The effects of weak direct current electric fields on three species of sharks

Supervisors: Craig O’Connell, Thomas Goreau (Global Coral Reef Alliance)
Sharks have existed in our oceans for approximately 400 million years. There exist over 500 known species of sharks, with only 32 being dangerous to humans. Although there have been negative encounters from sharks on humans, on a global scale, humans have had a much greater negative impact. Many shark populations have been greatly reduced by overfishing, bycatch, the shark finning industry, and nearly one quarter of shark species are considered vulnerable, endangered, or critically endangered by the International Union for Conservation of Nature (IUCN). Furthermore, the degradation of coastal ecosystems (e.g. coral reefs) with the consequent destruction of nurseries and breeding areas is also a factor contributing to the worldwide decline of shark population. For example, it is estimated that 20% of coral reefs have been destroyed, 24% are in imminent risk of collapse, and another 26% are threatened. In order to try to reverse coral reef degradation, an innovative restoration method, called Biorock® was invented and patented by Prof. Wolf Hilbertz and Dr. Thomas J. Goreau. This technology utilizes low-voltage direct currents (between 1.2 and 12.0V) in order to cause limestone deposition on conductive materials. These electric fields produced by low-voltage currents may impact shark behavior, since it is well known that sharks can feel extremely weak electrical fields generated by other animals through specialized electroreceptors called ampullae of Lorenzini. Therefore, in the present study, the behaviors of three species of sharks towards low voltage direct current electric fields similar to those produced by Biorock technology were assessed. The target species were: the bull shark (Carcharhinus leucas), Caribbean reef shark (Carcharhinus perezi), and blacknose shark (Carcharhinus acronotus). Furthermore, this study assessed if the behavioral responses were different between sharks and teleosts, considering the lack of electroreceptors in teleosts. The study was conducted during the months of February and March 2015 in Bimini, Bahamas. The experimental design contained four treatments, which constituted one trial: control (no apparatus), procedural control (apparatus with no electricity), electrode treatment (apparatus connected to power), and post-electrode treatment (apparatus disconnected but with residual charge). For each treatment, 25 pieces of bait were used and the following behaviors were recorded for both sharks and teleosts: feeding, avoidances, accelerations, bites, and bumps. Data was analyzed on a per-species basis. A total of 30 trials (20 at Alicetown Channel and 10 at Triangle Rocks) were conducted. The mean voltage was 4.7V for the experimental electrode treatment and 1.38V for the post-electrode treatment. The results of this study showed that weak direct current electric fields, such as those produced by Biorock reefs, can significantly alter the behavior of the three shark species that were studied in this experiment. More specifically, the electrode and the post-electrode treatments had a significant influence on the feeding behavior of these shark species by reducing or inhibiting the feeding. Likewise, both treatments also revealed higher avoidance ratios. Corroborating to that, the control treatment and the procedural control treatment (no electrical charge) yielded higher feeding ratios and lower avoidance ratios. In conclusion, the electrical fields, such as those produced by Biorock reef restoration projects, can elicit avoidance behaviors and changes in feeding patterns of interacting sharks. Although the Biorock technology exhibits promise in coral reef restoration, based on the findings from the present study, it is essential that future studies assess shark behavioural patterns around these devices on a larger-scale. Should large-scale changes in feeding patterns and increased avoidance behaviour be observed as in the present study, it may greatly call into question the future utility of the technology as the deterrence of apex predators may negatively impact ecosystem balance.