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Portland Study Shows Urban Watershed Can Support Healthy Population Of Native Fish
Posted on Friday, March 02, 2018 (PST)

A watershed in the south Portland suburbs was found to have a healthy and diverse population of native coastal cutthroat trout equal to cutthroats in a pristine mountain stream, according to a recent study.


The study found that native coastal cutthroat in Tryon Creek, a tributary of the Willamette River downstream of Willamette Falls and upstream of Portland, Ore. are represented by all age classes, that they migrate in and out of local streams, are genetically diverse, are free from disease, and that these characteristics of the Tryon Creek trout population were similar to non-urban populations of the trout in the Mt. Hood National Forest.


Tryon Creek is one of the largest urban watersheds in Portland, Oregon, a city of more than 650,000 people, where developed land accounts for 55.6 percent of total land use, the study says. The stream itself is located in the Tryon Creek State Natural Area and has about 43 kilometers (27 miles) of open streambed and 5 kilometers (3 miles) in culverts or pipes


“We found that an urban watershed can support a healthy population of environmentally sensitive, native fish,” said Brook Silver, fish biologist with the U.S. Fish and Wildlife Service and one of the study’s authors. “This example of a relatively healthy fish population in an urbanized landscape demonstrates the value of habitat conservation. As cities continue to grow, it will be important to preserve and maintain habitat that serves to protect sensitive, threatened, and endangered species.”


The study says that the effect of urbanization is often called urban stream syndrome with symptoms such as altered flow dynamics, passage barriers, poor water quality and degraded food webs, as well as an urban condition known as the heat island effect. After protections were put in place and habitat improvements were made in and around Tryon Creek, however, the stream is less prone to these symptoms than other urban watersheds.


“An urban stream can support a healthy population of coastal cutthroat trout” was published in the journal Urban Ecosystems.


Silver’s co-authors are Michael Hudson, fish biologist, Christian Smith, geneticist, Kenneth Lujon, fish biologist, and Timothy Whitesel, fish biologist, all with the Service; and Melissa Brown, environmental specialist with the City of Portland.


Native fish species found in the creek are cutthroat and, possibly, resident rainbow trout, the study says. Anecdotal evidence is that steelhead once spawned in the creek and that juvenile coho and chinook salmon are using the lower area of the creek near the Willamette River for temporary rearing.


“Of these species, Coastal Cutthroat Trout appear to be the most likely self-sustaining natural population of fish in the stream,” the study says.


The study was done after completing a number of fish habitat improvements. Some of those improvements by the City of Portland protect the sometimes steep-banked creek from erosion, others improved the floodplain fill, hardened creek banks and created shallow water refuges with large wood and boulder installations, as well as projects that replanted riparian areas, Silver said.


In 2008, the Oregon Department of Transportation retrofitted the culvert under Oregon State Highway 43 with a new baffle system to improve fish passage and provided habitat restoration. In 2010, the city improved floodplain connectivity below the Highway 43 culvert.


“Making large connected habitats a priority for preservation and improvement may be essential to protecting sensitive species,” Silver said. “In growing urban areas, it will be important to integrate the conservation of healthy stream habitats with watershed planning.”


The study was done to monitor the effectiveness of these habitat improvements made by the Service and City of Portland’s Bureau of Environmental Services.


“As part of our monitoring, we surveyed the Coastal Cutthroat Trout population above the Highway 43 culvert,” Silver said. “We wanted to see if the population health of Tryon Creek’s Coastal Cutthroat Trout was similar to that of populations of Coastal Cutthroat Trout located in relatively pristine ecosystems.”


Rehabilitation of essential habitat in developed urban watersheds, such as Portland’s, can theoretically improve fish abundance and diversity, the study concludes.


“In the case of Tryon Creek, it may be particularly important that rehabilitation efforts and habitat quality are relatively high in the lower portion of the watershed,” it says. “This helps promote the ability for cutthroat trout to have connectivity with ecosystems outside of the watershed.”


Areas where large connected spaces are preserved and maintained, such as the Tryon Creek State Natural Area, may be fundamental to protecting sensitive, threatened or endangered species in urban habitats, the study says.


“We realize not all urban watersheds have the same potential to support healthy fish populations, but it is possible where opportunities exist,” Silver said. “As cities grow, assets like fish and water quality often get overlooked.  Planning smart, conservation-minded development will be important to minimize impacts to natural resources and protect what remains.”


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