Sharks, metal, and food?

Many populations of sharks have plunged over the last few decades.  Huge numbers are removed from the ocean each year, but many of these are not even the fish the fisherman are trying to catch.  We set out to see if a certain type of metal could help keep sharks away from bait so the sharks can stay in the ocean, where they belong.  A group of metals called lanthanide metals react with seawater creating an electric field different from the weak electric fields created by living things.  Unlike most other fish, elasmobranchs (sharks, skates, and rays), can detect electric signals and use them to find their food (see From form to function: Stingray sensory superstars, but faced with these much stronger electric fields produced by lanthanide metals they could swim away.  

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Experiment

First we needed to make sure that there were no difference in sensitivity to electric fields between the two shark species we were working with, the spiny or piked dogfish, Squalus acanthias (above left), and the dusky smooth-hound or smooth dogfish, Mustelus canis (above right).  To do this we tested the sharks in a similar setup to the stingrays measuring their response to weak electric fields (dipole electrodes above right).  Next, we gave them some choices.  We attached a tasty piece of food to a lanthanide metal (Neodymium, Nd), to stainless steel, and to acrylic to see which piece of food they would choose, and if the lanthanide metal would keep the sharks away.  We tested one of the species, M. canis, one at a time and in small groups to see if some potential competition might affect their choices.   We did all of these experiments in tanks at the Marine Biological Laboratory, MBL, in Woods Hole, Mass.

Results

Both species were just as sensitive to electric fields as the stingrays I had tested before, but when it came to the lanthanide metal, they behaved differently!  Not only were there differences between the two species, but also within the same species tested alone vs. in groups!  When tested one at a time, sharks ate less food from the lanthanide metal, but when other sharks were around, they ate food from Nd more often than from the steel or acrylic!  So where does that leave us with keeping sharks off of fishing hooks... while these metals may not work with every species in every situation, there is some evidence that sharks will avoid food attached to these metals and other types of electrical and magnetic signals so stay tuned!  For more details on this study see "Publications" in About Laura.

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