Who knew that baby salmon were such explorers?
The long-held understanding that baby salmon emerge from the streams where they hatched to head out to sea actually is missing a far more complex story — and a far more interesting one, scientists explain in a new paper.
It turns out the intrepid baby fish, no longer than a pinkie finger, explore multiple streams miles apart, traveling from river mouth to river mouth, in what amounts to a connected meta-nursery.
Akin to the human understanding that it takes a village to raise a child, similarly, it takes a connected coastline of functioning, healthy rivers to bring up a baby salmon, the paper revealed.
The findings show that the typical model of conservation that focuses only on the natal streams of individual fish populations may overlook the importance of freshwater habitat elsewhere. This emerging concept of connected habitat is perhaps a more accurate way to understand what baby salmon need. It’s also a concept that’s increasingly important as some salmon populations struggle with stressors, from excessive heat to human development.
The discovery of the wandering baby salmon was a complete surprise.
“The first couple of times, you kind of write it off: Did a merganser eat it and eject it, or what?” said scientist Todd Bennett, referring to a fish-eating duck. “It was like, hey, this is kind of crazy, do we believe that?” But as he and other scientists recorded more and more tagged baby fish in streams where they were not born, they realized something had to be up.
These, said Bennett, a research field biologist in the watershed program at the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center in Seattle, were not fish dropped by a predator. Not dead fish at all. These were live salmon, using drainages beyond their natal stream in their early life stages. The scientists recorded hundreds of such movements, several times per fish, and between drainages more than 40 miles of saltwater shoreline apart.
The behavior is all the more remarkable because baby salmon undergo a complex physiological change as they emerge from freshwater to spend the next stage of their life at sea. Yet apparently they can tolerate switching back and forth between freshwater and salt at this stage of life.
The findings were recorded in 20 years of research, tracking the movements of tagged coho salmon and rainbow and cutthroat trout in Deep Creek, West Twin River and East Twin River on the Olympic Peninsula and of coho in California’s Redwood Creek, drainages that enter Humboldt Bay, and the Klamath River.
The advantages to an early life wander for the baby fish could be multiple, the authors explain. Exploring the conditions in other estuaries gives the baby fish a larger selection of food supplies and environmental conditions than sticking in one place.
And if a stream gets blown out in a flood, the baby fish can beat it to more hospitable rearing areas instead of getting ground up in rolling logs and rocks or stranded in a pool if a stream avulses to a whole new area in its flood plain.
“The takeaway for me is the landscapes and the seascapes are much more connected than we thought,” said Stuart Munsch, fish biologist at NOAA’s Northwest Fisheries Science Center, and lead author of the paper.
“This is not part of the paradigm, this is not what they are supposed to do. I was taught juveniles swim to sea, they don’t come back, that is not a thing,” Munsch said.
“Now I see it differently. It is the whole landscape that is working together, and the state of all those habitats could potentially matter to a population, no matter where that population originates.”
It was exciting to learn a bit more about how Pacific salmon — a species intensely studied for a century — do what they do.
Key questions remain of course. How prevalent is this behavior? How far upstream do the wandering baby salmon go? So many questions. The paper may underestimate the frequency and extent of baby salmon’s travels because they could only be detected in selected locations with tag readers.
The findings show the value once more of long-term scientific study in the field, Munsch said. “If you collect these data sets, it is an opportunity to discover things.”
