SWIMMING UPSTREAM: COHO SALMON ARE HEADING TOWARD EXTINCTION IN URBAN STREAMS—AND IT’S BECAUSE OF STORMWATER
Following urban watershed restoration efforts throughout the Puget Sound Region in the 1990s, ecologists and biologists were surprised to realize that restoration efforts had little positive effect on coho populations. In fact, adult female coho returning to urban streams and rivers in Puget Sound exhibit pre-spawn mortality ranging from 60 to 100%. Scientists have been able to rule out many factors, including temperature, dissolved oxygen, and disease; stormwater runoff appears to be the primary culprit. In fact, studies have demonstrated that exposure to urban stormwater runoff is sufficient to cause pre-spawn mortality within 3 hours. That’s horrific—and it’s particularly troubling considering that female coho return to freshwater streams in the fall. And in the Pacific Northwest, rains start in earnest in October. The stormwater runoff generated during that first flush contains the months of buildup from roads, parking lots, and roofs, and it flows right into salmon habitat.
In cities across the world, upwards of 10,000 individual compounds deposited on impervious surfaces—which are only increasing as the world becomes more developed—is transported to our streams via stormwater runoff and is harming salmon. Coho salmon seem to be the most impacted, both at the pre-spawn stage through embryonic stages. Unfortunately, these are the stages dependent upon freshwater streams and lakes, which see the most intense concentrations of stormwater runoff.
On the other hand, streams and rivers with limited, if any, impact from human interference continue to see robust populations of salmon, which suggest that high mortality rates are limited to urban areas and freshwater systems; the adult/maturity stages of salmon spent in the ocean seem to be doing just fine.
How do we help those fish who have no choice but to return to toxic waters? It may be easier than it sounds.
Identifying those outfalls that are the worst offenders and move upstream. In other words, implement source control. We can do this by deploying sensors that monitor the volume and quality of stormwater leaving the stormwater system and entering the stream.
Amazingly, simply filtering the polluted runoff through a bioretention system works swimmingly (pun intended)! Anything from tree wells designed to capture runoff, to rain gardens and swales constructed along roads and parking lots.
Installing pervious asphalts and concrete; Washington State University’s Stormwater Center in Puyallup, Washington, has conducted an incredible number of experiments testing the design, price, and efficacy of such systems. With the cost of maintenance and repair associated with traditional surfaces, impervious surfaces are becoming much more cost-effective.
And it could be as simple as implementing better maintenance of our right-of-ways and our stormwater infrastructure. Some major cities use high-efficiency sweepers to help collect pollutants before they drain to waterways, which can collect up to 90% of metal toxins like copper and zinc. Even standard street sweepers operated on a regular schedule significantly reduce the volume of contaminants, and annual (if not more frequent) catch basin cleaning does the same.
Granted, the costs to implement these solutions certainly add up over time. But using innovative technology to prioritize outfalls and problem areas offers a chance to solve, oh, let’s say 80% of the problem with 20% of the effort and cost. When you look at it that way, it’s definitely worth a try!