SCIENCE FILE / An exploration of issues and trends affecting science, medicine and the environment : Secrets of the Salmon : The fish spend their lives far out in the ocean, but almost always find their way back to the stream they were born in. Slowly, scientists are figuring out why and how.
The salmon have returned again this year to the more than 2,000 streams of southeastern Alaska, just as they have every season for millions of years.
Salmon generally return to the same river or creek in which they were born and fight valiantly against the current as they move upstream, where they spawn and die. With a sense of awe, human intruders can watch this classic struggle from the stream banks, and they are often moved to tears. It always ends the same way, with death and the regeneration of life.
For decades, scientists have tried to unravel the secrets of the salmon. Why do these large fish find it imperative to return to the same stream where they were born to produce their young? And how do they find their way home after traveling for thousands of miles through the Gulf of Alaska and the North Pacific Ocean?
“There are more unknowns than knowns,” says William Heard, a fisheries biologist with the National Marine Fisheries Service’s Auke Bay Laboratory in Juneau. But he adds that much has been learned in recent years, and the salmon is slowing giving up its secrets.
The lab is perched on the banks of Auke Creek, a small stream just a little longer than a football field that is wall-to-wall salmon during the spawning season. In a good year, 20,000 salmon may come home to Auke Creek to reproduce and die.
It is from that crystal clear creek that scientists have extracted some of their most astounding answers.
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Fisheries geneticist A.J. Gharrett and salmon biologist William W. Smoker of the University of Alaska have found that the salmon that spawn in Auke Creek are amazingly faithful to their natal stream.
The two scientists used a genetic marker, somewhat like a blood group, to distinguish young Auke Creek salmon from others in the area. Eighteen months later, when the fish returned from the sea, the scientists were able to determine if some had strayed into other streams.
To the surprise of many of their colleagues, they found that the marked fish returned only to Auke Creek. “They spawned at almost exactly the same spot” where they had been born, says Gharrett, and they returned at precisely the same time of the year. They did not even stray into a similar stream that is only half a mile away.
Why such faithfulness? Gharrett and Smoker believe the salmon that begin their lives in Auke Creek instinctively know that their offspring will have a better chance of survival if they return to the same place. Even such things as the size of the pebbles in the creek bed can influence survival rates, because the new eggs are genetically conditioned to cling to pebbles of a certain size. They might simply wash past a sandy bottom and back out to sea, for example, if they had been programmed for small rocks. Other factors, such as temperature and natural chemicals in the water, can also influence survival rates.
So the reproductive odds are better for the salmon of Auke Creek if they return to their birthplace, the scientists say, and the salmon apparently know that.
If they were to pick the wrong stream, chances are that more of their offspring would die.
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Such fidelity, however, may not always be the case. Elsewhere in Alaska, salmon are believed to return to the same area to spawn, but not necessarily to the same stream. New streams exposed by receding glaciers in Glacier Bay, 40 miles west of Juneau, are being recolonized by salmon within 10 years, so some salmon are obviously straying into the new creeks.
The scientists believe that in some areas, such as earthquake-prone Prince William Sound, streams are less stable than Auke Creek, which has provided a reliable spawning bed for many generations. It may be that salmon born in the creeks along Prince William Sound may instinctively know that their particular stream may not be there when they return, and thus some straying is essential to survival.
No one knows that for certain, however. It is, as Smoker put it, “armchair biology.”
The fact that scientists do not know all the intimate details should not be surprising. Only in recent years have they begun to understand how salmon know when to return, and how they find their way home from hundreds of miles away.
Auke Creek, for example, produces one of five types of Pacific salmon, known as the “pink” or “humpy.” The pink has a two-year life cycle, and is the most abundant salmon in the ocean.
After spending their first winter in the fresh water of the creek, young salmon emerge in the spring when the sea water is warm enough to provide nutrients. They travel out into the Gulf of Alaska, where they mature.
In the summer of the following year, somehow they know it is time to head home. “The cue that it’s time to go home is probably day length,” says Smoker. “As the days start to get shorter, they leave off feeding and return to the spawning streams.”
Initially, they are guided by the Earth’s magnetic field, and possibly by the direction of the morning sun, scientists believe. But that only tells them the general direction in which to head.
They probably use physical clues--literally the lay of the land--but that still would not tell them which stream is theirs. Many streams in Alaska would appear very similar, even to a salmon.
Scientists had puzzled over that for years until pioneering experiments by biologists at the University of Wisconsin provided the answer. Scientists there theorized that salmon were able to detect very subtle differences in the odor of streams.
They stuffed wads of cotton into the noses of a few salmon and let them go in Puget Sound. The fish became hopelessly lost. “It turned out that fish have an extremely high level of olfactory acuity,” says Heard. “They make a dog look like he doesn’t have a nose.”
That sense of smell allows the returning salmon to find the stream that has chemical and biological odors that are the most similar to the one in which its life began. Gharrett says he has seen that process at work.
“They stick their nose up a lot of different streams before they actually pick a stream to spawn in,” he says.
Sometimes a salmon travels a considerable distance upstream before deciding it is the wrong one and pulling out to find a better prospect.
Scientists believe that most of the time, a salmon succeeds. It returns to the same stream, and if Auke Creek is typical, to the same spot at the same time of the year as its own life began. There it spawns and dies, the long, tortured journey finally at an end.
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Pacific Sockeye Salmon
Sockeye salmon from Southeast Alaska mature in the Gulf of Alaska. One of several species of Pacific salmon--all of which follow different migration patterns--sockeyes range far out into the Aleutian Islands chain before returning to their natal streams. The males of this hooked-snout species often become bright red. Females are dirty olive to light red, and darker on the sides than males. Sockeyes enter rivers, usually those that are fed by lakes, from March to July. Spawning takes place in lakes or immediately adjacent in inlet or outlet streams from August to December. Their ocean food consists of plankton and small crustaceans.
Source: Encyclopedia of Aquatic Life.
