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Fishery Bulletin 107(3) 
Figure 5 
Time taken for cut pieces of menhaden 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. During these trials, seven juvenile sandbar sharks 
( Carcharhinus plumbeus) were maintained in a circular fiberglass tank (7 m 
diameter, 1.8 m deep). Eight trials (four with the electropositive metal bar 
and four with the stainless steel bolt) were conducted every other day. The 
deterrent effect of electropositive metal was present over a greater number 
of days compared with the previous situation when 14 sharks were present 
in the tank. The difference is assumed to be due to increased competition for 
food which limited the repulsive effect of electropositive metal in the latter 
circumstance. 
boundary of the area that is avoided (Fig. 1), we surmise 
the latter to be the case. Stoner and Kaimmer (2008) 
reach similar conclusions with respect the deterrent 
effect of electropositive metal on spiny dogfish sharks. 
The effective range of deterrence (-100 cm) for juvenile 
sandbar sharks is, however, considerably larger than 
that for spiny dogfish sharks (10-20 cm) (Stoner and 
Kaimmer, 2008). Whether this is due to differences 
in water temperatures (~10°C for spiny dogfish sharks 
versus 22-29°C for juvenile sandbar sharks), mass or 
shape of bars, specific composition of the electropositive 
metals, or species differences remains to be determined. 
The number of electrosensory pores present in sandbar 
sharks is approximately twice that in spiny dogfish 
sharks (2317 versus 1262, respectively; Cornett, 2006) 
which may explain the difference in the distances that 
these sharks were deterred. 
Because of the limited range of deterrence, electro- 
positive metal would have to be placed near every hook 
in pelagic longline gear, although it appears that it 
could be placed at distances that are unlikely to inter- 
fere with capture of the targeted fishes. It is unknown 
if electropositive metal could protect hooked fishes from 
depredation by sharks, which is a significant problem 
(Gilman et al., 2008; Mandelman et al., 2008). 
Feeding deterrent experiments 
Stoner and Kaimmer (2008) theorize that the pres- 
ence of electropositive metal is irritating or possibly 
interferes with the ability of sharks to locate a food 
item. We hypothesize that irritation is the more likely 
reason the bait was not attacked within three minutes 
during the initial trials with electropositive metal in 
our feeding experiments. The tank was brightly lit and 
the water was essentially free of suspended particles 
because of the extensive filtration. We therefore con- 
tend that the sandbar sharks located the bait primar- 
ily by vision (although olfaction may also be involved). 
Moreover, the pieces of cut menhaden would obviously 
not have the bio-electric signals emitted by living 
organisms (Haine et al., 2001). Further investigation 
into the exact mechanism(s) underlying the effect of 
electropositive metals as is clearly warranted. 
Our specific experimental procedures were designed 
to ensure that feeding motivation remained high and 
thus to minimize the influence of feeding motiva- 
tion on our results. Any influence of competition on 
feeding motivation could not be controlled however, 
except by altering the number of sharks in the tank. 
Competition is well known to increase feeding mo- 
tivation (Ryer and Olla, 1991; Eklov, 1992) and we 
assume that it likewise lessens the deterrent effects 
of electropositive metal. Increased feeding motivation 
due to competition could, therefore, explain the short- 
lived deterrent effects of electropositive metal when 
14 sharks are present in the tank. Our observation 
that the deterrent effect lasts longer during the tri- 
als when only seven sharks are present supports this 
contention. 
