1997 Sammamish River Sockeye Salmon Fry Production Evaluation Annual Report

The numbers of adult sockeye salmon returning to the Lake Washington system are estimated as they pass the Ballard Locks, and as spawners in the Cedar River, primary tributaries to the Sammamish system, and on certain beaches. The majority of the spawning has occurred in the Cedar River, but in three recent years (1992, 1994, and 1996), biologists have estimated that a quarter to a third of the Lake Washington Basin sockeye have spawned in the Sammamish River Basin (Egan and Ames WDFW memo, 1997) . Over the other twelve of the last fifteen years for which escapement estimates are available for all areas, the Cedar River accounts for an average of 88% of the total spawners (range = 82% to 98%). This interannual variation may result from differential survival as a function of spawning and emergence timing relative to stream specific hydrology. In addition to run timing differences, recent electrophoretic analysis indicates that the sockeye which spawn in the Sammamish system are genetically distinct from the larger Cedar River population.

In 1992, as part of a multi-agency effort to determine the cause(s) of the decline in the Lake Washington sockeye run, we began enumerating sockeye fry production from the Cedar River. Measuring the population at this lifestage and location separates freshwater survival into its two major components; spawning which takes place in the river and rearing which occurs in the lake. Over the past six broods, natural spawners in the Cedar River have produced fry populations to the lake of 0.7 to 27.1 million. We have determined that the severity of peak flows is the primary factor controlling survival from spawning to fry emigration in this system. Annual estimates of the numbers of fry entering the lake are also needed to understand the complex ecological relationships which regulate juvenile sockeye survival during their year in the lake. Because the Sammamish system may account for a significant portion of the fry entering Lake Washington in some years, an estimate of this production component is also needed to understand the dynamics of the combined population.