Fish Facts is a regular segment this season on the Bristol Bay Fisheries Report where we dive into fish science, ecology, and research, and swim the salmon life cycle from the open ocean to home streams and rivers.
Deep in your inner ear sits a tiny calcium carbonate structure called an otolith. These little stones work as sensors of movement and gravity in our bodies, helping us balance. Fish have otoliths too. And what’s remarkable about a salmon’s otolith is the story it tells.
Daniel Schindler is a fisheries biologist with the University of Washington’s Fisheries Research Institute, where researchers have been studying salmon otoliths in the Nushagak watershed for over 70 years.
“The otolith allows us to do some detective work,” he said. “To figure out, of the fish we're seeing now that are caught in the fisheries, where they were born and what part of the watershed was most important for them.”
Schindler says a salmon’s growth is recorded inside the otolith, like the rings of a tree.
“And that information we extract and use to understand their ecology,” he said. “We can tell how old the fish were, how many years they spent in freshwater, how many years they spent in the ocean. And then also ask, well, how different is the age composition from one stream or one beach to the next?”
He says as salmon grow, the chemistry of the surrounding water becomes incorporated into the otolith. He says it’s like a chemical fingerprint of the waters a salmon swam through.
“We can take the otolith, which is about the size of maybe a quarter of your pinky finger now, and we can cut it flat, and then polish it,” Schindler said. “And we can look at the strontium isotopes, and match the strontium isotopes in the natal piece of the otoliths to the very predictable strontium ratios throughout the Nushagak, and determine which tributary that fish was born in.”
Schindler says these otoliths provide clues not just about where salmon were born and return to spawn, but also when.
“The biggest insight that we've seen is that fewer fish are spending two years growing in the lakes before they go to the ocean,” he said. “So almost all of them are leaving after one full year of growth in the lakes.”
Young sockeye feed on zooplankton, which are booming in the Nushagak watershed’s lake systems. Schindler says over the last 70 years, the amount of zooplankton in these lakes has almost doubled.
He says this explosion of food for salmon is likely at the root of faster salmon growth rates, last year’s record-breaking run, and earlier salmon departures for the ocean. And, these insights help paint a picture of the ecosystems salmon depend on.
“The reason that's important from a conservation standpoint is that what we learned through that work is that the whole Nushagak matters,” Schindler said. “And it doesn't matter at every given year, because some tributaries may be particularly important in a single year. But over the course of time, the whole watershed is contributing at one point or another.”
The Fisheries Research Institute’s work continues this summer, with researchers collecting and analyzing thousands more otoliths to help us better understand the secret lives of salmon.
