Field investigation of rare-earth metal as a deterrent to spiny dogfish in the Pacific halibut fishery

Abstract

Spiny dogfish ( Squalus acanthias) comprise a significant unwanted bycatch on demersal longlines set for halibut, sablefish ( Anoplopoma fimbria), and Pacific cod ( Gadus macrocephalus) in shelf waters of the east and west coasts of North America. Recently, rare-earth magnets and metals have been shown to have deterrent effects on sharks. These effects are likely the result of magnetic or electric fields created by these materials in seawater, which are sensed and avoided by sharks. Our earlier laboratory studies showed that attack rates by spiny dogfish on baits protected with cerium mischmetal (a rare-earth alloy) were reduced and suggested that this metal might reduce unwanted bycatch of spiny dogfish in setline fishing for Pacific halibut ( Hippoglossus stenolepis). We conducted a field study near Homer, Alaska in September 2007 with three hook treatments interspersed on 36 longline sets. These included standard circle hooks used in the halibut fishery, hooks with small pieces of cerium mischmetal attached above the hook, and hooks with a similar (but inert) mild steel piece above the hook. Fewer dogfish were caught on hooks with mischmetal than on either of the two other treatments. Reductions in catch of longnose skate ( Raja rhina) also occurred on hooks protected with mischmetal. However, halibut catch did not increase with protected hooks. Limitations in using mischmetal in commercial operations are expense, hazardous nature, and relatively rapid hydrolysis in seawater.