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2004 Progress Report - Genetic relationships among anadromous and resident Oncorhynchus mykiss in Cedar River, Washington: Implications for steelhead recovery planning

Category: Fish/Shellfish Research and Management - Fish/Shellfish Research

Date Published: June 2004

Number of Pages: 36

Author(s): Anne R. Marshall, Maureen Small, and Steve Foley


We initiated this research to assist development of a steelhead (anadromous Oncorhynchus mykiss) restoration plan for approximately 17.5 miles of previously blocked habitat in Cedar River, a Puget Sound drainage in Washington. Landsburg Dam (RM 21) had blocked anadromous fish passage since 1900. Construction of adult and juvenile fish passage at Landsburg Dam was completed in 2003, and the restored accessibility of upstream habitat was expected to benefit steelhead. Steelhead abundance in this watershed had been critically low during the last 12 years, leaving potentially few fish to naturally re-colonize the opened habitat. Rainbow trout (resident O. mykiss) are present throughout the river, and appeared abundant in below-dam areas. Genetic relationships between anadromous and above- and below-Dam resident O. mykiss were unknown.

Several studies (Docker and Heath 2003; Pearsons et al. 2003; McCusker et al. 2000; Zimmerman and Reeves 2000) have shown that native resident and anadromous O. mykiss within a drainage are closely related, and likely to interbreed at some level. However, non-native (California-origin) hatchery rainbow trout had been released historically in the watershed, especially in Lake Washington. We needed to determine the genetic relationship between Cedar River resident and anadromous O. mykiss to understand what role the trout resource might play in restoration of the anadromous population.

Factors most critical to the success of Cedar River steelhead have not been determined, but several habitat-related and migration behavior research projects are underway. Adult and juvenile steelhead must migrate through Lake Washington and approximately five miles of an engineered waterway through Seattle, which includes a shipping locks (“Ballard Locks”) and associated fish passage facilities, to reach Puget Sound. Although the recent (1996 to 2002) drop in steelhead abundance was acute, there has been a less severe but steady longer-term decline in four-year (typical generation length) rolling average abundance. Genetic change in Cedar River steelhead may be an important factor in the decline of the anadromous population, as well as various habitat-related problems. For example, genetic change could result from selection against particular life-history trajectories or types, interbreeding with other populations or species, or, if not interbreeding, a consistently small number of successful spawners, which would reduce genetic diversity.

The current low abundance of anadromous adults could be viewed as a severe impediment to population restoration. However, it is conceivable that resident adults are capable of producing seaward-migrating smolts. It is relatively well-known that residual, precocial males occur in steelhead populations. In a Hood River, Oregon steelhead reproductive success study using DNA pedigree analysis techniques, researchers estimated that about 40% of returning steelhead had non-anadromous male parents (Ardren 2003; Blouin 2003). In an on-going breeding study using Grande Ronde Basin (OR) steelhead and trout, crosses between resident trout and between trout and steelhead all produced out-migrating smolts (Ruzycki et al. 2003). Smolt production capability of factors. Our genetic study examines their heritage by comparing resident adults from above- and below-Dam areas to anadromous adults presumed destined for Cedar River, from the ancestrally-related Green River population, and several relevant hatchery populations. The environmental capacity to support both life-history types or strategies is a major issue that should be evaluated subsequently.

Although above-Dam O. mykiss have been land-locked since 1900, below-Dam resident adults may be a recent ecophenotype (Zimmerman and Reeves 2000) of the steelhead population that is dominant under particular conditions, but could alternatively produce smolts. Puget Sound streams below migrational barriers generally are inhabited nearly exclusively by anadromous O. mykiss populations according to WDFW biologists, in contrast to inland Columbia Basin rivers where anadromous and resident fish typically co-exist. The steelhead life-history was likely the dominant natural state of O. mykiss in Cedar River when it was a Green River tributary, prior to its diversion into Lake Washington. Recent Cedar River and Lake Washington conditions may have favored fish expressing resident life-histories causing these phenotypes to become abundant coincidentally to a period of poor ocean survival for regional steelhead.

Previously, genetic analyses had been conducted on juvenile (age 1 parr) O. mykiss sampled during May 1993 and 1994 in mainstem Cedar River (Maple Valley area) below Landsburg Dam, on juveniles sampled in 1994 above Landsburg Dam, and on rainbow trout sampled in 1994 from Chester Morse Lake (Phelps et al. 1994; Phelps and Baker 1995; Phelps et al. 1997). Parr below Landsburg Dam were presumed to be steelhead progeny based on the assumption that steelhead was the primary life-history form in that area. Genetic data were collected by analyzing allelic variation at allozyme gene loci. Significant temporal variability in allele frequencies occurred between the two lower Cedar River juvenile samples, but they were more similar to each other than to other O. mykiss samples included in comparative analyses (Phelps et al. 1997; WDFW unpublished data). Below-Dam juvenile O. mykiss were genetically closer to Green River juveniles than they were to O. mykiss in the two upstream Cedar Basin locations. Phelps and Baker (1995) described Cedar River O. mykiss above Landsburg Dam as being relatively similar to downstream O. mykiss, except that the population appeared to contain alleles from non-native rainbow trout hatchery strains. Also, three above-Dam fish appeared to be O. mykiss-O. clarki (cutthroat trout) hybrids. They surmised that Chester Morse Lake O. mykiss likely had some ancestry from an exotic rainbow trout hatchery strain.

These earlier genetic results are informative in a number of ways, but do not elucidate the relationship between resident and anadromous O. mykiss below Landsburg Dam. The juveniles sampled could have included offspring from resident and/or anadromous parents. Various possible scenarios could be hypothesized (100% anadromous offspring, 50% offspring each from non-interbreeding anadromous and resident populations, 100% resident offspring, etc.) and the results examined under those conditions, but this is not definitive as to the source and behavior of resident O. mykiss. Above-Landsburg Dam resident O. mykiss necessarily produced the juveniles sampled in 1994. Their genetic results suggested the persistence of a legacy from steelhead remaining upstream after dam construction.

Hybridization and other interaction with cutthroat trout are important issues relative to the status of O. mykiss populations in Cedar River and other Lake Washington drainages. According to WDFW biologists, cutthroat trout are abundant in Lake Washington and appear to favor and succeed at an adfluvial life-history strategy (migration between streams and lake) versus anadromy. Within this system, cutthroat trout are potential competitors with and/or predators on O. mykiss. Cutthroat trout are known to hybridize with O. mykiss in anadromous zones of Puget Sound and other western Washington streams (Campton and Utter 1985; Hawkins 1997; Wenburg and Bentzen 2001; Young et al. 2001). In un-modified natural habitats cutthroat trout and O. mykiss are ecologically distinct and remain reproductively isolated through geographic and temporal differences in spawning behavior. The significant human-induced habitat changes in Cedar River and Lake Washington could have disrupted natural isolating mechanisms. Hybridization with cutthroat trout is a significant alteration of the O. mykiss gene pool and likely has an especially negative impact on the steelhead phenotype.

The over-riding goal of this project is to understand genetic population structure of Cedar River/Lake Washington O. mykiss so that managers can design and implement strategies that effectively conserve and recover native steelhead and rainbow trout resources. Our study design targets adult fish at or near reproductive age and employs several types of DNA markers expected to be highly informative. Our major research objectives to be accomplished through two years of sampling are:

  1. Determine genetic relationships between adult anadromous and resident O. mykiss in the Cedar River watershed
  2. Determine genetic relationships between Cedar River outmigrating O. mykiss and potential parent groups
  3. Evaluate genetic relationships between O. mykiss above Landsburg Dam in former anadromous fish habitat and those below the dam
  4. Evaluate genetic relationships between Cedar River O. mykiss and Green River wild and hatchery steelhead populations
  5. Estimate extent of cutthroat trout and O. mykiss hybridization in all sampled groups
  6. Based on population structure inferred from sample data, estimate effective population size for Cedar River steelhead

This progress report summarizes our work on this research project during 2003 and the first half of 2004, including the genetic analysis of all samples collected in 2003. Sampling for 2004 is already underway.