Stillwater River bounty

We’re about a month away from the first mayfly hatch, and it’s getting to be time for a fisherman’s thoughts to turn to casting a fly on one of the magnificent wild trout streams along the Beartooth Front.

It’s something we take for granted, but a recent study by a team of researchers from the US Geological Survey (USGS), including two based in Montana, found that changes in climate directly and consistently influence trout populations worldwide.

In the study, titled “Impacts of Climatic Variation on Trout: A Global Synthesis and Path Forward,” lead author Ryan Kovach and his colleagues provide the first global literature review of trout responses to climate change over time.

Ryan Kovach

Key patterns emerged from the existing data. In particular, lower summer streamflows were associated with reduced growth, survival, and abundance across trout species, locations, age classes, and even continents. As global warming brings diminished winter snowpack, lower summer streamflows and increased temperatures, the impact on local trout populations will be significant.

Measuring surface water
Summer streamflow is a matter of significant concern along the Beartooth Front.

The Surface Water Supply Index (SWSI) is a complex measure that incorporates both hydrological and climatological features into a single index value for river basins. It includes snowpack, streamflow, precipitation, and reservoir storage into a single number centered on zero, with a range between -4.2 and +4.2. The map below, from the state of Montana web site last summer, depicts the SWSI for all river basins in the state as of August 1, 2015.

As you can see, both the Stillwater and Clarks Fork Yellowstone basins are extremely dry, with SWSI indexes of -3.8 and -3.6 respectively. (To see this, click on map to enlarge, and look at legend on left. Stillwater is 44, Clarks Fork Yellowstone is 46.)

Prospects for next summer are not particularly good, as current snowpack is about 90% of normal. Today, at the peak of winter snowpack, the SWSI for the Stillwater is -1.7, and for the Clarks Fork Yellowstone -1.2.

Continued low streamflows and increased temperature due to climate change will have a sustained impact on the viability of trout populations in our rivers.

Natural resource management is critical
“The key message is that climate does have a strong influence on trout populations, and this is something we have observed over time,” said Kovach in a recent interview with the Flathead Beacon. “We’ve seen that year-to-year variations in streamflow and temperature can have an impact on trout populations. This highlights that climate and climate change is not speculative, it is realized. It is something that we need to confront now in terms of natural resource management, but also from a societal perspective.”

With regard to climate change, there are many things we can’t control and some we can. Our rivers are going to be lower and water temperatures will be warmer over time. This will inevitably impact the rainbows, browns and cutthroats in our rivers. It is essential that we do everything we can to protect those waters from negative outside influences, including mineral extraction.

You can read the USGS study by clicking here.