Fish are returning to Bristol Bay by the millions—the final stage in an epic journey home. But how do fish know where their natal stream is, or how to get there?
Tom Quinn is a professor at the University of Washington, who’s spent his career working to understand how salmon navigate, including Bristol Bay sockeye. He says the process of homing begins at the first stage of a salmon’s lifecycle.
“It all starts when young salmon hatch,” he said. “And during that first stage, as they are incubating in the water, their sense of smell is picking up chemical signals that are unique to that body of water.”
Quinn says it’s not as simple as remembering what a home stream smells like. As they make their way downstream, young salmon are constantly cataloging new smells along every leg of the journey.
“It has taken in a series of what we might call olfactory waypoints as it's moving downriver,” he said, referencing the Wood River system as an example, “They would learn the odors of the outlet of the lake as smolts heading down to sea. Then they would come to the confluence of the Wood River and the Nushagak River and they would learn that, so they could then retrace those odors in reverse.”
Quinn says it’s no small feat that salmon can hold onto these memories of smell after years in the ocean, especially when stream compositions can shift.
“Fish have to not be distracted by the fact that there might be more leaf litter, less leaf litter, runoff, hillside slumps where there's a bit more silt and dirt—all the sorts of things that cause streams to change a bit from year to year,” he said.
Once salmon hit salt water, their navigational tools expand. Quinn says that salmon, like many other migratory species, have a sense of direction related to the Earth’s magnetic field.
“They use the apparent transit of the sun across the sky as a compass, we call it sort of a sun compass, and this magnetic sense,” Quinn said. “How they integrate these things is tricky and impressive.”
While in the ocean, sockeye are widely distributed across the North Pacific and mix with salmon stocks from all over. But when it’s time to head for home, the majority of Bristol Bay sockeye arrive in the area within a few short days.
“The fish that are farther away have to either leave earlier or swim faster in order to catch up,” he said. “And that implies again some sense of where they are–-a map, a compass, and a calendar.”
Once they’re closer to freshwater, Quinn says salmon switch their navigational systems back to remembering those familiar smell waypoints.
Some of his research looks at how close returning salmon get to the exact place they were born. His team studied this in a tiny pond connected to a Wood River tributary called Hanson Creek. There, they exposed salmon eggs to a pulse of heat that leaves a distinct mark on their otoliths, the small ear stones in a salmon’s head.
“Four years later, we went back, and a significant number had made their way not just to Hanson Creek, but this tiny little side pond of Hanson Creek.” He added, “That's pretty precise!”
Quinn says there are still more questions to explore, like which chemicals in the water are important for a salmon’s smell, or why some salmon stray from their home streams.
“We know quite a bit, but there's also quite a bit we don't know,” he said. “Regardless of our ignorance, the fish get it right almost all the time.”
As of July 8th, over 10 million fish in Bristol Bay have made it past the fishing district’s gillnets and counting towers, on their way home to spawn the next generation.
Get in touch with the author at jessie@kdlg.org
