90 Sharks: The Animal Answer Guide
light organs on their sides and bellies. Capitalizing on this predator-prey
interaction are Cookiecutter Sharks. Cookiecutters are also biolumines-
cent, but for devious reasons. The Cookiecutters hide among the mesope-
lagic fishes, apparently blending into the bioluminescent background, and
ambush their prey. Cookiecutters aren’t feeding on zooplankton or meso-
pelagic fishes but on the predators of the vertical migrators. Unlike most
sharks that chop up their prey and swallow large mouthfuls, Cookiecutters
feed one small bite at a time, taking a single, circular, half-dollar-size plug
out of their victims’ sides. (See “What kind of teeth do sharks have?” in
chapter 2 and “Do sharks chew their food?” in chapter 7.)
Because many sharks are highly migratory species, moving around
and through entire ocean basins, they encounter fishing boats from many
countries. This exposes them to capture, repeatedly. A shark may be pro-
tected by local or national fishing regulations when near the shoreline, but
the high seas are largely unregulated, and few police exist to enforce any
laws on the books. As a result, pelagic (open-ocean) sharks are heavily over-
fished (see chapter 10).
How do sharks navigate?
Moving across an ocean and back again, returning to the exact same
locale after a nine-month absence, as was the case with Nicole the White
Shark, requires a highly sophisticated navigational system. So does arriv-
ing at the right place and time to feed on prey that is available for only a
short period. Tiger Sharks show up in the northwestern Hawaiian islands
just as young Blackfooted and Laysan albatross are taking their first, some-
times unsuccessful and therefore fatal, flights. Tiger Sharks do the same
thing at Raine Island, Great Barrier Reef, Australia, just as green sea tur-
tles appear by the thousands to lay eggs. At Raine Island, the Tigers scav-
enge heavily on dead turtles that had been stranded at low tide on exposed
reefs. Some Tiger Sharks occur year-round at both locations, suggesting
that Tiger Shark populations consist of resident and transient or migratory
individuals. Whale Sharks are also famous for showing up at different lo-
cales around the tropics just when reef fishes or invertebrates are about to
spawn. Manta rays do the same. White Sharks arrive at the Farallon Islands
off San Francisco in late autumn to feed on young elephant seals that arrive
there at about that time.
How sharks accomplish these and other precise long-distance naviga-
tional feats remains a mystery. Our best guess is that they rely on their
acute electric sense, navigating via geomagnetic cues related to the earth’s
magnetic field and its anomalies and gradients (see “How do sharks detect
electric fields?” in chapter 2). This kind of navigation means that sharks
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