How salmon find their way back to their birthplace to reproduce after migrating across thousands of miles of open ocean has mystified scientists for more than a century. But now marine biologists at think they may have discovered the secret.

The earth’s magnetic field varies across the globe, with every oceanic region having a slightly different magnetic signature. At the beginning of their lives, salmon may read the magnetic field of their home area and ?imprint? on it. By noting the unique ?magnetic address? of their birthplace and remembering it, the fish may be able to distinguish this location from all others when they are fully grown and ready to return years later to lay their eggs.

Biologist Kenneth Lohmann says, “What we are proposing is that natal homing can be explained in terms of animals learning the unique magnetic signature of their home area early in life and then retaining that information.”

Just why marine animals WANT to migrate such vast distances to return to their own birthplace, sometimes bypassing other suitable locations along the way, is not known. Scientists speculate that natal homing evolved because individuals that returned to their home areas to reproduce left more offspring than those that tried to reproduce elsewhere.

Lohmann says, “For animals that require highly specific environmental conditions to reproduce, assessing the suitability of an unfamiliar area can be difficult and risky?these animals seem to have hit on a strategy that if a natal site was good enough for them, then it will be good enough for their offspring.”

Earth’s magnetic field changes slightly over time and thus the salmon can probably only aim for the general region of their birthplace. Once they get close to the target, other senses, such as vision or smell, may be used to pinpoint specific reproductive sites.

But what if they can’t go home again? When young salmon are transported downstream on boats?to get them out of the way of a dam, for instance?they can lose the ability to migrate back to their breeding grounds.

Biologist Matthew Keefer says, “Juveniles trying to get back to sea usually go over the spillways or past the dam?s turbines.” Going past a dam’s turbines, however, can kill many young fish. In response, management try to help salmon and steelhead trout avoid dams altogether by transporting juveniles past dams toward the ocean on river-faring barges.

But Keefer has found that this free ferry ride can create problems when the juveniles grow up. When compared to fish that migrated naturally, transported juveniles have lower survivorship as adults and were less likely to find their way home. Maybe all salmon will be farmed in the future.

Salmon aren’t the only ones in trouble. Oceanographers surveying unexplored depths of the Pacific in small submarines have found disturbing declines in sea-life populations and evidence that human impacts have stretched down deeply in the gulf. They discovered some new kinds of fish, but along with the excitement of discovery came disturbing signs of human impacts in the gulf?s depths, and, in particular, signals that overfishing has decimated ecosystems. Large schools of fish documented in earlier expeditions have vanished, impacted by fishing and pollution.

Oceanographer Octavio Aburto-Oropeza says, “We have lots of evidence of ghost nets with trapped animals at many depths, along with pollution, including beer cans, in each deep location we studied.”

Fellow submarine researcher Steve Drogin discovered a hydrothermal vent at a depth of 450 feet with water temperatures reaching 266 Fahrenheit degrees. He says, “It felt to me like walking into the middle of a forest fire, with flames shooting out.”

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