Reducing bycatch could save millions of animals, reduce fishing costs
Dr. Peter Bushnell of Indiana University South Bend catches a juvenile sandbar shark off the Virginia coast for the study. (Credit: Stuart Schroff)
pattern (red lines) of a sandbar shark in the presence of three lead fishing
weights suspended in the tank at approximately the 5 o’clock position.
The image was produced with a digital video camera mounted directly
above the tank. (Photo Credit: Leonie Smith, Bangor University, Wales
pattern of a sandbar shark in the presence of three bars of palladium
neodymium suspended in the tank at approximately the 5 o’clock
position. The dark circle is present only to allow the software
recording the fish’s swimming path over a one-hour interval to work
correctly. (Photo Credit: Leonie Smith, Bangor University, Wales)
Sharks in captivity avoid metals that react with seawater to produce an
electric field, a behavior that may help fishery biologists develop a
strategy to reduce the bycatch of sharks in longline gear. Shark
bycatch is an increasing priority worldwide given diminished
populations of many shark species, and because sharks compete with
target species for baited lines, reducing fishing efficiency and
increasing operating costs.
A recent study by NOAA scientists and colleagues on captive juvenile
sandbar sharks showed the presence of an electropositive alloy, in this
case palladium neodymium, clearly altered the swimming patterns of
individual animals and temporarily deterred feeding in groups of
sharks. Rare earth metals have previously been reported to deter
spiny dogfish from attacking bait due to interactions with the shark’s
electroreceptive system, which detects weak electric fields including
those generated by their prey. Electric fields generated by
electropositive alloys are believed to deter or repel sharks by
overloading their sensory systems.
sandbar sharks would generally not approach the metal ingots closer
than about 24 inches, nor attack pieces of cut bait suspended within
approximately 12 inches,” said Richard Brill, a research scientist at
NOAA’s Northeast Fisheries Science Center and head of the Cooperative
Marine Education and Research (CMER) Program at the Virginia Institute
of Marine Science. “This study clearly shows the alloy has the
potential to repel sharks from pelagic longline fishing gear so they
are not caught as bycatch, but the optimal size and shape of the alloy
and other factors needs to be determined. This is a promising step.”
to now, electronic devices capable of repelling sharks have been large
and not practical for use on longline fishing gear. The current project
was undertaken to determine if small ingots of a relatively inexpensive
electropositive alloy were repulsive to sharks under controlled
laboratory conditions. If repulsion occurs consistently in the lab, the
next step would be to conduct field trials.
says juvenile sandbar sharks were used in the study because they are
readily available in the estuaries along Virginia’s coast, do well in
captivity, feed easily, and their constant forward motion makes it easy
to measure changes in their swimming patterns. “They are good
models for the species of sharks that are a significant problem in
pelagic or open ocean longline fisheries worldwide.”
estimated 11 to 13 million sharks are caught worldwide as bycatch each
year, sometimes more than the targeted fish species. Sharks, part of
the taxonomic group known as elasmobranchs which also includes skates
and rays, generally have slow growth and reproductive rates and late
sexual maturity. These factors result in an inability of shark
populations to support high rates of fishing mortality, or slow
population recovery. There is concern among scientists and fishery
resource managers that severe reductions in elasmobranch populations
could restructure marine ecosystems.
The sandbar shark, Carcharhinus plumbeus,
is one of the largest coastal sharks in the world and can reach lengths
of eight feet and weigh as much as 200 pounds. Sandbar sharks are
usually found in shallow coastal waters including bays and estuaries in
tropical and temperate waters around the world. In the western Atlantic
Ocean they range from Massachusetts to Brazil, with the waters of the
lower Chesapeake Bay considered a major nursery ground. Humans
are their main predator.
“Our results were very
promising but need further study,” Brill said. “The alloy we used,
palladium neodymium, appears to be a good alternative to more expensive
metals. It is also machinable and is reasonably resistant to corrosion
in seawater. How long the metal will last before corroding and how long
it will repel sharks in the field, however, needs to be determined.”
The lab experiments were conducted at the Virginia Institute of Marine
Science using juvenile sand sharks up to five years old caught in
surrounding waters and brought to an outdoor holding tank.
In addition to Brill, the study included scientists and students from
Indiana University, Bangor University in Wales, Hampton University in
Virginia, the University of Hawaii, and the research firm Shark Defense
LLC of Oak Ridge, NJ, which develops shark repellents. Principal
funding was provided by the Pacific Islands Fisheries Science Center of
NOAA’s National Marine Fisheries Service in Honolulu, with logistical
support from the VIMS Eastern Shore Laboratory.
This study by Brill and colleagues is among the first to rigorously
test the use of rare earth materials on repelling elasmobranchs, and
supports a recent study using metal alloys to repel spiny dogfish
conducted by NOAA researchers in Oregon.
Results of the sandbar shark study were presented at a
NOAA-sponsored shark deterrent workshop in Boston
earlier this month.
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