Fish/Shellfish Research and Management - Fish/Shellfish Research
Date Published: December 2001
Number of Pages: 46
Author(s): Dave Seiler, Greg Volkhardt and Lori Kishimoto
Declining adult sockeye salmon returns in the late 1980's and early 1990's prompted the creation of a multi-agency broad-based program to investigate causes for these declines. To determine which portion of the freshwater habitat was having the greatest impact on sockeye survival, a sockeye fry production study 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 1999, the eighth year of this project. Study objectives in 1999 included estimating the season total migration of wild and hatchery sockeye fry into Lake Washington, estimating the survival rate from egg deposition to lake entry, estimating survival of hatchery fry by release group, and estimating the incidence of hatchery fry in the population at lake entry.
As in previous years, a floating inclined-plane screen trap was operated nightly in the lower 1/2- mile of the Cedar River. A portion of the sockeye fry migrating into Lake Washington were captured in this trap. To estimate the capture efficiency, over the season 54 groups of 1,266 to 3,175 dye-marked fry were released upstream of the trap. The recapture rates were correlated with flow to develop a relationship that would predict nightly capture efficiency. Dredging in the lower Cedar River in 1998 substantially changed the channel morphology which reduced both the capture efficiency and the strength of the flow-based capture efficiency relationship. A number of trap position adjustments were made to improve capture efficiency. Nightly migration was estimated by dividing the nightly catch by either the flow-based estimates of capture efficiency or the mean capture efficiency rate depending on the position of the trap.
Over the season, 9.6-million hatchery sockeye fry were released into the Cedar River from three locations. To enable separation of hatchery and wild fry in the catch, thermal marks were intentionally placed on the otoliths (bony structures in the inner ear of fish) of hatchery fry by manipulating water temperatures in the hatchery. On the nights of and following hatchery releases, fry caught in the trap were sampled for thermal marks to determine the proportion of hatchery fish present.
Over the 126 night trapping period, nearly 757,000 sockeye fry were captured in the traps. From this catch and the estimated capture efficiencies, we estimated a total of 18.5-million wild and hatchery sockeye fry entered Lake Washington in 1999. This estimate was based on two assumptions; 1) it assumed that daytime migration (when the trap was not operated) accounted for 10% of the total daily migration (estimated from daytime trapping in 1998), and 2) that an estimated 73,000 sockeye fry migrated between an assumed January 1 migration starting date and the date that trapping began (January 23).
Out of the total estimate of 18.5-million sockeye fry that entered Lake Washington, we estimated that 9.5-million were wild fry. Of the 9.6-million hatchery fry released upstream of the trap, we estimated that 9.0-million (94%) survived to enter the lake.
Migration timing for wild sockeye fry was about average compared to the other seven years studied. In 1998, we determined that median migration timing for wild sockeye fry was found to be correlated with February stream temperatures. The median stream temperature in 1999 was 6.7C, also about average for the eight years evaluated so far.
The survival rate from egg deposition to lake entry for wild sockeye fry was estimated by dividing the wild fry migration estimate by the potential egg deposition (PED). PED was estimated from the 1998 sockeye escapement estimate by assuming and even sex ratio and an average fecundity of 3,176 eggs per female (estimated from hatchery spawners). The escapement estimate of 50,000 adults yielded a PED of 79.4-million eggs. The resulting egg-to-migrant survival rate was estimated to be 11.96%, the fourth highest found over the eight years studied. Over this period, egg-to-migrant survival has been negatively correlated with the highest daily average winter streamflow occurring during egg incubation. Survivals were found to be lowest (1.91%) for the 1995 brood which experienced a peak flow of 7,310-cfs, and highest for the 1992 brood (15.62%) where peak flows only reached 1,570-cfs. Peak flows affecting the 1998 brood were estimated at 2,720-cfs.