Back From The Brink
This summer, USGS Western Fisheries Research Center and collaborating scientists were delighted to find that the abundance of wild born juvenile fall Chinook salmon migrating out of the Snake River has increased by almost 2 orders of magnitude within the last 30 years.
Historically, between 2-6 million adult Pacific salmon and steelhead returned every year to spawn in the Snake River and its tributaries. Fall Chinook salmon accounted for about 500,000 adult returns. Salmon and steelhead are a Northwest icon. They support fisheries and are central to tradition, culture, and sustenance for Northwest Tribes. Moreover, these fish support the entire food web, from eagles to killer whales.
Since the final construction of the Lower Snake River dams in the 1970s, 13 species of salmon and steelhead have been listed ‘threatened’ according to the Endangered Species Act (ESA). This includes the Snake River fall Chinook salmon population that declined to as low as 78 wild-born adults in 1990. The number of wild-born juveniles out-migrating each year was previously unknown.
Due to the low number of adults spawning in the wild, USGS-developed statistical models estimate that as few as 50,000 juveniles migrated out of the Snake River basin in 1992. In 2023, that number reached 1.5 million young outmigrants from approximately 5,600 wild born and 26,800 hatchery born parents!
This huge progress is likely a result of several factors, including reduced harvest, improved water flow through the dams, predator control, hatchery fish contributing to spawning in the wild, improved juvenile passage structures, and periods of favorable ocean conditions and food availability Future research will determine which of these factors contributed most to this population’s comeback. This will give us actionable steps managers can take to help this at-risk fish population.
One fish... two fish... red fish... blue fish! Have you ever tried to count the fish in a river? It is far more challenging than you’d think. First of all, you can only count something you can see, but fish live underwater and you have to catch them to see them! This means scientists must account for all the fish they don’t catch. It gets even harder if you want to just count wild fish, because fish hatcheries only mark a certain percentage of the fish they release by clipping a fin, making differentiating between types of fish very difficult.
Fish population estimates are made by combining these of counts of captured fish with complex technologies such as Passive Integrated Transponders (PIT tags). Fish are injected with an internal microchip that is activated when the fish passes close to a special antenna, migrating out as juveniles and back in as adults. USGS used the data from these tags to help determine how many fish of each of type passed Lower Granite Dam each year.
Complex statistical models, like the ones in the publication, Back From the Brink, are used to take all of these factors into account to come up with the most accurate wild born fish population estimates possible. How’s that for counting fish?
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