394 
Analysis of permanent magnets 
as elasmobranch bycatch reduction devices 
in hook-and-line and longline trials 
Email address for contact author: coconnell2@umassd.edu 
1 Present address for contact author: School of Marine Science and Technology 
University of Massachusetts Dartmouth 
New Bedford, Massachusetts 02740 
2 Coastal Marine and Wetlands Studies 
Coastal Carolina University 
Conway, South Carolina 29526 
3 SharkDefense Technologies, LLC 
P.O. Box 2593 
Oak Ridge, New Jersey 07438 
4 Florida Keys Community College 
5901 College Drive 
Key West, Florida 33040 
Abstract — Previous studies indicate 
that elasmobranch fishes (sharks, 
skates and rays) detect the Earth’s 
geomagnetic field by indirect magne- 
toreception through electromagnetic 
induction, using their ampullae of 
Lorenzini. Applying this concept, 
we evaluated the capture of elasmo- 
branchs in the presence of permanent 
magnets in hook-and-line and inshore 
longline fishing experiments. Hooks 
with neodymium-iron-boron magnets 
significantly reduced the capture of 
elasmobranchs overall in comparison 
with control and procedural control 
hooks in the hook-and-line experi- 
ment. Catches of Atlantic sharpnose 
shark (Rhizoprionodon terraenovae ) 
and smooth dogfish (Mustelus canis) 
were significantly reduced with 
magnetic hook-and-line treatments, 
whereas catches of spiny dogfish 
( Squalus acanthias) and clearnose 
skate (Raja eglanteria ) were not. 
Longline hooks with barium-ferrite 
magnets significantly reduced total 
elasmobranch capture when compared 
with control hooks. In the longline 
study, capture of blacktip sharks 
( Carcharhinus lirnbatus ) and southern 
stingrays ( Dasyatis americatia ) was 
reduced on magnetic hooks, whereas 
capture of sandbar shark (Carcha- 
rhinus plumbeus) was not affected. 
Teleosts, such as red drum (Sciaenops 
ocellatus), Atlantic croaker ( Micro - 
pogonias undulatus), oyster toadfish 
( Opsanus tau), black sea bass (Cen- 
tropristis striata), and the bluefish 
(Pomatomas saltatrix), showed no 
hook preference in either hook-and- 
line or longline studies. These results 
indicate that permanent magnets, 
although eliciting species-specific 
capture trends, warrant further inves- 
tigation in commercial longline and 
recreational fisheries, where bycatch 
mortality is a leading contributor to 
declines in elasmobranch populations. 
Manuscript submitted 14 April 2011. 
Manuscript accepted 5 July 2011. 
Fish. Bull. 109:394-401 (2011). 
The views and opinions expressed 
or implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Craig P. O'Connell (contact author ) 1 2 3 
Daniel C. Abel 2 
Eric M. Stroud 3 
Patrick H. Rice 3 4 
Elasmobranch (sharks, skates, and 
rays) populations are being subjected 
to large-scale anthropogenic mortal- 
ity, resulting in significant population 
declines of numerous species (Musick 
et al., 1993; Stevens et al., 2000; 
Baum and Myers, 2004). Directed 
commercial fisheries for shark meat 
and fins, combined with substantial 
bycatch, are thought to be the main 
cause of elasmobranch mortality 
(Bonfil, 1994; McKinnell and Seki, 
1998; Megalofonou et al., 2005; Pois- 
son, 2011). Furthermore, recreational 
impact is increasing as charter boats 
have redirected their efforts to shark 
fishing to compensate for the lack of 
teleost targets such as tuna, marlin, 
and snapper (Anderson, 1990; Musick 
et al., 1993; NMFS 1 ). The decline of 
several elasmobranch populations is 
particularly significant because these 
animals are K-selected species, and 
therefore populations do not rebound 
1 NMFS (National Marine Fisheries Ser- 
vice). 1991. Draft Secretarial shark 
fishery management plan for the Atlan- 
tic Ocean (19 April 1991), 127 p. U.S. 
Dep. Commer. NOAA, NMFS, Southeast 
Regional Center, St. Petersburg, FL. 
quickly with changes in management 
practices (Smith et al., 1998). 
One strategy for managing shark 
fisheries and reducing bycatch is to 
employ repellents that selectively re- 
pel elasmobranchs but do not repel 
target species. A promising line of 
research involves the use of perma- 
nent magnets to create an abnormally 
strong electrical stimulus to over- 
whelm the acute electrosensory sys- 
tem of elasmobranchs and thus repel 
them (Rigg et al., 2009; O’Connell et 
al., 2010, 2011). This electrosensory 
system, comprising many individual 
ampullae of Lorenzini, is used to de- 
tect minute electrical impulses for 
detection of prey and may also pro- 
vide geolocation information (Murray, 
1962; Kalmijn, 1982; Klimley, 2002). 
In laboratory trials, Rigg et al. 
(2009) evaluated the effects of perma- 
nent magnets on five elasmobranch 
bycatch species: scalloped hammer- 
head ( Sphyrna lewini ); Australian 
blacktip shark ( Carcharhinus tilsto- 
ni ); gray reef shark (C. amblyrhyn- 
chos ); milk shark (Rhizoprionodon 
acutus ); and the speartooth shark 
( Glyphis glyphis ); as well as the bar- 
ramundi (Lates calcarifer), a teleost. 
i, 
