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The Washington state Department of Fish and Wildlife will study the main stem of the Chehalis River near Centralia and Chehalis this spring as it looks for ways to control invasive bass species and protect one of the river’s most vulnerable native fish species — Chinook salmon.
Senior fish scientist with WDFW John Winkowski presented to the Chehalis Basin Board during the board’s March 5 meeting on his team’s upcoming bass study.
For about two months this spring, Winkowski will lead a team in a “pilot suppression study” at unknown sites along the Chehalis River.
WDFW staff will use electric shock to remove three kinds of invasive bass species along a portion of the Chehalis River between where it joins with the Newaukum River west of Chehalis and where it joins with the Skookumchuck River on the southern edge of Fort Borst Park in Centralia. Teams will visit sites three times throughout the roughly two-month study period, collecting bass each time and using their numbers to estimate the total numbers present in the river.
According to Winkowski, the study will help the department identify the current number of bass in this portion of the river, what percentage of the population can reasonably be removed and how their removal will impact the number of Chinook salmon in the stream. Winkowski’s team will also be monitoring the area for other changes to the ecosystem stemming from bass removal.
“There have been examples where management has come in and removed a predator and then undesired results occur that can create more of a problem than at present,” Winkowski said.
During the meeting, Winkowski also explained why the department is so focused on bass. According to the fish scientist’s presentation, smallmouth bass, largemouth bass and rock bass have grown in numbers in the Chehalis River in recent years, especially in the parts of the river near Lewis County’s Twin Cities — the warmest part of the river. Smallmouth bass and largemouth bass also appear to consume large numbers of Chinook salmon, which are already facing dwindling numbers.
Bass, unlike many native salmon species, thrive in warmer waters. Studies done by the state show their numbers growing rapidly compared to other predator fish in just the last 20 years. Numbers presented to the board last month showed that bass are now the most abundant fish predator in the river compared to the previously dominant predator the northern pikeminnow, which accounted for more than 50% of that population just 20 years ago.
Smallmouth bass and largemouth bass in particular appear to have an outsized impact on Chinook salmon, with the department finding that 15% of the smallmouth bass diet is made up of Chinook salmon.
Winkowski’s presentation showed that 41% of smallmouth bass collected in a previous study had eaten at least one Chinook salmon smolt and some had eaten as many as 17.
About 34% of largemouth bass in the same study had eaten at least one Chinook salmon smolt and some had eaten as many as eight. Rock bass appear to be a smaller threat with just 19% of those caught having eaten a Chinook smolt.
A rough calculation from Winkowski backed up by data from previous studies on the issue showed 1,000 smallmouth bass in a 10-mile stretch of the river could consume anywhere between 35,000 and 75,000 juvenile salmon smolts in a two-month period, possibly reducing the number of reproducing adults by as many as 750 to 1,500. Those numbers do not include how many Chinook might be eaten by largemouth bass or rock bass.
“It’s likely to be a significant impact is sort of the take home where we’re at,” said WDFW Fish Scientist Mara Zimmerman.
According to an earlier presentation from Zimmerman on the status of Chinook salmon in the basin, around 15,000 fish returned to the basin and were able to spawn, or lay eggs, in 2024. The impact is especially significant for a sub-group of Chinook salmon that return to the river in the spring and are seeing increasingly small spawning return numbers, under 2,000 in recent years.
Bass species, which were introduced in the 1800s and 1900s for sport fishing, have long been a cause for concern for WDFW, and the department has stepped up efforts to study their numbers and impacts in the Chehalis River in the recent past. Roughly a year ago, the department radio tagged about 120 smallmouth bass to study where they go in the river.
A previous presentation from Winkowski cited a study from the state that suggested smallmouth bass could eat as much as 35% of juvenile Chinook salmon traveling from their spawning grounds to the ocean.
