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Monitoring salmon populations

PUD employees near smolt trap on Sultan River
PUD environmental coordinators Andrew McDonnell & Kyle Legare near smolt trap on Sultan River

As part of the license requirements for the Jackson Hydroelectric Project, the PUD monitors salmon populations in the Sultan River. Each year in the fall, adult salmon return from the ocean to spawn. During spawning, the female deposits her eggs into a nest created in the gravel. The nest, also called a redd, protects the eggs as they develop into little fish, called fry. As the fry emerge from the gravel, they begin their journey to the estuary and eventually the ocean. One way the PUD monitors salmon populations is by using a smolt trap. Smolt refers to a young salmon (or trout), when it becomes silvery and migrates to the sea for the first time.

Since 2012, PUD biologists have operated a smolt trap in the lower Sultan River, 0.2 miles upstream of the confluence with the Skykomish River in the City of Sultan. The trap is operated during the out-migration period from January through June.

In 2024, the PUD began operating its smolt trap early on December 4, 2023. The smolt trap faces upstream and the cone in the front of the trap rotates as water passes through. A portion of the juvenile salmon out-migrating from the Sultan enter the cone and are funneled to the back. At the back of the trap, fish are contained in a live box where PUD biologists sort, count and then release fish each day the trap operates.

The smolt trap results are compiled in the Excel spreadsheet below. There are 2 tabs in the spreadsheet: Current Year and Historical Data.


  • The table and chart will be updated bi-weekly from December 2023 through June 2024, with out-migration information from the Sultan River smolt trap. The table and chart show the number of salmon caught per hour each week the trap operates. This measure is calculated by dividing the total number of a fish species caught in one week by the total number of hours the trap fished in that week. This method accounts for the fact that the trap is fished for different amounts of time each week – generally anywhere from 20-80 hours per week.  Weekly fish catch totals alone can be misleading because less fish are likely to be caught in weeks that the trap has fished for less hours – regardless of how many fish actually migrated out of the river. The fish per hour measure displayed here corrects for this and provides a more accurate way to compare weekly data within a season and between years. The table also shows the total number of different fish species caught each week, as well as the number of hours the trap fished per week.


  • Figure 1 allows viewers to compare the current trend in out-migration, which shows the average out-migration timing of each salmon species since monitoring began in 2012.
  • Table and Figure 2 summarize and display by year the catch per hour of Chinook, chum, and pink salmon.
  • Table and figure 3 show the total number of juvenile Chinook, chum, and pink salmon that out-migrated from the Sultan River by year, calculated from mark and recapture studies.

View Smolt Trap Data >
Last Updated 5/16/24

Sultan River smolt trap
Sultan River smolt trap
Albino chum salmon caught at smolt trap
Albino chum salmon caught at smolt trap
Conjoined twin chum salmon caught in trap
Conjoined twin chum salmon caught in trap
An overnight catch in the live box
An overnight catch in the live box