East Meets West Fisheries managers set separate quotas for bluefin tuna in the eastern and western Atlantic. But researchers who affixed tracking devices onto tuna off the coast of New England found that almost one in three swam clear across the ocean. Obviously, tight quotas on "western Atlantic" tuna don't protect these fish in eastern waters. © Richard Herrmann / Seapics.com

The open ocean has no obvious landmarks—just water, water everywhere. Yet some creatures navigate easily across this trackless expanse, on seasonal migrations.

Why make such perilous and energy-demanding journeys? Some migrants just go where the food is. The ocean is like a desert, where the oases are patches of food. Other migrants commute between breeding and feeding grounds. Blue whales feed in polar waters, then travel to the tropics, where pregnant females give birth.

How do these migrants find their way? Lobsters, turtles and whales have internal magnetic compasses. Close to the coast, turtles use another tool: they sniff their way to nesting beaches. For other ocean travelers, the mechanism of migration remains a mystery.

	Nine species of hammerhead shark exist, all of them migratory. Scientists think that scalloped hammerheads (Sphyrna lewini) navigate using electro-receptors in their oddly shaped heads to detect changes in the Earth's magnetic field. On annual migrations, underwater mountains or "seamounts" may serve as stepping stones along the invisible path. © R & V Taylor / Bruce Coleman, Inc.

CONSERVATION: Migrants Know No Borders
The very middle of the ocean is no refuge from human activity. Pollution reaches this far-off realm, and so do fishing boats, with trawls and nets that are miles long.

In recent years fishing boats from many nations have increasingly targeted sharks, largely because their valuable fins are used to make shark's fin soup in Asia. Because of overfishing, many shark species are experiencing drastic declines. New regulations protect some sharks when they swim in U.S. waters. But on their seasonal migrations, many sharks are left unprotected move through international waters.

Even when international laws are in place, the oceans are so vast that enforcement is a challenge. One place where international cooperation has been a success is in the waters of the North Pacific. Here, vast drift-nets, set for fish, were also catching and killing enormous numbers of sharks, seabirds, marine mammals, dolphins and turtles. The United States, Canada, Japan and Russia are now cooperating to enforce a United Nations' moratorium on these so-called "curtains of death."

	Powerful Signals It may look like a backpack, but it's really a satellite tag—and it broadcasts data to help scientists track this endangered leatherback turtle on her Atlantic Ocean migration. All too often, leatherbacks get tangled and drown in fishing nets. With more information on turtle migration routes, fisheries managers could take measures to reduce mortality. © Mark Conlin / Seapics.com

Long-Distance Data
Many trans-oceanic migrants share an adaptive trait—they're big. But despite their size, they are often elusive. Now new technology is helping scientists learn more about the amazing annual journeys these animals make.

Being big is a big advantage: large animals have a lot of storage space. Loggerhead turtles rival a dining-room table in size; great white sharks are as bulky as a family van; and blue whales are the biggest animal alive on Earth. Whales, for example, feed voraciously to pack on a thick layer of blubber before they migrate. Stored calories sustain them on their long-distance travels.

A technology called satellite tagging has shed new light on the movements of these giants of the ocean highways. With older radio tags, the only way to track an animal once it was tagged with a transmitter was to follow behind, within radio range. So it was expensive, and difficult, to collect data for more than a few days.

Satellite tags record data for weeks or months, then beam the information to an orbiting satellite. In turn, the satellite relays the figures straight to the scientist's desktop computer.