ORIGIN AND STRAYING OF HATCHERY WINTER STEELHEAD
IN OREGON COASTAL RIVERS
Schroeder, Kirk, R.,
Robert B. Lindsay, and Ken R. Kenaston. 2001. Origin and straying of hatchery
winter steelhead in Oregon coastal rivers. Tans. Am. Fish. Soc. 130: 431-441.
ABSTRACT
We evaluated the origin and straying of hatchery steelhead Oncorhynchus mykiss among 16 rivers on the Oregon coast to examine rearing or release practices that might contribute to straying. Data were collected on the returning adults of three brood years that had been differentially marked and released as smolts in 1990-1992. The percentage of strays averaged 11% (range 4-26%) of the samples of hatchery and wild fish in 11 streams where hatchery steelhead were released. Stray hatchery fish composed a mean of 22% (range 9-43%) in 5 streams without hatchery releases. The two predominant factors that contributed to straying were releases of stocks transplanted from their natal basins and releases into adjacent basins. Releases of transplanted stocks into adjacent basins accounted for 41% of the strays, while releases of transplanted stocks into nonadjacent basins accounted for 29% of the strays. Local stocks of steelhead released into adjacent basins accounted for 16% of the strays. The incidence of straying by hatchery fish and its widespread occurrence in Oregon coastal rivers present genetic and ecological risks to wild populations of winter steelhead. Strategies to reduce straying may include using local brood stocks, rearing and releasing fish within their natal basins, reducing the numbers of hatchery fish released, and eliminating some hatchery releases altogether.
QUOTES FORM TEXT
Homing of adult anadromous salmonids to their natal stream has been recognized as an important adaptation in establishing and maintaining distinct spawning populations through reproductive isolation. Mature fish that migrate to and spawn in a stream other than the one where they originated are considered strays. Straying is a natural behavior that enables salmonids to colonize new habitat, to avoid locally unfavorable conditions, to maintain genetic diversity within stocks, and to perpetuate metapopulations. The genetically distinct structure of anadromous salmonid populations suggests that the successful reproduction of strays naturally occurs at low levels. However, straying of hatchery fish concerns fish managers because of the potential negative impacts on wild populations from interbreeding between wild and hatchery fish.
Stray hatchery fish can have genetic and ecological effects on wild fish populations. Hatchery fish that stray and hybridize with wild fish can reduce genetic diversity between populations and decrease the fitness of wild populations through the displacement or breakdown of locally adapted gene complexes. Gene flow increases between non-native hatchery and wild populations when hatchery strays successfully spawn with wild fish, and it can result in the reduced frequency and subsequent loss of locally adapted alleles. Hatchery strays spawning in several rivers can also result in a genetically homogenized population. A potential ecological effect of stray hatchery fish on wild fish is competition in spawning and rearing areas. In addition, large numbers of strays can mask trends in the population abundance of wild fish and bias estimates of the survival and exploitation of wild and hatchery stocks.
Concerns about the genetic and ecological impacts of hatchery fish on wild fish have led to proposals for assessing and altering hatchery programs as well as to policy changes in fish management agencies. (For example, the Oregon Wild Fish Policy (1978, 1992) and the Native Fish Conservation Policy (2002).
Based on all streams sampled, transplanted stocks of hatchery steelhead accounted for a higher proportion of strays than local stocks. Strays from releases of steelhead transplanted outside their natal basins accounted for 70% of strays reported in Oregon coastal basins, with release of transplanted stocks into adjacent basins accounting for 41% of the strays. Releases of transplanted stocks composed 42% of the annual smolt releases in Oregon coastal basins. Strays from transplanted releases were predominant in 6 of 16 streams we sampled. Local stocks reared and released in their natal basins accounted for another 12% of the strays from 7% of the smolt releases and composed the majority of strays in two streams. The composition of strays in the remaining four streams was not predominated by a release type.
The incidence of straying was lower when examined at the scale of an ESU than at the smaller scale of individual basins. Of hatchery steelhead from known release groups, those straying from the neighboring ESU composed 2%of all hatchery fish in the northern ESU and 4% of hatchery fish in the southern ESU. The average number of hatchery steelhead smolts released into northern ESU basins were twice the average released into southern ESU basins. The percentage of steelhead from a neighboring ESU was highest in basins closest to the ESU boundary. About 15% of all strays sampled within an ESU were from the neighboring ESU. The incidence of stray steelhead in our study was lower at the geographic scale of ESUs than at the scale of individual basins.
In Oregon coastal basins the mean percentage of strays from transplanted stocks was about twice that from local stocks. The incidence of straying in Vancouver Island streams increased an average of eight times when steelhead were transplanted to other streams rather than locally released. Other studies also report that locally adapted populations stray less than transplanted populations in the cause of chinook salmon. Pascual and Quinn (1994) reported accurate homing of Rogue River salmon transplanted to the Columbia River but considerable straying within the Columbia.
The straying patterns of hatchery steelhead among Oregon coastal basins suggest that the potential for gene flow between hatchery and wild fish is greatest in basins that are geographically proximate to the basin where the hatchery fish are released.
In addition to the effects on nearby populations, the occurrence of long-distance straying in our study (24% of the strays) indicates that hatchery releases can potentially influence wild fish populations over a large geographic area. Even a low rate of gene flow from distant hatchery populations can reduce the genetic diversity of wild populations.
Marking reduces the survival of fish and can affect estimates of straying. In this study, almost all hatchery fish were marked with a combination of fin and maxillary clips, which would have reduced their survival. This probably decreased our estimates of the percentage of strays in the total run in most basins.
Although straying is a natural phenomena in anadromous salmonids, the level of straying by hatchery steelhead and its widespread occurrence in Oregon coastal rivers raises concern about the long-term genetic impacts on local wild populations of winter steelhead.
In addition, large numbers of strays to a hatchery pose a risk to the genetic integrity of local stocks of hatchery fish. If straying hatchery fish cannot be identified, they could be incorporated into the local hatchery stock. The genetic integrity of locally adapted populations can decrease with the rates of gene flow from stray hatchery fish as low as 5-10% (Emlen 1991; Felsenstein 1997). (Note: the Oregon Native Fish Conservation Policy (2002) allows a 10% stray rate)
Stray hatchery steelhead composed 10% or more of the steelhead sampled in 10 of 16 Oregon coastal streams. Of particular concern is the high percentage of strays (9-43%) in streams where no hatchery fish were released.
Because hatchery steelhead tend to return to their release locations, releases into tributaries rather than into the main stem may increase homing within and among basins, but the residualism of hatchery smolts should be evaluated. Reducing the numbers of hatchery fish released or eliminating some hatchery releases altogether would also decrease the numbers of strays.