Brill et al.: The repulsive and feeding-deterrent effects of electropositive metals on Carcharhmus plumbeus 
303 
During the second set of experiments 
with fewer sharks in the tank (seven ani- 
mals versus 14), the deterrent effect of 
electropositive metal was apparent until 
day 8 and it did not completely disappear 
until day 12 (Fig. 5). 
Longline trial experiments 
Of the juvenile sandbar sharks captured, 39 
were female, 26 were male, and there was 
one individual where sex was not recorded. 
Sharks ranged in size (standard length) 
from 47 to 130 cm, and had a median 
length of 72.5 cm. Only one cownose ray 
(Rhinoptera bonasus) was captured, the 
other rays were either Gymnura spp. or 
Dasyatis spp. Two Atlantic sharpnose 
sharks ( Rhizoprionodon terraenovae) and 
nine clearnose skates (Raja eglanteria) 
were captured during the one gear deploy- 
ment made outside the lagoon system. 
The ratio of sharks caught on hooks 
near plastic to sharks caught on hooks 
near electropositive metal (2.6:1) was 
significantly different from the predicted 
ratio of 1:1 if the presence of the latter 
had no deterrent effect (chi square test 
P=0.001, df=l, x 2 =10.78). In other words, 
electropositive metal near the hooks re- 
duced the catch rates of sharks by 62% 
(Table 1). In contrast, the numbers of 
rays caught on hooks near plastic and on 
hooks near electropositive metal were not 
significantly different from the expected 
ratio of 1:1 (chi square test P- 0.67, df=l, 
X 2 =0.39), indicating that the presence of 
electropositive metal had no deterrent ef- 
fect. The low number of clearnose skates 
captured precluded any definitive con- 
clusions. However, the essentially equal 
numbers of skates caught on each hook 
type (Table 1) implies that the presence 
of electropositive metal does not deter this 
species. 
Discussion 
Repulsion experiments with individual sharks 
Because juvenile sandbar sharks showed no reactions to 
lead fishing weights (other than to avoid running into 
them), we concluded that lead fishing weights exert no 
significant repulsive effect. In contrast, juvenile sand- 
bar sharks generally avoided approaching the electro- 
positive metal bars presumably because they produce 
mild irritation. Whether this irritation is chemical or 
electrical (i.e. , stimulation of the sharks electrorecep- 
tive system) is unknown. Given the apparent definitive 
Trial number 
Figure 4 
Time taken for cut pieces of menhaden ( Brevoortia tyrannus) to be 
attacked when suspended 30 cm below a stainless steel bolt (filled 
circles) or an electropositive metal bar (open circles). The line was 
removed from the tank if the bait was not attacked within three 
minutes. Trials were conducted with 14 juvenile sandbar sharks ( Car - 
charliinus plumbeus) maintained in a circular fiberglass tank (7 m 
diameter, 1.8 m deep). Fourteen trials (seven with the electropositive 
metal bar and seven with the stainless steel bolt) were conducted 
every other day for the first 14 days, suspended for seven days, and 
then two additional trials were run to test whether the electropositive 
metal near the bait, as seen during the initial trails, would continue 
to deter the sharks. 
Table 1 
Catch of sharks (primarily juvenile sandbar sharks, Car- 
charhinus plumbeus ), rays (primarily Gymnura spp. and 
Dasyatis spp.) and clearnose skates ( Raja eglanteria) by 
bottom longline gear. Pieces of electropositive metal, or 
pieces of plastic of similar dimensions, were placed within 
10 cm of the hooks. Hooks in proximity to electropositive 
metal or to plastic pieces were deployed in equal numbers 
and in an alternating pattern during each set. 
Sharks 
Rays 
Skates 
Hooks near 
electropositive 
metal 
Hooks near plastic 
16 
42 
10 
13 
