Effects of barium-ferrite permanent magnets on great hammerhead shark Sphyrna mokarran behavior and implications for future conservation technologies

Cp O’Connell,Sy Hyun,Sh Gruber,P He

doi:10.3354/esr00629

Cp O’Connell, Sy Hyun + Show 2 more

Open Access

https://doi.org/10.3354/esr00629

Copy DOI

Abstract

The great hammerhead shark Sphyrna mokarran is an endangered species that is exposed to several sources of anthropogenic mortality, including beach nets. Although not a major contributor to S. mokarran mortality, beach nets are utilized in several locations to minimize the potential harmful interaction between sharks and beachgoers. To address this mortality, perma- nent magnets have been employed to determine if these materials can deter sharks away from netted areas. The present study examined the effects of barium-ferrite (BaFe12O19) permanent magnets on S. mokarran behavior under several environmental and biological conditions. In the bait experiment, feeding frequency significantly decreased and avoidance frequency significantly increased with the magnet treatment, with exposure quantity yielding an increase in feeding fre- quency, although this effect was not statistically significant. For the barrier experiment, entrance frequency significantly decreased and avoidance and pass-around frequencies significantly increased with the magnet treatment, with heterospecific density also being a significant pre - dictor of entrance frequency. The findings demonstrate how permanent magnets can modify S. mokarran behavior and how this behavior is modified based on situational context. Since sev- eral other sphyrnid species are caught in beach nets more frequently than S. mokarran (e.g. scal- loped hammerheads S. lewini), the present results may serve as a model for these other sphyrnid species and illustrate the potential conservation implications of future magnetic deterrent barrier technologies.

Highlights

The great hammerhead shark Sphyrna mokarran is the largest of all sphyrnid species
Throughout experimentation, sea surface temperature was 22.8°C ± 1.69, salinity was 36.95 ppt ± 0.79, water visibility was 16.82 m ± 5.6, conspecific density ranged from 1 to 3, and heterospecific density ranged from 1 to 8
Throughout experimentation, sea surface temperature was 23.1°C ± 1.85, salinity was 37.09 ppt ± 0.65, water visibility was 16.79 m ± 6.08, conspecific density ranged from 1 to 3, and heterospecific density ranged from 2 to 6

Summary

Introduction

The great hammerhead shark Sphyrna mokarran is the largest of all sphyrnid species. This species is geographically widespread and has a range that extends throughout coastal and pelagic ecosystems associated with warm temperate to tropical waters (Gilbert 1967, Taniuchi 1974, Compagno 1984, Hammerschlag et al 2011). Endang Species Res 26: 243–256, 2015 berg et al 2006) This mortality is concerning due to the life history characteristics of S. mokarran, such as (1) low fecundity in comparison to other marine organisms, such as teleosts (Stevens & Lyle 1989), (2) late sexual maturity (Cliff 1995, Last & Stevens 2009), and (3) slow growth (Piercy et al 2010). It was recently voted to list S. mokarran in the CITES (Convention of the International Trade in Species of Wild Flora and Fauna) Appendix II, indicating that strict controls of mortality and trading are required in order to reduce over-exploitation of this species

Methods

Results

Discussion

Conclusion