Brill et al.: The repulsive and feeding-deterrent effects of electropositive metals on Carcharhinus plumbeus 
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Our data also imply that tolerance of electropositive 
metals can be learned, and that this learned behavior 
is retained for at least seven days. It is unknown how 
often individual sharks encounter pelagic longline gear, 
but it is unlikely to be anywhere near the frequency of 
our feeding trials with captive sandbar sharks. For this 
reason we propose that learned tolerance of electroposi- 
tive metals will unlikely diminish their deterrent effect 
when used with pelagic longline fishing gear. 
Longline trial experiments 
From our longline catch data (Table 1), it is clear that 
the presence of electropositive metal near hooks is a 
strong deterrent to juvenile sandbar sharks, but not 
to rays. In recent studies where similar methods were 
used resulted in either a smaller reduction in catch rates 
(20%) of spiny dogfish sharks (Kaimmer and Stoner, 
2008) than we observed, or in no statistically significant 
reduction (Tallack and Mandelman, in press). Surpris- 
ingly, Kaimmer and Stoner (2008) also recorded a large 
reduction (46%) in the catch of longnose skates (Raja 
rhina) due the presence of electropositive metal near 
longline hooks, whereas we saw no indication of a repul- 
sive effect on clearnose skates. 
The sensitivity of the electroreceptor system has been 
studied in a broad range of elasmobranchs (reviewed 
by Montgomery, 1988; Kalmijn, 2003) and there is no 
evidence of a lesser sensitivity in rays when compared 
to sharks. More specifically, the sensitivity of the elec- 
troreceptor system in the sandbar shark, the blacktip 
reef shark ( Carcharhinus melanopterus, family Car- 
charhinidae), and the mangrove whipray ( Himantura 
granulata, family Dasyatidae) are roughly equivalent 
(1 to 4 nV/cm; Haine et al., 2001; Kajiura and Holland, 
2002). By implication, therefore, the catch rates of all 
the elasmobranch species interacting with the longline 
gear should be reduced equally, but clearly are not. The 
species-specific responses of sharks, skates, and rays to 
electropositive metal may reside at the receptor level 
(Tricas and New, 1998), the level of central processing, 
or simply reflect different behavioral tolerance related 
to feeding motivation. Kaimmer and Stoner (2008) and 
Tallack and Mandelman (in press) both speculate that 
the abundance of dogfish results in strong competition 
for food and increased aggressiveness, and that these 
limit the repulsive effect of electropositive metal. Our 
results showing a longer lasting repulsive effect of 
electropositive metal during feeding experiments when 
fewer sharks are present in the tank (Fig. 4 and 5) 
support this contention. Assessing the specific differ- 
ences between various species of sharks, skates, and 
rays could clearly be a fruitful area of investigation. 
Health and environmental safety concerns 
with use of electropositive metals in fisheries 
The electropositive metals used in our experiments 
are mixtures of lanthanide elements (e.g., lanthanum, 
cerium, neodymium, and praseodymium) that are collec- 
tively known as the “rare earth” elements, although they 
are not particularly rare (Bulman, 1994). Lanthanide 
elements are generally considered nontoxic to mam- 
mals primarily because they are not easily absorbed if 
ingested (Haley, 1965; Bulman, 1994). Their accumula- 
tion in animal tissue is therefore generally very low to 
negligible even for animals in long-term feeding trials, 
and transfer to humans through foodstuffs is likewise 
very low (Redling, 2006). We therefore conclude that 
the use of electropositive metals as elasmobranch deter- 
rents would pose little if any toxicity to fishing crews 
handling the material, or to the food safety of targeted 
fish species. Lanthanide elements are also used as crop 
fertilizers and animal feed performance boosters for 
poultry, sheep, cattle, pigs, fish, and prawns; and in a 
variety of medical applications such as antimicrobial 
agents, MRI imaging, burn and cancer treatments, and 
for countering hyperphosphatemia in renal dialysis 
patients (Fricker, 2006). 
Lanthanide elements injected intravenously can be 
toxic, however, because they cross cell membranes by 
passing through calcium channels, and because they 
have high affinity for calcium binding sites on biological 
molecules (Haley, 1965; Bulman, 1994). It is therefore 
at least possible that extensive distribution of lantha- 
nide elements in the marine environment could impact 
invertebrate species (e.g., mollusks and crustaceans) 
that routinely incorporate calcium into their shells and 
exoskeletons. 
Conclusion and future directions 
Improving gear selectivity (i.e., reducing shark bycatch 
and depredation) is considered a high priority in pelagic 
longline fisheries because of its ecological and economic 
benefits (Gilman et al., 2008, Mandelman et al., 2008). 
The use of electropositive metals appears promising in 
this regard. However, the specific composition, mass, 
and shape of the composite metal deterrent representing 
an optimal compromise between a high deterrent effect 
and a long useable durability in seawater remain to be 
ascertained. In conjunction with at-sea trials, behavioral 
assays with captive juvenile sandbar sharks would pro- 
vide an effective means for testing and optimizing the 
use of electropositive metals. 
Acknowledgments 
Funding for this project was provided by the Fishery 
Biology and Stock Assessment Division, Pacific Islands 
Fisheries Science Center, National Marine Fisheries Ser- 
vice, NOAA; the National Shark Research Consortium 
(NOAA/NMFS Grant no. NA17FL2813); and an Indiana 
University South Bend SMART grant to R. Sundaram. 
We also gratefully acknowledge the entire staff of the 
Virginia Institute of Marine Science Eastern Shore Lab- 
oratory for their continuing and genuine hospitality and 
technical support. All animal capture, maintenance, 
and handling procedures were approved by the College 
