Spromberg, J. A., Baldwin, D. H., Damm, S. E., McIntyre, J. K., Huff, M., Sloan, C. A., … & Scholz, N. L. (2015). Coho salmon spawner mortality in western US urban watersheds: bioinfiltration prevents lethal storm water impacts. Journal of Applied Ecology. DOI: 10.1111/1365-2664.12534 (Open Access)
Coho salmon (Oncorhynchus kisutch) is one of the six species of (delicious) Pacific salmon (Figure 1). Adult Pacific salmon are famous for returning to the freshwater stream of their birth to spawn once they reach sexual maturity. Coho salmon spawn in streams from central California through Canada and up to Alaska. Adults begin their migrations from the Pacific Ocean into freshwater in the fall, coinciding with increased seasonal rainfall. The increased rainfall means less of the salmons’ habitats will be dried up, but it also means increased nonpoint source pollution (more on this in a moment). Populations of Pacific salmon as a whole have been declining for several decades, Coho salmon in California, Washington, and Oregon have fallen low enough to earn a listing as “threatened” under the Endangered Species Act. One factor in their population decline is a high rate of mortality of adults prior to spawning in some urban streams.
We are all familiar with the idea of a pipe directly pumping toxic waste into the water, but a lot of pollution in marine and aquatic ecosystems comes from less obvious sources. Nonpoint source pollution describes all pollution without a clear entry point into the aquatic environment. Common nonpoint source pollution includes runoff from agricultural land (pesticides, fertilizers) or urban runoff caused by water running across roads, parking lots, and building and collecting all the grit and grime as it goes before it makes its way to a body of water. Nonpoint source pollution has been shown to cause reductions in biodiversity and species abundance in many aquatic habitats, but is rarely tied to sudden fish kills. This study directly links toxins from highway runoff to premature death in Coho salmon. But there is good news: there may be a relatively simple way to address this problem.
The researchers in this study set out to test if pollutants in Coho salmon spawning streams derived from rain water runoff were responsible for killing adult Cohos before they are able to spawn. Previous field studies have observed over 50% of adult fish dying in California, Washington, and Oregon before successfully spawning. However, nonpoint source pollution toxins have not yet been identified as the cause.
To test this hypothesis the researchers exposed small groups of four salmon to contaminants commonly found in urban rain water runoff. The researchers did this with three main experimental groups. First, they collected runoff water directly from the downspouts of an urban four-lane highway in Seattle, Washington. This water contained polycyclic aromatic hydrocarbons (PAHs) commonly found in oils and gasoline, as well as heavy metals such as lead, zinc, cadmium, copper and nickel. The second experimental group consisted of artificially polluted water. For this test, the researchers made contaminated water with combinations of stock solutions of PAHs and heavy metals to match and even slightly exceed the concentrations found in the rain water collected. This was done to determine if PAHs and heavy metals in rain water runoff were directly responsible for fish mortality or if something else in the runoff was potentially the cause. The third experimental group consisted of the rain water runoff collected from the same highway that was filtered through a simple soil filter before administering to the fish. This was done to test if simple soil filters can sufficiently reduce contaminants to prevent fish mortality. Lastly, control groups of fish were exposed to the normal water at the lab to determine if the lab environment or handling stress caused any mortality in the fish.
The findings from this study were quite striking. Every fish exposed to the polluted highway runoff water died within 24 hours of exposure (many within only 4 hours), but none of the fish in the control groups died during the experiments.
Surprisingly, fish exposed to the artificially polluted water did not suffer mortality, even when pollutant concentrations were greater than the highway runoff water. This suggests that the PAHs and heavy metals alone in the complex mixture of water collected from the highway are not responsible for causing death in the salmon and the causal agent has not yet been identified. But wait, there is good news… Although highway runoff water resulted in the deaths of all salmon exposed to it, simply running this same water through a soil filter before exposing the salmon to it resulted in 100% survival! This relatively simple, cheap, and natural filter was successful at reducing the concentrations of toxins in the urban runoff water (Figure 2).
This study was an important step towards understanding why adult Coho salmon have been dying prior to spawning in California, Washington, and Oregon. The current state of more than 50% of fish dying before spawning puts these populations at serious risk of extinction. The researchers demonstrated that urban runoff pollution is capable of causing mortality of adult salmon. This points to the need to reduce nonpoint source pollution in the spawning streams of these fish which are surrounded by urban environments. The most encouraging aspect of this study was that a simple soil filtration method was sufficient to prevent mortality. However, this method is in the early stages of development and thus it is not yet clear what size of filter area would be needed to sufficiently reduce contaminants to nonlethal levels. Furthermore, the toxin(s) specifically responsible for causing death have not yet been identified. Nonetheless, we now know for sure that major roadways and other impervious surfaces right alongside waterways is a really bad idea.
Derrick is pursuing a Ph.D. in the Organismic and Evolutionary Biology Program at the University of Massachusetts Amherst. He is interested in anadromous fish migrations, how aquatic organisms interact with their physical environment, and the impact of human development on natural systems.