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Fishery Bulletin 109(4) 
analysis. Four species of teleost fishes were captured: 
Atlantic croaker ( Micropogonias undulatus: control 
(0=3, procedural control ( PC)=2 , magnet (M)=l), oys- 
ter toad fish (Opsanus tau: C=l, PC =2, M=2), black sea 
bass (Centropristis striata: C=0, PC=0, M=l), and the 
bluefish ( Pomatomus saltatrix: C=2, PC=1, M=l). There 
was no significant difference in the total number of tele- 
ost fish captured between control and procedural control 
treatments (^ 2 =0.077, P=0.7815) nor between control 
and magnetic treatments f^ 2 =0.077, P=0.7815; Table 3). 
Discussion 
Magnets were associated with a species-specific catch 
in elasmobranchs in both longline and hook-and-line 
studies. Longline hooks treated with neodymium-iron- 
boron magnets had no effect on any captured elasmo- 
branchs (Table 1). Longline hooks with barium-ferrite 
permanent magnets produced a reduction in capture 
of C. limbatus and D. americana, whereas all other 
species were either not affected or were data-deficient 
(Table 2). In the hook-and-line study, neodymium-iron- 
boron magnets reduced the capture of two species, R. 
terraenovae and M. canis, compared with controls and 
procedural controls (Table 3). Teleost species were cap- 
tured in both experiments and capture rate did not vary 
with treatment type. 
Longline study 
Barium-ferrite magnets repelled elasmobranchs, where- 
as neodymium-iron-boron magnets did not. Neodym- 
ium-iron-boron magnets (Nd 2 Fe 14 B) contain neodym- 
ium from the lanthanide group of elements, as well as 
iron (a ferromagnet) and boron. The neodymium-iron- 
boron magnets (grade N52) used in our study produced 
a maximum flux of 14,800 gauss at their surface. 
Barium-ferrite permanent magnets (BaFe 12 0 19 : grade 
C8) are also alloys with a solidified structure and pro- 
duce a maximum flux of 3850 gauss at their surface. 
A species-specific difference in catch was observed 
when using barium-ferrite magnets. Capture of C. lim- 
batus and D. americana was significantly associated 
with control hooks; however, capture of C. plumbeus was 
not affected. Species-specific differences may be due 
to morphological (i.e., ampullary pore density or canal 
depth) or behavior (i.e., foraging strategy) (see addition- 
al discussion below). Because D. americana is a benthic 
elasmobranch whose vision is not the primary sense 
in locating buried prey (Raschi, 1986; Jordan, 2008; 
Jordan et al., 2009), especially in the turbid waters of 
our study sites, we hypothesize that D. americana may 
rely more heavily on electroreception, and therefore the 
strong induced current produced by the barium-ferrite 
magnets elicited a repellent response. O’Connell et al. 
(2010) conducted a study which examined the effects 
of grade C8 barium-ferrite permanent magnets, identi- 
cal to the magnets used in the present longline study, 
on D. americana and found that the feeding response 
of this species was highly correlated with procedural 
control and control regions, and there were significantly 
greater quantities of avoidance behaviors toward the 
magnetic regions. Similarly, Rigg et al. (2009) showed 
that ferrite magnets induce repellent responses in five 
elasmobranch species, S. lewini, C. tilstoni, C. ambly- 
rhynchos, R. acutus, and G. glyphis. These findings 
support the results obtained from field trials in the 
present study. 
In addition to these results, it is unclear why C. 
limbatus catch was significantly associated with con- 
trol hooks and C. plumbeus catch was not. One pos- 
sible explanation for this result may be animal size 
and maturity. The size of the animal is directly cor- 
related to ampullary canal length, resulting in dif- 
fering electroreception capabilities (Sisneros et al., 
1998; Sisneros and Tricas, 2002). Studies show that 
as the Atlantic stingray (Dasyatis sabina) and clear- 
nose skate ( Raja eglanteria ) mature there is a gain 
of electrosensory primary afferents and, presumably, 
neural sensitivity (Sisneros et al., 1998; Sisneros and 
Tricas, 2002). More specifically, the neural sensitivity 
of R. eglanteria was five times greater in juveniles 
and eight times greater in adults than in embryos, 
(Sisneros et al., 1998). Similarly, in D. sabina, the 
neural sensitivity is three times greater in juveniles 
and four times greater in adults than in embryos 
(Sisneros and Tricas, 2002). All C. plumbeus cap- 
tured in this experiment were juveniles (Sminkey and 
Musick, 1995), whereas all C. limbatus were adults 
(Killam and Parsons, 1989). It is possible that in the 
case of these two species, maturity was an important 
characteristic in determining the success of magnetic 
repellents and therefore could also explain why mag- 
nets successfully repelled D. americana, which were 
all adults. Intraspecific comparisons between animal 
maturity and repellent success could not be made 
because only one size class per species being was 
present in the catch; therefore we could not accurately 
conclude whether or not animal maturity reflects the 
effectiveness of the magnets as repellents. 
Supporting our C. plumbeus findings, Brill et al. 
(2009) found that juvenile C. plumbeus catch was sig- 
nificantly reduced with the use of electropositive met- 
als on longline hooks; however, preliminary labora- 
tory investigations have demonstrated that juvenile C. 
plumbeus quickly habituate to magnetic stimulation 
(R. Brill, personal connnun. 2 ) — a finding that serves 
as a possible explanation for the observed C. plumbeus 
results in our study. Lastly, differences in C. plumbeus 
and C. limbatus results may be an artifact of small 
sample size. 
Hook-and-line study 
Neodymium-iron-boron magnets polarized through the 
longitudinal axis repelled M. canis and R. terraenovae 
2 Brill, Richard. 2009. Virginia Institute of Marine Science, 
PO Box 1346, Gloucester Point, Virginia, 23062. 
