Final Report: Ross Lake Rainbow Broodstock Program, Upper Skagit Reservoir Fish Community Surveys and Management Plan

The 1991 Settlement Agreement for re-licensing the Seattle City Light upper Skagit hydroelectric project provided for development of a native rainbow trout broodstock from Ross Reservoir to supplement the Gorge and Diablo reservoir trout fisheries. This provision was based on the premise that a paucity of spawning habitat limited fishery recruitment, and on a long history of fish stocking. Moreover, it provided for enhancement of rainbow trout fisheries in waters draining to the lower Skagit River with the stock presumed to be historically present and native to the basin.

The development of an enhancement program for native rainbow trout needed to be consistent with principles of hatchery reform. Given the fragmentation of this upper Skagit population by hydro-electric dams and the lack of functional spawning and rearing habitat in Diablo and Gorge reservoirs, WDFW elected to manage rainbow trout above Gorge Canyon as a single population. This approach is consistent with recent genetic studies of O. mykiss in the Skagit Basin, which reveal strong genetic similarities among the Diablo, Gorge, and Ross Lake populations. Spawning rainbows inhabiting Ross Lake and its tributaries account for more than 90% of the overall population. Although program fish would likely account for at least 50% of the rainbow spawners in the lower reservoirs, they would account for no greater that 10% of the total spawning population even if they accounted for 100%, yielding a maximum proportion of Hatchery Origin Spawners (pHOS) estimate of < 0.1. Equal numbers of wild and F2 rainbow used for broodstock replacement yield a proportion Natural Origin Brood (pNOB) estimate of 0.50, generating a conservative proportionate Natural Influence (PNI) estimate of 0.83.

In 2002 the Washington Department of Fish and Wildlife (WDFW) began implementation of the program. The department focused on four primary activities: broodstock development; fish production; investigation of the life history and status of Ross Lake rainbow trout populations to inform broodstock management; standardized baseline surveys of Diablo and Gorge reservoirs; and the development of management recommendations for enhancing reservoir fisheries.

WDFW assessed several tributaries to Ross Lake including Ruby, Big Beaver, Roland, Dry, Lightning, Little Beaver, and Silver creeks for potential sources of rainbow trout spawners. Larger tributaries proved difficult for live capture of incoming spawners while many of the smaller tributaries lacked populations large enough to provide sustainable collection opportunity. Only two smaller tributaries, Roland and Dry creeks, met both criteria.

The first collection of wild broodstock originated from Roland Creek in June of 1999 when WDFW had conducted a pilot study to evaluate fish collection, spawning, and rearing techniques for this program. Beginning in 2002, WDFW made annual collections of wild broodstock from Roland and Dry creeks. Within three years Marblemount Hatchery conducted its first egg take from hatchery-reared Ross rainbow, and began releasing fish into Diablo and Gorge reservoirs. From 2006 through 2009 production ranged from 200,000 to 345,000 eggs. Annual releases of upper Skagit rainbow trout ranged from 1,000 to 286,000 fish into Diablo and 2,040 to 4,000 fish into Gorge. Several alpine lakes within the Skagit basin also received plants of Ross Lake rainbow between 2006 and 2009.

As the project progressed, the Ross Lake rainbow broodstock program evolved. Annual collections from wild populations were used to produce each cohort of hatchery broodstock which in turn produced the quantity of fish needed to stock the reservoirs. Information concerning age class structure of the donor populations was used to develop the spawning protocols implemented in the hatchery. Annual monitoring of age class structure of the natural Dry and Roland creek populations from 2002 through 2004 indicated that the majority of males spawned at Age 2 and the majority of females spawned at Age 3 with Age 2, 3 and 4 males and females present in the populations. This diversity of spawning ages within the populations and the dominance of different brood years for males and females suggested strong inherent mechanisms for genetic out-crossing. Matrix spawning and outcrossing males and females by brood year were implemented to maximize genetic diversity of fish produced in the hatchery.

The Roland Creek population exhibited substantially larger returns than the Dry Creek population every year from 2002 through 2008 except in 2003 when the escapement for Dry Creek was larger. Escapement for Roland Creek ranged from 276 to 854 for spawners and averaged 470 spawners across a seven year period. Escapement for Dry Creek ranged from 103 to 330 spawners and averaged 216 spawners across the same period. Broodstock collections for 2002, 2003, and 2004 ranged from 80 and 150 fish. After 2004 collection targets were reduced to 25 fish per year based on the recommendations of WDFW geneticists.

Dry Creek consistently recruited spawners earlier, peaked earlier, and exhibited a narrower timing window than Roland Creek, presumably due to its steeper, smaller watershed and associated hydrologic characteristics. The cumulative spawning curves for both populations exhibited multiple peaks, with the initial peak for Dry Creek being its most prominent and the second peak for Roland Creek being its most prominent. In Roland Creek the onset of spawning correlated with increasing stream temperature to about 10°C, and with declining flows modulated by the attenuation of snowmelt runoff.

Prudent fish management necessitated a baseline assessment of abundance, growth and condition of fish in the reservoirs. The WDFW conducted comprehensive surveys of the fish populations in Diablo Reservoir in August 2005 and Gorge Reservoir in 2006 to assess these fish community attributes. These communities were sampled with standardized combinations of variable mesh horizontal and vertical gill nets under a systematic sampling regime. Angler reports of declining growth rates in rainbow trout in Ross Reservoir led WDFW to conduct an index survey there in August 2006 with a standardized combination horizontal gill net sets. Although a fundamentally different research question in Ross necessitated a replicated index sampling approach to maximize sample size, standardization of sampling gear allowed for population comparisons.

Species composition in all three reservoirs was comprised of rainbow trout (oncorhynchus mykiss), non-native eastern brook trout (salvelinus fontinalis), and two native char species, dolly varden (salvelinus malma) and bull trout (salvelinus confluentus). however, positive differentiation of dolly varden and bull trout was not possible in the field. therefore they are referred to collectively throughout this report as native char. in diablo rainbow trout were dominant in number and biomass, comprising 47.1 and 51.9% of the sample respectively. rainbow trout also dominated the species composition in Gorge and Ross by number, 68.5% and 40% respectively, while native char accounted for the majority of biomass in the catch. In addition to these species, red side shiners (Richardsonius balteatus) were documented in Ross Lake for the first time in 2006 where they were captured in small numbers in the fine mesh panels of horizontal gill nets and accounted for 2% by number and less than 1% by biomass. Catch per unit effort for rainbow trout was highest in Ross Lake at 21.1 fish per night followed by Diablo at 13.4 fish per night and then Gorge at 8.9 fish per night.

Age 1, 2, and 3 rainbow trout were well represented in all three reservoirs, and growth rates for these fish were highest through Age 1, nearly identical to fish collected from tributaries to Ross Lake. Growth of Age 2, 3 and 4 rainbow trout collected from Diablo and Gorge fell consistently below Ross Lake rainbow collected in the index survey, but not significantly so. Condition, expressed in relative weight (Wr), was below the national 75th percentile (Wr = 100) for all size classes but acceptable for the post-spawn season in which they were sampled.

Proportional stock densities for native char for Diablo, Gorge and Ross, calculated based on published values for bull trout, were 60 ± 28, 80 ± 22, 77 ± 17 respectively. While Age 1, 2, and 3 native char made up the majority of the catch, several Age 5, 6, 7, and 8 fish were also sampled. Regular observations of char exhibiting phenotypic characteristics of both native char and eastern brook provoked collection of tissue from all char sampled from Diablo in 2005 for future analysis of the relative contributions of Dolly Varden and bull trout to the presumed native char complex of Diablo Lake, and to provide samples for future investigations of the potential for hybridization with eastern brook trout. Time and resources precluded similar collections in Gorge and Ross. The genetic analysis of tissue samples collected from 50 adult char in Gorge Reservoir and over 100 adult char in Ross Lake, collected by Seattle City Light, found that most fish in Ross Reservoir were bull trout, and that the majority of fish in Gorge Reservoir were bull trout (Smith 2011). Bull trout / Dolly Varden hybrids were found to be present in Ross Lake and Gorge Reservoir, while bull trout / eastern brook trout hybrids were also found in Gorge Reservoir.

Catch rates of rainbow trout in Diablo and Gorge support the premise of lower abundances of rainbow trout in the two reservoirs, but may also be explained by increased emigration opportunity due to the spill regimes of the lower reservoirs. Lower growth rates may be influenced by overall cooler water temperatures and shorter water residence times in the lower reservoirs but could also reflect lower system productivity. Regular standardized monitoring of these rainbow trout populations will be necessary to guard against overstocking. Diablo and Gorge reservoirs should be surveyed at least once every four years or one trout generation. Systematic sampling to maximize independence of net set samples should be conducted with variable mesh, multiple panel experimental gill nets. Mesh sizes should range from half-inch stretched to 2 1/2 inches stretched and include at least three intermediate sizes. Calculations of species composition by number and biomass, species distribution, catch per unit effort, stock density indices, length frequency by age class, condition factor expressed in terms of relative weight, and scale-based age class analysis will provide useful information on both community and population level variability and change over time.

Initially, a conservative rate of 80,000 Ross Lake rainbow fingerlings served as a starting point for the Gorge and Diablo Reservoir fish stocking program. This was based on stocking densities applied to relatively unproductive alpine lakes then adjusted to an annual program. Rearing fish at Marblemount Hatchery through the winter and planting larger fish in the spring was intended to reduce competition with eastern brook and predation by sub-adult native char. However, seven years of investigation on Ross Lake have led to more comprehensive understanding of what constitutes a robust native fish community under the conditions imposed by the hydroelectric projects. In the Ross system numerous streams of varying order produce a diversity of habitats and life history opportunities for rainbow trout and native char. Both species use the larger streams to rear for 1 to 2 years. Rainbow trout also use smaller streams and migrate to the lake in large numbers during their first summer.

Gorge and Diablo are extremely limited in both spawning and rearing habitat for rainbow trout. The inability of stream habitats to produce adequate numbers combined with low productivity and perpetually constrained forage to drive growth result in poor recruitment into the fishery. In order to provide a stable fishery, a minimum density of 20 to 30 fish (greater than 300mm TL)/acre would be desirable. A multifaceted management approach that augments multiple trophic levels of these populations would be recommended to achieve these fishery objectives.

Therefore the stocking strategy should employ the use of both fry and fingerlings. Initial stocking rates under this strategy should be approximately 160 fish/acre for 1,200 fish per pound (fpp) fry in September and about 80fish/acre for 200 fpp fingerlings in May, resulting in about 158,000 fry and 79,000 fingerlings for Diablo and 34,000 fry and 17,000 fingerlings for Gorge. An additional 10,000 fry should also be produced for the stocking of alpine lakes within the Skagit Basin and 5,000 for broodstock replacement. The total production would be 303,000 rainbow trout for the program. These initial stocking densities should be adjusted based on growth and condition data collected from the monitoring program and harvest data collected through regular creel survey efforts. Introduction of Ross Lake rainbow trout into Diablo will enhance the sport fishery, reestablish downstream dispersal opportunity for rainbow trout populations from the upper to the lower Skagit, and may increase the long-term viability of rainbow trout in Gorge and Diablo reservoirs.

Founded on a framework of adaptive management, the primary purpose of this program is to enhance recreational opportunity by increasing the numbers and quality of rainbow trout available to anglers in Gorge and Diablo reservoirs. Information gathered during standardized stock assessment surveys will provide metrics for abundance and quality as evidenced by increases in unbiased catch per unit effort estimates. However, direct fisheries measures should also be employed. Pre-stocking catch rates of less than one fish per angler and one fish per hour could certainly be improved. Minimum target catch rates for year round lowland lakes of 2.5 fish/angler would be a meaningful goal for boat anglers on Gorge and Diablo reservoirs.