Impact of Reservoir Elevation during the Spawning Season on the Distribution of Bull Trout Redds

Abstract

AbstractAdfluvial Bull Trout Salvelinus confluentus reside in Chester Morse Lake (CML), a major reservoir in the City of Seattle water supply system on the central west slope of the Cascade mountain range, Washington. Annual surveys over a 12‐year period (2000–2011) were used to investigate the spatial distribution of adfluvial Bull Trout redds in river systems of CML relative to reservoir elevation. Because reservoir elevations were lower in the spawning season than during the incubation period, some redds placed in river reaches near the reservoir were vulnerable to cumulative effects from impending inundation as reservoir levels increased and as fine sediments settled across gravel substrates. Higher average elevation of CML during the peak period of spawning caused significantly lower percentages of Bull Trout redds in lowermost river reaches, where fluctuating water levels could adversely affect incubating eggs (R 2 = 0.73). Annually, the proportion of redds distributed within habitat reaches where inundation, as indicated by mean CML elevation during the incubation period, could affect them, varied from 1% to 18%. However, a greater proportion of annual Bull Trout redds (10–68%) fell within the maximum reservoir elevation during the incubation period. These results suggest that maintaining moderate reservoir elevations during the spawning season and holding the reservoir near this elevation through the incubation period can lower the effect on incubating Bull Trout redds. The implications for water level management to benefit adfluvial Bull Trout derived from this data set could also be applicable in similar systems, especially where knowledge of temporal and spatial distribution of salmonid spawning is available.Received November 13, 2012; accepted June 17, 2013