Brill et al : The repulsive and feeding-deterrent effects of electropositive metals on Carcharhinus plumbeus 
301 
when the fish was further than 100 cm from the elec- 
tropositive metal bars or lead fishing weights. 
Feeding deterrent experiments 
Groups of sharks maintained in the outdoor circular 
fiberglass holding tank were used to determine the 
ability of electropositive metals to deter sharks from 
attacking bait. Individual pieces of cut menhaden were 
placed 30 cm below a single electropositive metal bar 
(-2 cm x 2 cmxlO cm and of the same composition 
described previously) by using a monofilament nylon 
fishing line. For control trials, pieces of cut menha- 
den were placed 30 cm below a stainless steel bolt of 
approximately the same dimensions as the electroposi- 
tive metal bar. Baits were attached to the monofila- 
ment line with light twine that allowed the bait to be 
removed by the sharks with moderate effort. Hooks 
were not used because of the risk of injuring the sharks 
and the likelihood that hooking would influence the 
willingness of the sharks to attack baits in subsequent 
trials. The line (with the bait and stainless steel bolt 
or electropositive metal bar) was suspended near the 
center of the tank and in approximately the middle 
of the water column. The order of presentation was 
randomized. 
During each trial, the line was immediately removed 
from the water after the bait was attacked and the 
time from presentation to attack was recorded. The 
line was also removed from the water if the bait was 
not attacked within three minutes. In either case, five 
minutes were allowed before the next trial was begun. 
Because of the number of sharks in the tank, the ra- 
pidity of the attacks, and the frequent shark-shark 
interactions, it was not possible to identify which indi- 
vidual attacked the bait or to quantify specific changes 
in behavior as the bait was approached. 
Two separate series of experiments were conducted. 
In the first, 14 actively feeding juvenile sandbar sharks 
were present in the holding tank and 14 trials (seven 
with the electropositive metal and seven with the stain- 
less steel bolt) were conducted every other day over a 
14-d period. The sharks were fed to satiation at the 
end of each set of trials, but not on the days between 
experiments. Two additional sessions were run one 
week after the completion of the first 14 sessions. The 
original group of sharks was then released and re- 
placed with seven naive individuals. Eight trials (four 
with the electropositive metal bar and four with the 
stainless steel bolt) were conducted every other day, 
over a 12-day period. As in the previous experiments, 
the sharks were fed to satiation at the end of each set 
of trials, but not on the days between experiments. 
Feeding trials were run at approximately the same 
time everyday (late afternoon). 
Longline experiments 
Bottom longline fishing gear was used to test the ability 
of electropositive metal to influence shark catch rates 
in the field. Longline trials were conducted during the 
summer months (July and August 2008). The gear was 
deployed a total of 26 times (two deployments per day) 
and all deployments except for one were in the tidal 
lagoon system adjacent to the eastern shore of Virginia 
(an area of tidal creeks and broad marshes separated 
from the Atlantic Ocean by a series of barrier islands 
to the east). One longline set was made in the ocean 
immediately offshore of the barrier islands. All longline 
sets were conducted during daylight hours and the gear 
retrieved after two hours when conditions permitted. 
Gear deployment schedules were primarily based on 
weather, as well as crew and vessel availability, rather 
than on time of day or tidal state. Experiments were 
generally not undertaken on consecutive days. 
Approximately 40 hooks were deployed during each 
set. The monofilament dropper lines were two meters 
long and were terminated with steel circle hooks (10/0 
or 11/0) baited with pieces of menhaden. Dropper lines 
were connected to the mainline at 10-m intervals to 
help ensure that each dropper fished independently. 
Small pieces (approximately 30-70 g initial weight) 
of electropositive metal comprising lanthanum (28%), 
cerium (53%), neodymium (15%), and praseodymium 
(4%) were attached to the dropper lines approximately 
10 cm from the hook by using plastic zip ties. The elec- 
tropositive metal pieces, cut in cross section from the 
ingots supplied by the distributor (Hefa Rare Earth, 
Vancouver, Canada), were approximately 2-cm thick 
plates (-30-60 cm 2 surface area per side). Plastic 
pieces, of approximately the same shape and surface 
area were attached at the same positions to control for 
any visual deterrent or mechanical effects. Lines with 
electropositive metal near the hook and with a plastic 
piece near the hook were attached to the mainline in 
an alternating pattern and in equal numbers during 
each gear deployment. This arrangement allowed the 
resultant catch data to be analyzed with a chi-square 
procedure based on the expectation that equal num- 
bers of sharks would be caught on hooks near a plastic 
piece or on hooks near electropositive metal, if the 
latter did not alter shark behaviors. 
Captured sharks were brought into the boat, hooks 
were cut in two places to help ensure that they would 
be shed quickly, standard length was measured, and 
sex was noted. These sharks were then immediately re- 
leased. Clearnose skates ( Raja eglanteria ) were treated 
similarly. Large rays (orders: Rajiformes and Myliobati- 
formes) were released without removing them from the 
water to ensure crew safety, and were therefore usually 
not identified to species. 
Results 
Repulsion experiments with individual sharks 
In the presence of the lead fishing weights, sharks 
swam predominately around the periphery of the tank, 
showed essentially no avoidance response, and fre- 
