298 
Abstract — Reducing shark bycatch 
and depredation (i.e. , damage caused 
by sharks to gear, bait, and desired 
fish species) in pelagic longline fisher- 
ies targeting tunas and swordfish is 
a priority. Electropositive metals (i.e., 
a mixture of the lanthanide elements 
lanthanum, cerium, neodymium, and 
praseodymium) have been shown to 
deter spiny dogfish ( Squalus acanth- 
ias, primarily a coastal species) from 
attacking bait, presumably because of 
interactions with the electroreceptive 
system of this shark. We undertook to 
determine the possible effectiveness 
of electropositive metals for reducing 
the interactions of pelagic sharks with 
longline gear, using sandbar sharks 
(Carcharhinus plumbeus, family 
Carcharhinidae) as a model species. 
The presence of electropositive metal 
deterred feeding in groups of juvenile 
sandbar sharks and altered the swim- 
ming patterns of individuals in the 
absence of food motivation (these indi- 
viduals generally avoided approaching 
electropositive metal closer than -100 
cm). The former effect was relatively 
short-lived however; primarily (we 
assume) because competition with 
other individuals increased feeding 
motivation. In field trials with bottom 
longline gear, electropositive metal 
placed within -10 cm of the hooks 
reduced the catch of sandbar sharks 
by approximately two thirds, com- 
pared to the catch on hooks in the 
proximity of plastic pieces of similar 
dimensions. Electropositive metals 
therefore appear to have the poten- 
tial to reduce shark interactions in 
pelagic longline fisheries, although 
the optimal mass, shape, composition, 
and distance to baited hooks remain 
to be determined. 
Manuscript submitted 31 October 2008. 
Manuscript accepted 2 March 2009. 
Fish. Bull. 107:298-307 (2009). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
The repulsive and feeding-deterrent effects 
of electropositive metals on 
juvenile sandbar sharks 
( Carcharhinus plumbeus) 
Richard Brill (contact author ) 1 
Peter Bushnell 2 Rumya Sundaram 2 
Leonie Smith 3 Eric Stroud 5 
Coley Speaks 4 John Wang 6 
Email address for contact author: rbrill@vims.edu 
1 Cooperative Marine Education 
and Research Program 
Northeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
166 Water Street 
Woods Hole, Massachusetts, 02543 
Present address: Virginia Institute 
of Marine Science 
PO Box 1346 (mail) 
Route 1208 Greate Rd. 
Gloucester Point, 
Virginia 23062 
2 Department of Biological Sciences 
Indiana University South Bend 
1700 Mishawaka Avenue 
South Bend, Indiana, 46634 
3 Department of Biological Science 
Bangor University 
Bangor Gwynedd, LL57 2DG, UK 
4 Department of Marine Science 
Hampton University 
Hampton, Virginia, 23668 
5 Shark Defense Technologies, LLC 
PO. Box 2593 
Oak Ridge, New Jersey 074 3 8 
6 Joint Institute for Marine and Atmospheric 
Research University of Hawaii at Manoa 
1000 Pope Road 
Honolulu, Hawaii, 96822 
The worldwide bycatch of sharks is 
estimated to be 260,000-300,000 
metric tons annually (11.6 to 12.7 
million individual sharks) (Bonfil, 
1994; Camhi et al., 1998). In pelagic 
longline fisheries targeting tunas 
and swordfish, it is not uncommon 
for the number of sharks caught to 
exceed that of the desired fish species 
(Stevens, 1992; Bonfil, 1994; Gilman 
et al., 2008). Shark populations are 
especially vulnerable to high rates 
of fishing mortality because of their 
slow growth rates, low reproductive 
output, and late sexual maturity. 
Once depleted, they also generally 
have slow rates of recovery because 
of these characteristics (Smith and 
Snow, 1998; Chen and Yuan, 2006). 
Scalloped hammerhead ( Sphyrna 
lewini), oceanic whitetip (Carcha- 
rhinus longimanus), and tiger shark 
(Galeocerdo cuvier ) populations have 
already decreased within the range 
from 60% to 99% of their historical 
biomass (Baum et al., 2003; Baum 
and Myers, 2004; Gilman et al., 2008), 
and these species are now included 
on the International Union for Con- 
servation of Nature (IUCN) Red List 
of Threatened Species (IUCN, 2008). 
Such severe reductions in elasmo- 
branch populations have the poten- 
tial to detrimentally restructure 
marine ecosystems (Jackson et al., 
2001; Myers and Worm, 2003; Worm 
et al., 2006; Myers et al., 2007). Sur- 
vival rates of pelagic sharks released 
from longline gear appear high for 
animals that are not moribund when 
the gear is retrieved (Moyes et al., 
2006). Nonetheless, reduction of both 
shark bycatch and depredation (i.e., 
shark damage to longline gear, bait, 
and desired fish species) is considered 
a priority (Gilman et al., 2008, Man- 
delman et al., 2008). 
Sharks (but not the large pelagic 
teleosts targeted by longline fisher- 
ies) possess a unique sensory system 
based on the ampullae of Lorenzini 
that can detect electric field gradi- 
