The use of permanent magnets to reduce elasmobranch encounter with a simulated beach net. 1. The bull shark (Carcharhinus leucas)

Abstract

Beach nets are protective nets used to minimize interactions between potentially dangerous sharks and beachgoers. Studies have demonstrated that beach nets are substantial contributors to elasmobranch mortality. One hypothesized solution to reducing this mortality is the use of permanent magnets. The present study examined the effects of grade C8 barium-ferrite (BaFe12O19) permanent magnets on bull shark (Carcharhinus leucas) behavior, a species frequently entangled in beach nets. To examine this effect, two experiments were conducted: bait and barrier experiments. Log-linear models, more specifically, Poisson regressions were used to test hypotheses pertaining to the effects of treatment type, conspecific density, water visibility, and year on shark behavior. For the bait experiment, the magnetic treatment significantly reduced feeding frequency and increased avoidance frequency, with Poisson regressions also demonstrating that conspecific density was a significant predicator of avoidance and feeding frequencies. For the barrier experiment, the magnetic treatment reduced entrance frequency and yielded an increased avoidance frequency, with Poisson regressions also demonstrating that water visibility was inversely correlated to entrance frequency. This study is the first to demonstrate that C. leucas can be deterred by permanent magnets and that magnet efficacy can vary based on situational context. While this study sheds light on the potential for permanent magnets as devices that may reduce C. leucas encounters with protective beach nets, research on magnet exclusion properties should be conducted prior to applying this concept to future shark exclusion technologies.