Smallmouth Bass Numbers Are Up in a Warming Washington River. Read the Fine Print.

Start with the limits, because they decide how much weight this study can carry. The authors compared fish surveys in Washington's Chehalis River against a single baseline year, 2007. They say themselves that the differences they saw could reflect short-term swings in abundance or in how easy the fish were to catch. With that on the table, here is what the study found about smallmouth bass in a river that has warmed and has run below average flow.

What the researchers looked at

The Chehalis is a large, free-flowing, salmon-bearing river in Washington. Smallmouth bass are nonnative there, and the authors say expanding bass habitat fuels concern for imperiled Pacific salmon because of known predation impacts.

The team did three things. They reconstructed a 35-year summer stream temperature record (1990 to 2025) and estimated how much of the river was thermally suitable for smallmouth bass, using an occurrence threshold from a distribution model. They compared juvenile bass growth across recent brood years and checked it against temperature and flow. And they compared electrofishing catch per unit effort, a measure of how many fish turn up per unit of sampling, in 2023 and 2025 against the 2007 survey.

What they found

Over those 35 years, August stream temperature rose 0.23°C per decade. Thermally suitable smallmouth habitat increased at a rate of 0.8% per decade, which the authors put at about 28.4 km.

Flow was below average over the past decade. Young bass growth was generally higher in the drought and heatwave years of 2015 and 2021, consistent with a negative relationship between growth and discharge. In plain terms, lower flow went along with faster growth. That is an association. Stream temperature itself did not show a significant relationship with growth.

The fish counts moved too. Smallmouth bass and rock bass catch per unit effort were significantly higher in the recent surveys than in 2007, while catch of northern pikeminnow was lower. The authors conclude that recent changes are "broadly favoring nonnative warmwater predators."

The fine print

This is where the headline gets complicated.

The fish comparison leans on one baseline year, as noted up top. The habitat estimate depends on a 19°C occupancy threshold taken from a distribution model, and the authors note that favorable rearing temperatures for smallmouth can approach 27°C, and that summer temperatures in the main-stem Chehalis are still below the 19°C threshold. They also point out that the pikeminnow decline is itself a complication: pikeminnow are an important predator of young salmon too, so the net change in predation on salmon is hard to read from these data.

Rock bass show up in the fish counts, but their response to warming was not measured. The authors say their similar thermal preferences suggest they may respond similarly.

The authors say similar dynamics are likely more widespread because similar hydroclimatic trends are occurring across many western U.S. watersheds. That is their inference, and this is one river and one study, not a settled picture of the whole West.

What this means for smallmouth anglers in warming rivers

This section is my read, not the study's.

I have caught plenty of smallmouth in the UP, and I have never fished the Chehalis or targeted salmon in Washington, so I can't tell you what this looks like from a boat there. What I can say is how I'd use it. If you fish a western river where smallmouth are established, this study is a reason to keep an eye on how those fish do in warm, low-water years. If you are a salmon angler, it is a reason to pay attention to what your state agency says about nonnative bass, since the authors frame the worry as predation on salmon.

The catch numbers here come from the researchers' own electrofishing surveys, not from anglers. Treat this as background on one river's fish community, not a fishing report.

[INTERNAL LINK: smallmouth bass fishing basics]

Citation

Winkowski, J. J., et al. (2026). Climate-driven shifts in stream temperature and discharge favor nonnative bass in a salmon-bearing watershed. North American Journal of Fisheries Management. https://doi.org/10.1093/najfmt/vqag082