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Newport Laboratory: Fisheries Behavioral Ecology Program

Reducing Elasmobranch Bycatch on Longline Gear: Laboratory and Field Studies With Rare-Earth Metal Deterrents

Figure 12, spiny dogfish snarled in a demersal longline
Figure 12.  Spiny dogfish snarled in a demersal longline.  Adult dogfish may be up to 1.2 m long and weigh up to 10 pounds.

Unwanted bycatch of elasmobranchs is a worldwide problem in both commercial and recreational fisheries. Sharks, skates, and rays compete with target species for baits and can occupy a large proportion of hooks set on longlines, reducing capture efficiency and increasing costs of operation. Also, it is now recognized that declining populations of elasmobranchs might result in unintentional changes in ecosystem structure in both coastal and offshore waters. Thus, new methods are needed to reduce elasmobranch bycatch.

Spiny dogfish are found in temperate and subarctic shelf waters worldwide. This small shark (<120 cm) has some economic and cultural significance in certain areas, but its abundance, toxic spines, and low market value make it a frequent nuisance species in both recreational and commercial fishing in both the Atlantic and Pacific Oceans (Fig. 12). Spiny dogfish often occur in large schools and cause great damage to fishing gear. They can make up more than 90% of the catch in surveys for Pacific halibut conducted by the International Pacific Halibut Commission (IPHC) at some locations off Alaska and British Columbia, where dogfish populations appear to be increasing.

During 2006, spiny dogfish was the single most common species caught off British Columbia and the northwest United States, occupying 15% and 5% of the hooks fished in those areas, respectively. The area off Kodiak Island in Alaska also had high dogfish catch (16% of hooks), barely exceeding halibut catch. Over 20 years ago, the IPHC recognized the negative effects of dogfish on setline catchability of halibut, observing more than a four-fold effect on catchability ratios between areas with low and high dogfish abundances.

In 2006, the World Wildlife Fund grand prize for “Smart Gear” was awarded for the discovery that rare-earth magnets can be used to repulse certain sharks. It has long been known that elasmobranchs respond to magnetic and electrical fields using the ampullae of Lorenzini (special sensing organs), but this was the first fishing-related application aimed at reducing shark bycatch. We also know that nonmagnetic rare-earth alloys can produce a shark repulsing effect. For example, alloys in the lanthanoid series are electropositive, giving up electrons in seawater to the more electronegative skin of a shark. While the exact mechanisms of deterrence are not understood, the potential for reducing dogfish bycatch is promising. However, to date, there has been no peer-reviewed experimentation to assess the efficacy of rare-earth materials in deterring sharks.

During summer 2007, experiments were conducted at the AFSC’s Newport Laboratory facility to test the hypothesis that rare-earth magnets and metals placed in close proximity to baits would reduce attacks on and consumption of baits by spiny dogfish. Trials were also conducted with Pacific halibut. Attacks on baits were tested in the presence of two different rare-earth materials (neodymium-iron-boride magnets and cerium mischmetal) both believed to affect the behavior of elasmobranchs.

Pairwise tests of the rare-earth materials with inert metal decoys showed that dogfish detected both rare-earth materials, but magnets provided no protection for baits in feeding trials. However, baits tested with mischmetal were attacked and consumed at a lower frequency than mimic treatments (to 70% reduction), and times to attack baits were significantly higher, as were numbers of approaches before first attack. The time differential between mischmetal and mimic treatments and the number of baits consumed converged with increasing food deprivation (to 4 days), but differences were significant for all deprivation levels.Cerium mischmetal appeared to be irritating to dogfish and may disrupt their bait detection and orientation abilities. In contrast, Pacific halibut showed no reaction whatsoever to the rare-earth magnets or cerium mischmetal, and we concluded that the latter may be useful in reducing spiny dogfish bycatch in the halibut fishery.

  Figure 13, attaching the cerium mischmetal triangles
Figure 13.  Cerium mischmetal triangles are attached near the eye of circle hooks set for Pacific halibut.

Following the encouraging results of laboratory trials, a fishing trial was conducted during September 2007, to test the efficacy of cerium mischmetal in deterring spiny dogfish in the field. The fishing trial was carried out off Homer, Alaska, in a joint effort between AFSC’s Fisheries Behavioral Ecology Program and the IPHC (Steve Kaimmer), with support from NOAA’s Bycatch Reduction Program. Thirty-six longline sets were made in locations known for both the presence of halibut and abundant dogfish. Each set contained three skates of gear, one with standard hook gear, one with hooks protected with cerium mischmetal (Fig. 13), and one with inert metal mimics.

Preliminary results indicate modest reduction in dogfish catch on the mischmetal-protected hooks, only about 17%. Catch of longnose skates was reduced 48%. Catch of legal-sized halibut was increased by 5%. While these differences are not large, it is clear that cerium mischmetal can affect catch rates of elasmobranchs. In our field trial, dogfish were caught on 35% of the hooks in short, 2-hour sets. Effectiveness in individual sets appears to be density-dependent, and we speculate that the deterrents will be more effective where dogfish are less abundant.

Some other limitations may affect the use of cerium mischmetal in commercial operations. The material is relatively expensive, it has some hazardous properties (e.g., it is a flammable solid!), and hydrolyzes in seawater. While experiments are currently being conducted in Hawaii, New England, and Florida, our study represents the first rigorous experimentation with rare-earth materials in a fishery application.

By Allan Stoner

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