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Bull Trout

Bull trout are listed as threatened under the Endangered Species Act.

Overview

Video of a female and male bull trout spawning in the Cedar River upstream of Chester Morse Lake.

Bull trout (Salvelinus confluentus), although related to trout (such as rainbow trout, Oncorhyncus mykiss), are actually a species of char, along with Dolly Varden (Salvelinus malma) and brook trout (Salvelinus fontinalis).

Bull trout in the Cedar River Watershed have been isolated from other char populations in the Puget Sound basin by Cedar Falls, a natural fish barrier, since the last glacial period over 13,000 years ago. Most bull trout in Chester Morse Lake live their adult life in the lake and spawn in the streams flowing into the lake. This is known as an adfluvial life history.

Some bull trout in the watershed may also have a resident life history, living their life entirely in a riverine habitat.

Eggs & Fry

Bull trout lay eggs in several river systems in the watershed, including the Cedar and Rex rivers and Rack, Boulder, and Cabin creeks.

They create redds (nests that bull trout build in the gravel where they lay their eggs) in close proximity to the reservoir in low gradient (<1%) stream habitat. Eggs develop in the gravels until young fry emerge, which has been documented in the Cedar River Watershed as beginning the last week of February.

Some fry reside in the streams for several years before moving to the reservoir, while others move directly to the reservoir soon after emerging.

Bull trout in Chester Morse Lake feed primarily on pygmy whitefish but also forage on sculpin, leeches, rainbow trout, younger bull trout and many species of invertebrates. Their diet likely is influenced by seasonal fluctuations in food availability.

Spawning

Bull trout begin spawning in tributary streams of Chester Morse Lake the last week of September and continue spawning into December and sometimes January. See maps of Bull Trout Spawning (pdf) and Rearing (pdf).

Bulltrout Spawning

See photos of the bull trout lifecycle.

The number of redds recorded annually by City biologists from 2000-2010 has varied from 236 to 587 redds per year. Although the number of redds fluctuates, the timing of when redds are found is consistent from year to year.

The peak of spawning activity usually happens during the last week of October through the first week of November. During this peak spawning time, scientists occasionally see fish in the river spawning.

Threats

Bull trout redds are susceptible to scour from high stream flows, and much of a year’s egg production may be lost in a big flood like the one that occurred during November 2006.

Some bull trout in the Chester Morse Lake population spawn in floodplain channels fed by groundwater, which are not as susceptible to scour. The high streamflows occurring in the mainstream rivers do not occur in these channels.

The water level in Chester Morse Lake typically goes up in late winter and spring, when the City stores water in the reservoir for its summer and fall water supply. When the lake rises and inundates the river channel in the lake delta, bull trout redds in the delta channels are vulnerable because they are no longer being flushed with oxygen-rich stream flow and may become covered with sediment.

Studies

The City is conducting several studies to better understand the ecology of this population of bull trout and the effects of reservoir management.

  • Bull Trout and Rainbow Trout Juvenile Movement Study: Scientists are following movements of young bull trout and rainbow trout using PIT tags. A PIT (passive integrated transponder) tag is about the size of a grain of rice and implanted in an individual fish. Each tag has a unique tag number that identifies the fish. As tagged fish are recaptured or move over stationary detection sites, we find out when and where they move.
  • Acoustic Telemetry Study of Bull Trout/Rainbow Trout/Pygmy Whitefish Behavior and Ecology: City biologists are using acoustic telemetry to follow movements of adult bull trout, rainbow trout, and pygmy whitefish in Chester Morse Lake and major tributary streams. Transmitters are surgically implanted in fish and monitored using an array of hydrophones that detect the acoustic signals.
  • Bull Trout Redd Inundation Study: Scientists are investigating the impacts of reservoir management on bull trout egg and fry survival. As lake levels rise, a portion of natural redd sites are flooded by rising water and a fine layer of sediment is deposited over the redd site. This study monitors artificially installed redds located in sites that vary in duration of inundation. Results will provide a basis to evaluate inundation effects on egg survival.

For more information contact dwayne.paige@seattle.gov.

Reports and Maps

Related Links

Other fish species in the watershed: