City of Seattle, Seattle Public Utilities, Bull Trout Fry Emergence Trapping 2023-current, Cedar River Municipal Watershed, King County, WA

Chester Morse Lake is managed for drinking water supply for the City of Seattle by Seattle Public Utilities (SPU). The reservoir was created in 1915 upon the completion of Masonry Dam, which raised the natural level of Cedar Lake from 1,538 feet to normal operational levels between 1,550 and 1,554 feet, with a maximum refill level of 1,565 feet. Construction of the dam removed several miles of potential stream spawning, incubation, and rearing habitat for an adfluvial population of bull trout, a species listed as threatened under the U.S. Endangered Species Act. The reservoir fluctuates widely across operational elevations during fall and winter storms. During reservoir refill in springtime, however, the reservoir will be continuously filled until reaching peak elevation (1,560 -1,565 feet) terminally inundating habitat where bull trout had previously spawned and embryos and alevins continue to develop in the streambed. Embryo incubation and fry emergence timing data are required to improve SPU's understanding of frequency and magnitude of operational impacts to bull trout embryos developing in stream habitats affected by reservoir inundation. This information enables empirically-based estimates for the number of embryos in the streambed vulnerable to impacts of inundation during fall and winter storms, which is an intermittent impact period for embryos; and reservoir refill, which is a terminal impact period for embryos. Between two and six hand-driven redd egg pocket water quality monitoring wells and quarantine fences (to avoid additional spawning and superimposition) were deployed near primary egg pockets the week of redd observation. Temperature and dissolved oxygen data were recorded in wells and weekly logger downloads informed biologists of incubation conditions (temperature and oxygen) and accumulated temperature units (ATU). Upon reaching approximately 500 ATU, fry emergence traps were deployed over quarantined redds and subsequently, cod ends were sampled weekly for fry count distributions to sample fry that had emerged from gravel and swam away from the redd. This sampling will continue indefinitely until all representative habitat types can be sampled adequately.