The Morning Commute: Killing More than Just Your Mood

Daniel Karp
Nov 26, 2019 · 4 min read

By Krista Schmidt

Have you ever considered the cost of traffic? As the human population grows, so do cities, which means more roads and more traffic. There are immediate costs of traffic, such as lost time, increased stress and anger, and reduced fuel efficiency. But what other impacts does traffic have on the world around us? A recent study shows that heavily trafficked roadways are the main culprit of increased Coho salmon die-off in the Puget Sound region in Washington state.

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Figure 1: Rush hour on Seattle’s Evergreen Point Floating Bridge. Source:

Why are Coho salmon important?

Coho salmon are an important species in the Puget Sound region for multiple reasons. They are vital to the traditional practices and beliefs of indigenous peoples. They help support a thriving recreational and commercial fishing industry. They also have a large ecological impact.

Salmon are born in inland rivers, then migrate out to sea. When they are adults, they return to these rivers to deposit their eggs and die. By doing so, they transport marine nutrients inland, which are then cycled through the ecosystem, increasing plant growth and providing nutrients to predators.

However, these inland rivers flow through some of the largest metropolitan areas in the Pacific Northwest. Furthermore, the small streams that salmon prefer to lay their eggs in are highly vulnerable to urbanization and changes in land use. In the Puget Sound region, studies have shown high mortality rates for coho salmon in urban streams for over a decade. If salmon keep dying at such high rates, they will likely suffer a population crash.

We know that salmon are dying, but what is causing it? Scientists have tried to identify which human activities are causing salmon death.

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Figure 2: Coho salmon in the White River near Mount Rainier Source:

Why are Coho dying?

Common contributors to fish die-offs include stream temperature, disease, or amount of oxygen in the water; however, these factors were not found to play a significant role in the coho die-off in the Puget Sound region. Instead, toxic stormwater runoff is the most likely driver in coho decline.

To test this, a study used computer modeling to see if changes in landscape attributes that impact stormwater runoff led to differing rates of salmon mortality. These attributes included precipitation, human population density, and road presence and density. To determine mortality rates, carcasses of females that had not yet laid their eggs were collected. Since death after laying eggs is normal, a dead female that retained her eggs would imply an unnatural death.

Overall, salmon mortality rate was higher in more urban areas. Furthermore, mortality increased as road density and traffic density increased. This means that the density of roads impacts mortality more than the overall land area taken up by roads.

When cities grow larger, the density of large roadways like highways and freeways increases disproportionately to the density of small, local roads. Moreover, heavily used roads like highways and freeways have a larger impact on salmon mortality due to the increased amount of contaminants deposited by vehicles. This means that the contaminants that are toxic to salmon most likely come from our vehicles rather than asphalt or pavement.

Roadway contaminants are able to enter rivers in runoff from precipitation events. But does the timing of precipitation events play a major role in the amount of toxic runoff entering the rivers? Surprisingly, seasonal rainfall only impacted mortality in the least developed areas. This is because contaminants on lightly trafficked roads take longer to accumulate, so a drought would allow more toxins to amass before being washed into a stream. These events of drought followed by heavy rain increase the concentration of contaminants in the water, thereby increasing salmon mortality.

Even though variable precipitation has a negligible effect on mortality in urban areas, this does not mean those areas are less contaminated. Toxicants on heavily trafficked roads accumulate much faster, meaning that the length of a drought before a storm event does not affect the level of toxicants entering a waterway. No matter what, large amounts of pollution from these roads enter the water.

So what can we do?

Unfortunately, this problem will only get worse if we do not act. Climate change is expected to reduce the snowpack that feeds these freshwater streams. This will increase the concentration of contaminants in these waterways, which will only exacerbate the coho salmon decline.

While heavily trafficked roads are shown to be the largest contributors of contaminants, we cannot simply stop building or using them. The pressures of human population growth do not allow for that. So how can we stop vehicular contaminants from entering our freshwater streams?

Clean water initiatives will need to be implemented. One strategy is green stormwater infrastructure, or GSI. A GSI that has proven to be successful is soil columns. These simple and inexpensive units are placed near the stream to filter chemical contaminants before they can reach the water.

Scientists have identified the locations that are most vulnerable to contamination, and thus coho die-off, but it is up to the people to implement the necessary initiatives to save this species in the Puget Sound region.


Feist, B. E., Buhle, E. R., Baldwin, D. H., Spromberg, J. A., Damm, S. E., Davis, J. W. and Scholz, N. L. (2017), Roads to ruin: conservation threats to a sentinel species across an urban gradient. Ecol Appl, 27: 2382–2396.

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