2000 Cedar River Sockeye Salmon Fry Production Evaluation Annual Report

Declining adult sockeye salmon returns to Lake Washington in the late 1980's and early 1990's prompted the creation of a multi-agency effort to investigate causes for this decline. To determine in which life-stage sockeye were experiencing lower survival, an evaluation of fry production was undertaken in the Cedar River beginning in 1992. Assessing the sockeye population at this location and life-stage separates freshwater production into river and lake components. This report documents our investigations during 2000, the ninth year of this project. As in previous years, the primary study goal was to estimate the season total migration of Cedar River wild and hatchery sockeye fry into Lake Washington. These estimates enable calculating the survival rate from egg deposition to lake entry, the survival of hatchery fry by release group, and the incidence of hatchery fry in the population at lake entry.

A floating inclined-plane screen trap was operated on 89 nights from January 20 through April 26 in the Cedar River at River Mile 0.9. This trap captured a portion of the sockeye fry migrating into Lake Washington. To estimate its capture efficiency, on 43 nights over the season, dye-marked fry were released upstream of the trap. Unlike previous years, regression analysis failed to show a relationship between flow and capture efficiency. Instead, capture rates declined over the season. We determined that significant predation in the half mile reach above the fry trap increasingly biased capture rates. Consequently, we estimated nightly migration with the capture rate derived early in the season while predation rates were low.

In total, 3.2 million hatchery produced sockeye fry were released into the Cedar River from February through March. Fry were released from two locations over 16 nights. All of the hatchery fry were internally marked via an innovative process involving deliberate variation of water temperatures during incubation to induce recognizable growth patterns in otoliths. Nightly hatchery fry migrations were estimated through a combination of analyzing otolith samples and through subtracting interpolated wild migrations from nightly total fry estimates.

Over the 89 nights that were trapped, 755,230 sockeye fry were captured. From this catch and the capture rate data, we estimated a total of 10.1 million sockeye fry entered Lake Washington in 2000. This production includes 8.1 million wild fry and 2.0 million hatchery produced fry. Relating this latter estimate to the 3.2 million hatchery fry released estimates that 63% survived to enter the lake. In-river survival of hatchery produced sockeye fry was related to migration distance. Fry released at river mile 13.9 and 1.9 survived to the trap at average rates of 48% and 77%, respectively.

Egg to migrant survival for the 1999 brood natural spawning sockeye was estimated at 9.5%. This rate is higher than that predicted by the relationship between peak flow and estimated egg to migrant survival. During incubation, on December 18, 1999, flow in the Cedar River peaked at 2,680 cfs. At this flow, the relationship derived with nine years of fry production evaluation and incorporating higher estimates of egg deposition, predicts an egg to migrant survival rate of 6.7%.