[env-trinity] CBB: Research Models Show Coho Salmon At High Risk In Urbanizing Watersheds

[Sari Sommarstrom]

THE COLUMBIA BASIN BULLETIN: Weekly Fish and Wildlife News
www.cbbulletin.com
July 29, 2011   -- Issue No. 584

Research Models Show Coho Salmon At High Risk In Urbanizing Watersheds

For a decade researchers in Seattle have worked to solve the mystery 
of why adult coho salmon are dying prematurely in urban streams when 
they return from the ocean to mate and spawn.

In a study published in Integrated Environmental Assessment and 
Management researchers used models to estimate the potential impact 
of urban land development on the salmon population in the decades ahead.

Stricken coho salmon show a syndrome of disorientation, equilibrium 
loss, and other symptoms of acute toxicity, which usually cause death 
within a few hours. In some watersheds as many as 90 percent of the 
returning fish are killed, yet juvenile coho and related salmon 
species remain unaffected, even in the same streams.

"Forensic evidence currently points to toxic chemical contaminants in 
urban storm water runoff as the likely cause of the recurrent fish 
kills," said Julann Spromberg, from the National Marine Fisheries 
Service. "However, urban runoff is a complex soup containing mixtures 
of many different chemicals and we've yet to identify the smoking 
gun. With the forensic story still unfolding, this study looked at 
how wild coho populations might be affected by an increase in the 
geographic extent and severity of the spawner mortality syndrome over time."

Spromberg and her colleague Nat Scholz focused their study on lowland 
urban streams around Puget Sound. As with the rest of the Pacific 
Northwest this area of lowland watersheds is undergoing substantial 
human population growth. By 2025, the population of the Puget Sound 
region is expected to rise from 4 million to 5.4 million people.

"Currently there are many relatively healthy river systems that 
provide high quality freshwater habitat for wild coho in this area" 
said Spromberg. "However this may change if these watersheds acquire 
the urban land use characteristics that typify the Seattle-area urban 
drainages where coho spawners are currently dying in high proportions."

The authors constructed a virtual coho population, with conventional 
rates of survival and straying based on the published literature. 
They then imposed spawner mortality on different segments of the 
population, based on actual rates observed from spawning surveys of 
urban creeks in recent years which allowed them to estimate timelines 
for local population extinction.

The models showed that the proportion of the population affected was 
a more important factor that the overall rate of mortality. One 
scenario that simulated rapid urbanization forecast an increase in 
pre-spawn mortality from 0 to 75 percent over 20 years.

When rates were increased to 75 percent over 20 years, the model 
predicted the extinction of a solitary coho population in 30 years. 
Lastly, the modeling effort revealed how production from high quality 
habitat areas within large river basins might mask local declines in 
coho abundance due to spawner losses.

"While our findings are for virtual populations, our models capture 
the basic dynamics of how wild coho may respond to the mortality 
syndrome at a population scale. This will help natural resource 
managers anticipate future threats to coho conservation and recovery 
in Puget Sound and elsewhere in the Pacific Northwest," concluded 
Spromberg. "Our next steps include incorporating storm water impacts 
on embryo and juvenile survival rates as well as refining the models 
by expanding the range of input data to reflect different types of 
future land uses."

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