O'Connell et at: Analysis of permanent magnets as elasmobranch bycatch reduction devices 
397 
Table 1 
Elasmobranch catch composition from longline gear with 
neodymium-iron-boron magnets in 30 sets. No significant 
differences were found between control and magnetic 
treatments for any of the species or all species combined. 
Species 
n 
No. of 
control 
treatments 
No. of 
magnet 
treatments 
Rhizoprionodon 
15 
7 
8 
terraenovae 
Carcharhinus limbatus 
6 
3 
3 
Carcharhinus plumbeus 
4 
2 
2 
Dasyatis americana 
4 
1 
3 
Negaprion brevirostris 
1 
0 
1 
Total elasmobranchs 
30 
13 
17 
(n= 30 sets): Atlantic sharpnose shark (Rhizoprionodon 
terraenovae), blacktip shark ( Carcharhinus limbatus), 
sandbar shark ( Carcharhinus plumbeus), southern sting- 
ray ( Dasyatis americana), and lemon shark ( Negaprion 
brevirostris). Total capture between magnetic and control 
treatments was not significant (^ 2 =0.533, P=0.4652), 
nor was there a significant difference in catch for R. 
terraenovae (^ 2 = 0.067, P = 0.7963), the only species 
for which sufficient catch allowed analysis by species 
(Table 1). No teleosts were caught on any hooks. 
Longline: barium-ferrite permanent magnets 
Seven different species were captured during the bar- 
ium-ferrite permanent magnetic trials (n- 54 sets): C. 
limbatus, D. americana, C. plumbeus, N. brevirostris, 
bonnethead shark (Sphyrna tiburo), blacknose shark 
( Carcharhinus acronotus), and one teleost — red drum 
( Sciaenops ocellatus). Elasmobranch catch with the use 
of barium-ferrite permanent magnets was significantly 
lower than the catch with controls ( j 2 =4.235, P=0.0396). 
Among individual species with sufficient numbers to 
analyze, catches of D. americana and C. limbatus were 
significantly greater on control hooks than on magnetic 
treatment hooks (^ 2 =4.455, P=0.0348). There was no dif- 
ference in the catch of C. plumbeus (j 2 =1.286, P=0.257; 
Table 2). 
Hook-and-line 
Six elasmobranch species were captured by hook-and- 
line: R. terraenovae, spiny dogfish (Squalus acanthias), 
smooth dogfish ( Mustelus canis), clearnose skate {R. 
eglanteria), C. limbatus, and scalloped hammerhead 
( Sphyrna lewini). 
For all species combined, there was no statistical 
significance in capture found between control and 
procedural control hooks: R. terraenovae (;^ 2 =0.419, 
P=0.5175); S. acanthias (j 2 =0.019, P=0.8907); M. ca- 
Table 2 
Elasmobranch catch composition from longline gear with 
barium-ferrite magnets in 54 sets. Asterisks indicate sig- 
nificant (P<0.005) differences between control and mag- 
netic treatments in chi-square analyses. 
Species 
n 
Control 
Magnets 
Dasyatis americana * 
ii 
9 
2 
Carcharhinus limbatus * 
ii 
9 
2 
Carcharhinus plumbeus 
7 
2 
5 
Negaprion brevirostris 
2 
2 
0 
Carcharhinus acronotus 
2 
0 
2 
Sphyrna tiburo 
1 
1 
0 
Total elasmobranchs* 
34 
23 
11 
Total teleosts 
4 
2 
2 
Table 3 
Elasmobranch catch composition from hook-and-line gear 
with neodymium-iron-boron magnets in 660 trials. Pro- 
cedural control data were not included because no signifi- 
cant difference in catch for control and procedural control 
treatments was observed. Asterisks indicate significant 
(P<0.005) differences between control and magnetic 
treatments in chi-square analysis. 
Species 
n 
Control 
Magnets 
Rhizoprionodon 
169 
67 
30 
terraenovae* 
Mustelus canis* 
21 
10 
1 
Squalus acanthias 
85 
31 
23 
Raja eglanteria 
16 
6 
3 
Carcharhinus limbatus 
7 
4 
0 
Sphyrna lewini 
2 
1 
0 
Total elasmobranchs* 
147 
119 
57 
Total teleosts 
16 
6 
5 
nis (x 2 = 0.222, P= 0.6374); R. eglanteria (j 2 = 0.2860, 
P=0.5930); C. limbatus (x 2 = 1.000, P=0.3173); and S. 
lewini (^ 2 =0.000, P=1.000). Therefore, direct compari- 
son between combined control and magnetic treatments 
was statistically warranted. 
Compared with control hooks, neodymium-iron-boron 
magnets significantly reduced elasmobranch capture 
(X 2 - 21.841, P=0.0001; Table 3). The capture of both 
R. terraenovae and M. canis was significantly reduced 
by magnets (M. canis: ^ 2 =7.364, P=0.0067; R. terrae- 
novae: j 2 =14.113, P=0.0002). Squalus acanthias and R. 
eglanteria catch was not significantly different between 
control and magnet treatments ( S . acanthias-. j 2 =1.185, 
P=0.2763; R. eglanteria-. ^ 2 =1.000, P=0.3173). Low C. 
limbatus and S. lewini catch did not allow experimental 
