By working toward innovative solutions, we have developed acoustic pingers that successfully warn dolphins and porpoises away from gillnets. More recently, we have successfully shown that metals may be usable as shark repellents in hook-based fisheries and we are exploring altering the chemistry of gillnets to reduce bycatch.
Investigating the Use of Metals as Shark Repellents
Sharks are frequently caught as bycatch by commercial and recreational fishermen when they are captured on baited hooks. Since sharks are very sensitive to the presence of metal in the water, we are working with several different types of electromagnetic materials to develop an effective shark repellent for fishing hooks.
Over a period of two months, we tested how each species responded to the various baits and metals. In each species-specific trial, we presented animals with squid-baited fake hooks. Each fake hook was “protected” by either the mischmetal, the magnet or an inert decoy (e.g. stainless steel). We ran tests under a variety of conditions in order to test the effects of hunger and shark density on the bait selectivity of each species.
We have not yet completed our statistical analysis, but initial data suggests that the repellent effects of each metal will likely vary from one species of shark to another. The spiny dogfish were more averse to the mischmetal, while the smooth dogfish was more discouraged by the magnet. Both species were less likely to be affected by the repellents if they were hungry, and neither species was affected by the density of sharks in the experimental tanks.
These initial results suggest that metallic repellents may help reduce the bycatch rate of specific shark species, but their repellent qualities may be too specific to work with all species of sharks. Additional research is needed to better understand how other species of sharks respond to these and other metallic substances.
Reducing Porpoise Bycatch in the Gillnet Fishery with Acoustic Pingers
In the 1990s, the Aquarium conducted an experiment showing conclusively that acoustic pingers reduced bycatch of harbor porpoise to almost zero. Another potential approach we are evaluating involves altering the chemistry of the net. With funding from the Lenfest Ocean Program, the Aquarium is evaluating the potential for gillnets impregnated with a small percentage of barium sulfate to reduce mammal and turtle bycatch, including in the artisanal croaker fishery of Argentina where many franciscana dolphin are killed.