Salmon are an iconic and valuable species in the Pacific Northwest, but they face many threats from human activities and environmental changes.
One of these threats is a toxic chemical that leaches from car tires and washes into streams and rivers when it rains.
A new study suggests that rain gardens, or specially designed gardens that capture and filter stormwater runoff, could be a simple and effective way to reduce the amount of this chemical entering salmon habitats.
The study was published in Environmental Science & Technology Letters.
How tires kill salmon
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The chemical in question is called 6PPD-quinone, and it is formed when a preservative added to tires, called 6PPD, reacts with ozone in the air, as per Phys.org.
6PPD is used to protect tires from breaking down due to ozone exposure, but it also creates a harmful byproduct that can be washed off the road surface by rain.
6PPD-quinone is highly toxic to coho salmon, a species that spawns in urban streams and rivers.
When exposed to this chemical, coho salmon suffer from rapid death syndrome, a condition that causes them to lose coordination, gasp for air, and die within hours.
Scientists estimate that 6PPD-quinone is responsible for killing up to 90% of adult coho salmon returning to spawn in some urban watersheds.
6PPD-quinone is not only harmful to coho salmon, but also to other fish species, such as rainbow trout and zebrafish.
It may also affect aquatic insects and plants that are part of the food web.
The chemical has been detected in waterways across the U.S. and Canada, as well as in Europe and Asia.
Rain gardens are landscaped areas that collect and filter stormwater runoff from roofs, driveways, parking lots, and roads, as per Eurekalert.
They are designed to mimic natural ecosystems, with layers of soil, gravel, plants, and mulch that slow down, absorb, and cleanse the water before it reaches the storm drain or the nearest water body.
Rain gardens can provide many benefits for urban environments, such as reducing flooding, enhancing biodiversity, beautifying neighborhoods, and improving water quality.
They can also help remove pollutants from runoff, such as metals, nutrients, bacteria, and pesticides.
According to the new study, rain gardens can also significantly reduce the amount of 6PPD-quinone entering salmon habitats.
The researchers conducted a field experiment in Vancouver, Canada, where they pumped water spiked with 6PPD-quinone onto a rain garden for four hours and measured the concentration of the chemical in the water draining from the garden.
They found that only about 2% to 5% of the chemical made it through the rain garden, while about 75% was captured by the soil and plants.
Using a computer model, the researchers predicted that the rain garden would prevent more than 90% of the chemical from directly entering salmon-bearing streams in an average year.
They also suggested that rain gardens could be strategically placed in areas where runoff from large highways flows into salmon habitats.
The researchers concluded that rain gardens are a promising green infrastructure solution for protecting salmon from toxic tire chemicals.
They also recommended further research on how rain gardens affect other pollutants in runoff, how different types of plants influence pollutant removal, and how long rain gardens can retain their effectiveness.
