180 Sharks: The Animal Answer Guide
a fish ecologist, and he informed us that he was not aware of any field tests
of the concept. Given that many other shark repelling schemes have not
worked, it at least seems worth trying.
Although most efforts at repelling sharks focus on reducing human in-
jury risk, recent research has taken a different approach, one more con-
cerned with shark welfare and conservation. Sharks are a large component
of the “bycatch” associated with baited hook fisheries that target nonshark
species, such as tunas and billfishes. The challenge, of course, is to discover
something that deters sharks but doesn’t reduce the attractiveness of the
bait to the targeted fish. Some promising albeit not totally effective meth-
ods capitalize on the great sensitivity of elasmobranchs to electrical fields.
Attaching magnets and electropositive metals to baits to produce strong
electric fields has reduced catches of Atlantic Sharpnose Sharks, Smooth
Dogfish, Blacktip Sharks, and Southern Stingrays but has had no effect on
catches of Clearnose Skates or Spiny Dogfish, the latter a pest species in
some fisheries (see “Are some sharks pests?” above). (As an aside, a neigh-
bor of your first author had read that sending electrical impulses down his
metal fishing line would increase his catch of salmon. Spiny Dogfish found
it irresistible.) Catches of Sandbar Sharks were reduced in one study of
bait magnets but not in another. Shark abundance also has an effect: solitary
sharks may be repelled, but groups of sharks are apparently highly motivated
to feed (see ”Do sharks feel pain?” in chapter 8) and are not discouraged.
Personal safety devices using electrical shocks have been developed for
divers, but a White Shark killed an Australian diver who was using one.
The electrical current only served to hinder his mates from pulling him out
of the water. Much obviously remains to be accomplished in the realm of
shark repellants.
Do sharks have diseases and are they contagious?
Cruising the web, you are likely to find statements such as “Sharks are
the only animals that almost never get sick: they are immune to almost ev-
ery known disease.” Sharks do appear to be immune to many diseases, in-
cluding some that infect bony fishes. But sharks do have illnesses, including
cartilage cancer and cancers such as melanomas and cutaneous fibrosarco-
mas (skin and connective tissue cancers).
Among the list of known shark ailments are a variety of bacterial dis-
eases. The bacterial genus Vibrio includes species that cause cholera and oth-
ers that cause gastrointestinal ailments often associated with undercooked
seafood. A Sandbar Shark (Carcharhinus plumbeus) that died in a public
aquarium was found to be infected with Vibrio damsela, focused in the kid-
neys. When injected experimentally under the skin of a Spiny Dogfish,
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