1998 Sammamish River Sockeye Salmon Fry Production Evaluation Annual Report

As part of the Lake Washington Studies, a multi-agency effort to investigate the recent decline in sockeye salmon abundance within this system, we began assessing fry production in the Cedar River in 1992. Because in some years as much as one third of adult sockeye spawning has occurred in Sammamish Basin tributaries, we also initiated fry monitoring in the Sammamish River in 1997. In this first year of the Sammamish study, we estimated that 953,000 sockeye fry were produced from the 60,000 adults that spawned in the fall of 1996. We attributed the very low egg-to-fry survival rate of just 1% to the record high flows (2,830 cfs) produced by a severe rain-on-snow event in early January 1997, which immediately followed a large ice storm. This report documents results from 1998, the second year that we evaluated production of sockeye salmon fry from the Sammamish Basin.

As in the previous year, we placed a barge that contained two inclined plane screen traps in the Sammamish River near Bothell (R.M. 4.0). From January 31 to April 25 1998, we operated the traps on 61 nights. Early and late in the season, when the migration was low, we trapped every other night. On 33 nights, we estimated capture rate via releasing groups of dye-marked fry upstream of the trap. Over the season, trap efficiency averaged 10.6 % and, as in 1997, was negatively correlated with flow. Expanding catches with the season average capture rate, and interpolating for the nights not fished, produced a season total estimate of 1,243,000 sockeye fry.

Relating this migration to the estimated deposition of 12 million eggs during fall 1997 yields an egg-to-fry survival rate of 10%. We attribute this rate, which is 10 times higher than we estimated for the previous brood, to the more moderate flows during incubation. While other previous years have had much lower peak flows, the peak flow of 1,060 cfs on January 24, 1998 was less than half of the record high flow registered on January 2, 1997 of 2,830 cfs in the Sammamish River at Bothell. Therefore, although we have measured production from just two broods in this system, given the extreme range in flows and the tenfold difference in survival rate, it appears that egg-to-migrant fry survival of sockeye in the Sammamish system follows a similar negative relationship with flow as we have developed in the Cedar River.