Fish/Shellfish Research and Management - Fish/Shellfish Research
Date Published: November 1997
Number of Pages: 19
Author(s): Dave Seiler and Lori Kishimoto
Adult sockeye salmon returns to the Lake Washington system have declined from peak runs in excess of 600,000 fish as recently as 1988, to under 100,000 fish in subsequent years. In 1991, a broad-based group comprised of representatives of local governments, the Muckleshoot Indian Tribe, state and federal fisheries agencies, academic institutions, and concerned citizens was formed to address this decline. Resource managers developed a program to investigate the cause(s) of the sockeye decline through research and population monitoring in combination with an artificial production program. Information generated by these efforts will be used to devise a restoration plan for Lake Washington sockeye salmon.
Existing management information indicated that marine survival has averaged 13.1%, varying eight-fold (2.6% to 21.4%) with no apparent decline over the data set, which begins with the 1967 brood. In contrast, however, survival during the freshwater phase has declined in recent years. For the 1985 through 1992 broods, freshwater survival (as indicated by the estimated numbers of presmolts produced per spawner) has averaged only 4.8. This rate is only one third of the average production rate of 14.1 presmolts per spawner for the previous 18 broods (1967- 1984).
The majority of sockeye production involves two freshwater habitats: the stream, where spawning, egg incubation, fry emergence, and migration to the lake occurs; and the lake, where the juveniles rear for one year before emigrating to the ocean as smolts. Measuring survival rates in both of these habitats requires quantifying the numbers of hatchery and naturally-produced sockeye fry entering Lake Washington as well as estimating the population as spawners and as smolts.
Production at the Landsburg Hatchery began with the 1991 brood. This brood, released in 1992, and all subsequent sockeye incubated at this hatchery, have been identified with thermallyinduced otolith-marks (Volk et al. 1990). We developed the trapping gear and methodology to estimate sockeye fry production from the Cedar River in 1992.
During the first three years of this evaluation, we determined that survival of hatchery fry from Landsburg to the trap was very low, often less than 10%. In these three seasons, however, flows during most upriver releases were at or near minimum levels. To avoid this high in-river mortality, beginning in the second year (1993), the majority of the hatchery production was transported and released in the lower river just upstream of Highway I-405. In 1995, we evaluated the effect of flow on survival using ten groups released over a range of flows. Results corroborated the earlier estimates, demonstrating that in-river fry survival is largely a function of flow.
Over the first five brood years of this evaluation we have also determined that the survival from egg deposition to fry emigration is a function of the severity of peak flows in the Cedar River during the time that the eggs are incubating in the gravel. Therefore, over the range of spawning population levels we have thus far evaluated, the numbers of naturally produced fry entering Lake Washington are the product of the survival rate and the number of eggs deposited. In 1996 an estimated 230,000 sockeye spawned in the Cedar River, over twice as many as in any of the previous five years.
This report documents the 1997 Cedar River Sockeye Salmon Fry Production Evaluation. This trapping project estimated the numbers of 1996 brood Cedar River wild and hatchery-produced fry that entered Lake Washington during 1997.