Electrochemical properties of lanthanide metals in relation to their application as shark repellents

• We quantified the electrochemical properties of various lanthanide metals in seawater.
• We calculated the distance at which sharks can detect the resultant electric field.
• Voltage decreases with distance as a power function, x−1.5.
• Lanthanides rapidly dissolve in seawater but at significantly different rates.
• Neodymium was ineffective at repelling two shark species from bait.

Sharks comprise a large portion of unwanted bycatch in longline fisheries worldwide and various technologies have been proposed to reduce elasmobranch bycatch without impacting the catch of target species. Recently, the naturally electrogenic lanthanide metals have been introduced as an elasmobranch-specific repellent. We quantified the voltage produced by six lanthanide metals in seawater, compared their dissolution rates, and performed a behavioral assay to determine their efficacy against two coastal shark species. We found that there was no difference in the voltage produced by the six tested metals and the voltage decayed as a power function (approximately x−1.5) with distance from the metal sample. We calculated that sharks should detect a sample of neodymium from a distance of 65–85 cm in seawater. Voltage was greatest in freshwater and decreased logarithmically with increasing salinity but did not differ above salinities greater than 10 ppt. The dissolution rate for the lanthanides varied from −1.6 to −0.2 g h−1 and as the metals dissolved, the voltage remained constant. In a behavioral assay, neodymium was ineffective at repelling bonnethead sharks (Sphyrna tiburo) tested individually and in groups, and juvenile lemon sharks (Negaprion brevirostris) in groups.