Behavioral responses to weak electric fields and a lanthanide metal in two shark species
The unintentional catch of sharks on hooks intended for other fishes is an economic, environmental and safety concern. Recent research has sought to repel sharks from baited hooks by applying various lanthanide metals and alloys to stimulate the elasmobranch electrosensory system. We present a simplified experimental methodology to test responses of two shark species to a single lanthanide metal. Behavioral responses to prey-simulating, weak electric fields were quantified to establish the sensitivity of the electrosensory system in Squalus acanthias (Linnaeus, 1758) and Mustelus canis (Mitchill, 1815). Both species detected electric fields < 1 nV cm, and responded similarly to other elasmobranchs previously studied. Sharks were then presented with food affixed to treatments of acrylic, stainless steel or neodymium (Nd) metal. S. acanthias only fed in groups and fed from Nd significantly less frequently than from either control. M. canis was tested both individually and in groups and, when alone, fed less from Nd, however, in groups they fed significantly more often from Nd. These results confirm variability in response to a lanthanide metal both across species and within a species in the presence of competition. Since observed differences are not due to differences in sensitivity, other factors appear to influence behavioral responses and may compromise the effectiveness of lanthanide metals for the reduction of shark bycatch.