Brill et a I.: The repulsive and feeding-deterrent effects of electropositive metals on Carcharhinus plumbeus 
299 
ents as small as 5 nV/cm (Haine et al., 2001). These 
ampullary receptors are most sensitive to frequencies 
from 1 to 8 Hz (Montgomery, 1988), are capable of 
detecting weak electric fields generated by neuromus- 
cular activity, and can guide sharks to prey in the 
absence of other sensory stimuli (Kajiura and Holland, 
2002; Kajiura, 2003; Collin and Whitehead, 2004). 
It should be possible, therefore, to develop effective 
deterrent procedures that could take advantage of the 
sharks’ electroreceptive sense. The procedures could 
then decrease the bycatch and incidental mortality 
of sharks and increase fishing efficiency and yield of 
the desired fish species. Strong electric fields have 
been shown to deter approaching sharks, presumably 
by overloading their electrosensory modality (Smith, 
1974, 1991; Cliff and Dudley, 1992). However, cur- 
rently available electronic devices for achieving this 
behavioral response are designed to protect humans 
and aquaculture structures from shark attack and are 
large, expensive, and not practical for deployment on 
longline fishing gear. There are no data on the mini- 
mum field strength needed to achieve electrosensory 
repulsion. 
Electropositive metals (generally mixtures of the lan- 
thanide elements praseodymium, neodymium, cerium, 
lanthanum, samarium, and yttrium) rouse juvenile 
lemon sharks (Negaprion brevirostris), nurse sharks 
( Ginglymostoma cirratum), and spiny dogfish sharks 
( Squalus acanthias) from tonic immobility when brought 
close to the head (Stoner and Kaimmer, 2008). Elec- 
tropositive metals have also been shown to deter spiny 
dogfish sharks from attacking baits in a tank study 
(Stoner and Kaimmer, 2008), and to reduce the catch of 
this species by 19% on bottom longline gear (Kaimmer 
and Stoner, 2008). Electropositive metals are assumed 
to stimulate the electroreceptive system by giving up 
cations to the more electronegative skin of the elas- 
mobranchs (Rice, 2008; Stoner and Kaimmer, 2008), 
although the exact mechanisms responsible for repul- 
sion are not known. 
Our studies are designed to determine if electroposi- 
tive metals affect the behaviors of juvenile sandbar 
sharks (Carcharhinus plumbeus) under both laboratory 
and field conditions. Sandbar sharks are highly suit- 
able for this line of research because they do well and 
feed readily in captivity. They are also an obligatory 
ram-ventilating species and their constant forward mo- 
tion makes it easier to measure changes in swimming 
patterns caused by electropositive metals, compared 
to species that remain motionless on the bottom for 
extended periods. More importantly, although pri- 
marily a coastal species (Conrath, 2005; Conrath and 
Musick, 2008), the sandbar shark is a member of the 
family Carcharhinidae (requiem sharks), which in- 
cludes many of the other shark species that frequently 
interact with pelagic longline gear (Mandelman et al., 
2008). Results with sandbar sharks should, therefore, 
provide a good indication of the efficacy of electro- 
positive metals for reducing shark bycatch in pelagic 
longline fisheries. 
Our experiments with captive sandbar sharks include 
tests of the ability of electropositive metals to influence 
the swimming patterns of individual animals in the 
absence of food motivation and to repel sharks from 
pieces of cut bait. The former is intended to quantify 
repulsive distances, and both are intended to provide 
data directly comparable with those obtained previously 
with spiny dogfish sharks (Stoner and Kaimmer, 2008; 
Tallack and Mandelman, in press). Our deployment of 
longline fishing gear in a tidal lagoon system used as 
a nursery area by juvenile sandbar sharks (Conrath, 
2005; Conrath and Musick, 2007) tested the ability of 
electropositive metal to deter sharks under field condi- 
tions and provided data comparable to data from recent 
studies where spiny dogfish sharks were targeted by a 
similar method (Kaimmer and Stoner, 2008; Tallack 
and Mandelman, in press). 
Materials and methods 
Experiments with captive animals were conducted 
during the summer months (June through August 2007) 
at the Virginia Institute of Marine Science, Eastern 
Shore Laboratory, in Wachapreague, Virginia. Juvenile 
sandbar sharks weighting up to ~5 kg (i.e., neonates to 
approximately 5 years old; Casey and Natanson, 1992) 
were captured with standard recreational hook-and-line 
fishing gear in the surrounding tidal lagoon system and 
transported to an outdoor circular fiberglass tank (7 m 
diameter, 1.8 m deep) as described previously (Brill et. 
al., 2008). The tank was supplied with sea water pumped 
from the adjacent tidal lagoon which was passed through 
sand filters to remove suspended particles, as well as 
phytoplankton and fouling organisms. Water from the 
holding tank was also continuously circulated through a 
separate set of sand filters, ultraviolet sterilizer, biofilter, 
and protein skimmer. Tank temperature and salinity 
over the course of the study (22-29°C and 30-33 %c, 
respectively) reflected that of the adjacent tidal lagoon. 
When not part of an active experiment, the sharks were 
fed pieces of cut menhaden ( Brevoortia tyrannus) every 
other day. All sharks were actively feeding before use 
in any trials. 
Repulsion experiments with individual sharks 
Experiments were performed on 10 sharks, and individu- 
als were not used more than once. For each replicate, an 
individual shark was transferred from the main holding 
tank to a smaller vinyl circular indoor test tank (3.6 m 
diameter, 0.67 m water depth) and allowed to acclimate 
for 24 hours. The test tank was supplied with seawater 
pumped from the adjacent tidal lagoon which was passed 
through sand filters. Temperature and salinity ranged 
from 22° to 29°C and from 30 %c to 33%e over the course 
of the study. 
An experiment consisted of three one-hr periods. At 
the start of the first hour, a string of three lead fishing 
weights was suspended in the tank to allow the shark 
