The migrations that make up the well-known salmon life cycle have long been described as one way at a time. Juvenile salmon hatch and swim down rivers to the ocean, where they grow and mature before returning to the same river to spawn the next generation.
Turns out that many young salmon do things differently, according to new research by NOAA Fisheries, Tribal, and university scientists. The findings were published in the peer-reviewed journal Frontiers in Ecology and the Environment.
They discovered that as many as 22 percent of juvenile salmon in California and Washington streams swam downriver to the ocean and then back up other rivers as many as 9 times. They reached rivers as far as 40 miles away along the coast. Instead of simply entering the ocean for good, they roam miles of coastline, moving between freshwater and saltwater and exploring rivers as they go, said Todd Bennett, a salmon scientist at NOAA Fisheries' Northwest Fisheries Science Center and coauthor of the new research.
Stretches of coast and their rivers form enormous salmon nurseries for the exploring juveniles, the scientists said. The researchers documented coho salmon, steelhead, and cutthroat trout using coastal rivers separated by salt water, and suspect other species may do the same.
"The landscapes are much more connected than we realized, and salmon take advantage of that," said Stuart Munsch, also a research scientist at the Northwest Fisheries Service Center and lead author of the new research. "This provides a more complete and accurate picture of the habitat they are using, which helps us make informed decisions as to how to promote their recovery."
Brief Forays and Longer Stays
Researchers first noticed fish that had been tagged with tiny microchips in one river swimming up different rivers many miles away. As they tracked more juveniles, the researchers found some entered unfamiliar rivers on brief forays, while others spent the whole winter in rivers beyond where they hatched. They documented traveling juveniles in streams on the Olympic Peninsula and in Northern California.
The capability to swim between unconnected rivers means that some juvenile salmon may even make temporary use of small streams with no spawning adults. The fish "form migratory pathways that are much more complex than is presently recognized and could access freshwater habitats where people do not expect them to be," the scientists wrote.
The wide-ranging juvenile fish likely improve the resilience of their populations by locating new and better habitats they can rely on as conditions change. Along the way they may also school with salmon from other areas, picking up clues on where to find the best refuge and prey. Heavy rains subject many streams and rivers in the Pacific Northwest to flows that can go from a trickle to a raging torrent in a matter of hours. This can shift available habitat by moving or carrying away instream wood and gravel. By moving between streams, salmon may find more stable habitat that can greatly increase their chances of survival.
At the same time, though, moving between freshwater and saltwater may expose the fish to more predators. "They're perfect food for lots of other species, so they are taking a risk but also finding some benefits as they go," Bennett said.
"They're searching for the best opportunities they can find," Munsch said. "By moving around, they are also spreading the risk as some may find alternative habitats that better support them as conditions change."
Scientists have long known that adult salmon returning upstream may sometimes stray into different rivers than where they hatched as juveniles. However, researchers said they need to monitor the fish further to tell if this may be related to the different rivers they might have explored as juveniles.
Some Salmon Roam Many Miles
In a series of streams on Washington's Olympic Peninsula, researchers documented almost 900 different juvenile coho salmon, steelhead, and trout moving between drainages. This included up to 22 percent of the juvenile fish they tagged and then later detected again, depending on the year and river where they were tagged. Most moved only between two different drainages. About 8 percent of the fish moved up to nine times among three different drainages with monitors to detect the tagged fish.
In Northern California, scientists documented 28 coho salmon moving between drainages. One of the fish swam from a tributary of Redwood Creek to a drainage feeding Humboldt Bay some 40 miles to the south. The scientists also reviewed other studies, finding evidence for similar movements of salmon species across three different continents. They hypothesize that this behavior is widespread.
The juvenile fish followed both well-traveled paths and lesser-used routes in and out of separate drainages over multiple years. The young salmon may disperse when streams become crowded, reducing competition between the fish and increasing their chances of survival.
The research may still underestimate how many fish move between drainages because they only included drainages with monitoring equipment. Other fish may move between unmonitored drainages where they might never be recognized, the researchers noted.
The findings suggest that salmon scientists and managers must be "willing to branch out from the normal and accepted paradigms you read in the textbooks," Bennett said. "We wouldn't know about this unless we happened to be looking. Even after so many scientists have studied salmon over many decades, they still figure out how to surprise us."
