The use of permanent magnets to reduce elasmobranch encounter with a simulated beach net. 2. The great white shark (Carcharodon carcharias)

Abstract

Beach nets are preventative devices that are utilized to minimize the potential interaction between a beachgoer and a predatory shark. One species, the great white shark (Carcharodon carcharias), the focal species for the present study and a protected species in South African waters, is often killed in beach nets within the KwaZulu-Natal (KZN) region. To address the issue of C. carcharias capture in beach nets and to reduce mortality of this species, two related experiments were carried out: the bait experiment and the magnetic-control barrier experiment. Both experiments were aimed to determine the effect of permanent magnets on C. carcharias. During the bait experiment, a total of twenty C. carcharias interacted with the control and magnetic apparatuses. The results indicate that avoidance and feeding behaviors were significantly associated with treatment type, suggesting that permanent magnets had C. carcharias deterrent capabilities. In addition, it was demonstrated that the likelihood of an avoidance behavior on the magnet-associated baits was not significantly correlated with water visibility or conspecific density. For the second experiment, results from stage I of the magnetic-control barrier experiment indicate that behavior was not associated with treatment zone; however, stage II indicated that behavior was significantly associated with treatment type. Results from the magnetic-control barrier experiment clearly demonstrate that although a visual barrier, such as the procedural control barrier, may be sufficient to deter C. carcharias from an area, the addition of permanent magnets provide additional successful deterrence of C. carcharias. This study demonstrates that C. carcharias are sensitive to strong permanent magnetic fields; therefore a large-scale experiment with a substantially greater sample size is warranted to investigate the potential of a non-invasive magnetic barrier to replace detrimental beach nets in KwaZulu-Natal, South Africa.