the shark.
To make things worse the existing studies on electrosensory repellents, be it electropositive metals or magnets, lead to highly inconsistent results. The repelling effect seems to suffer of large inter-species variations and for magnets the axis of polarization may have a severe but not yet understood influence. Also far from being answered is the question which species really are sensitive to these repellents and whether there is a detrimental effect on the behavior of sea dwellers, which are not elasmobranches alone.
O’Connell C. P., Stroud E. M., He P. “The emerging field of electrosensory and semiochemical shark repellents: Mechanisms of detection, overview of past studies and future directions” Ocean&Costal Management 2012 http://dx.doi.org/10.1016/j.ocecoaman.2012.11.005