O'Connell et al.: Analysis of permanent magnets as elasmobranch bycatch reduction devices 
399 
during the hook-and-line experiment. Other species of 
elasmobranchs did not show any significant responses 
to the magnetic treatment hook ( S . acanthias and R. 
eglanteria) or were data deficient (C. limbatus and S. 
lewini). 
The ineffectiveness of electrosensory stimuli on S. 
acanthias is supported by the results of Tallack and 
Mandelman (2009), who reported that the effective- 
ness of electrosensory stimuli was reduced owing to 
a high level of food deprivation (four days) for captive 
S. acanthias. Moreover, electrosensory stimuli had no 
affect in field studies involving this species. Because 
S. acanthias is found in dense schools, it is possible 
that the ineffectiveness of the magnetic stimuli in our 
experiment was due to factors such as social-facilitation 
(Guttridge et al., 2009). In teleosts fishes, social fa- 
cilitation due to increasing group size increased intra- 
specific feeding activity (Major, 1978; Ryer and Olla, 
1991); therefore these findings may correlate with our 
results for S. acanthias and indicate that high shark 
densities may influence conspecific feeding activity. 
Additionally, because S. acanthias may be found in 
dense schools, it is possible that the ineffectiveness of 
the electrosensory stimuli was due to the abundance 
of conspecific behavior and competition which simply 
overrode the electrosensory stimulation induced by the 
magnets. High densities of elasmobranchs have been 
previously postulated as a potential explanation for 
repellent ineffectiveness (Kaimmer and Stoner, 2008; 
Robbins et al., 2011) and therefore may explain our S. 
acanthias results. 
Contrasting with S. acanthias, M. catiis responded 
very differently to the treatment hooks and catch was 
significantly associated with control hooks. Kalmijn 
(1982) showed that M. canis was highly electrorecep- 
tive and oriented itself toward or bit electrodes, which 
mimicked the bioelectric fields produced by prey. An 
explanation for our findings may be that a stron- 
ger, induced voltage produced by an electrosensory 
stimulus, such as a barium-ferrite magnet, may repel 
M. canis. 
Unlike our hypothesis above for C. plumbeus catch on 
longlines, the relationship between catch and animal 
size or ampullary canal length is weak and therefore 
cannot be used to explain catch in the hook-and-line 
study. For M. canis, ampullary canal length as a result 
of stage of maturity may explain the hook-and-line re- 
sults because catch was significantly higher in controls 
and all animals were adults (Conrath et al., 2002); 
however, other examples of catch trends between an 
animal’s maturity and magnetic effectiveness do not 
exist. For example, all S. acanthias were mature adults 
(Hammock et al., 1985), yet no significant trends in 
catch existed. Also, although catch of R. terraenovae 
was significantly higher for control hooks, catch was 
mixed between juvenile and adults (Parsons, 1985) and 
no distinct catch relationship was observed between 
control and treatment hooks, therefore minimizing the 
potential of animal maturity as a pertinent indicator 
of the effectiveness of the repellents. 
Longline vs. hook-and line 
Differences in catch rates for longlines and hooks-and- 
lines may be due to species-specific responses, as we 
discuss above. Additionally differences in magnetic 
characteristics, namely the axis of polarization, may 
have led to significant catch trends. Neodymium-iron- 
boron magnets used on longlines were polarized through 
the diameter, whereas barium-ferrite magnets and the 
hook-and-line neodymium-iron-boron magnets were 
polarized through a longitudinal axis (height) — a part 
of the experimental design that we initially overlooked. 
Because the longline neodymium-iron-boron magnets 
were placed approximately six centimeters away from 
the hook, the measurable magnetic field did not fully 
protect the bait, (peaks in magnetic flux occurred side 
to side instead of surface-to-substrate), whereas in the 
barium-ferrite magnets and hook-and-line neodymium- 
iron-boron magnets, the magnetic field covered the 
entire bait and hook, which may have been sufficient to 
deter elasmobranchs from feeding on the baited hooks. 
Deployment methods may also be a possible expla- 
nation for these experimental differences. Because 
longlines are immersed (i.e., soaked) for longer time 
intervals, it is possible that elasmobranchs attracted 
by the bait were initially repelled but lingered owing 
to the continuous scent emanating from the bait. In 
this situation it is possible that sensory habituation to 
the magnetic field may have occurred, rendering the 
magnets less effective. Numerous studies have demon- 
strated that habituation is a common phenomenon in 
organisms subjected to repeated sensory stimulation 
(Myrberg et al., 1969; Myrberg et al., 1978; Givois and 
Pollack, 2000). Moreover, in a previous study, lemon 
sharks ( Negapi'ion brevii'ostris) repeatedly exposed to 
a magnetic stimulus reacted at first but became un- 
responsive after several exposures (O’Connell et al., 
2011). However, if repeated exposure to magnets during 
longline experiments resulted in sensory habituation 
with the use of neodymium-iron-boron magnets, this 
explanation is not supported by results with the use of 
barium-ferrite magnets, where there was significantly 
less elasmobranch catch on magnetic treatment hooks 
during the longline experiments. 
Conclusion 
In conclusion, magnets used as elasmobranch-selective 
repellents during both the longlining and hook-and-line 
fishing experiments produced positive result. Although 
the effectiveness of magnets may be influenced by 
animal density (Kaimmer and Stoner, 2008; Robbins 
et al., 2011) and by the level of satiation (Tallack and 
Mandelman, 2009), we found that magnetic polarization 
may effectively “protect” fish hooks and reduce unwanted 
elasmobranch capture in commercial and recreational 
fishing. Although promising, these results warrant fur- 
ther investigation before recommendations can be made 
to fishery managers and policy makers. 
