Efficacy of Lanthanide Metals to Reduce Shark Bycatch

Sharks possess an exquisitely sensitive electrosensory system that enables them to detect voltage gradients in their environment.  At close range, this sensory modality overrides other senses and provides the sharks with spatial information to localize their prey.  The electric field surrounding prey items is approximated by a dipole and sharks vigorously bite at prey-simulating dipole electric fields.  When naturally electrogenic lanthanide metals are submerged in a conductive seawater environment, they produce a local electric field 100-1000x greater than the bioelectric field of teleost fishes, which are typically consumed by the sharks as prey.  This unnaturally powerful electric field may be sufficiently strong to disrupt the sharks’ electrosense.  Behavioral assays demonstrated that various elasmobranch species were repelled from a food source when the lanthanide metal was present, but readily ate in the absence of the metal or in the presence of a non-electrogenic control metal like lead.  Teleost fishes were unaffected by the presence of the metals and readily ate from either target.  When tested in groups of 3-4 individuals, the aversion was muted and competition for the food led the sharks to more readily bite at the bait even if it was affixed to a lanthanide metal.  Responses varied by species and among individuals, with sphyrnid sharks typically demonstrating greater aversion to the lanthanides than their carcharhinid relatives.  Lemon sharks in particular demonstrated interest in the lanthanide metal and continued to bite at the metal even after the bait was removed.  The strong adversive reaction of at least some shark species to the metals suggests a possible utility of lanthanide metals as a bite deterrent which could reduce shark by-catch on long line fishing gear.