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200709100 - The evaluation of limiting factors on resident and anadromous salmonids in Lake Wenatchee, Washington

Sponsor: Washington Department of Fish and Wildlife (WDFW)

Budgets: FY07: $489,210 | FY08: $433,814 | FY09: $447,380

Short description: This project will evaluate predation, water quality and the available prey base on bull trout, spring chinook salmon and sockeye salmon survival in Lake Wenatchee. Bioenergetics modeling will quantify consumption rates of piscivores to determine impacts.

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Final Council recommendation (Nov 2006)

Funding category: Expense

Recommended budgets: FY07: $0 | FY08: $0 | FY09: $0


ISRP final recommendation: Fundable in part


The project sponsors provided information that partially addresses the ISRP's questions about how Lake Wenatchee compares to other large oligotrophic lakes in the Pacific Northwest. Overall, however, the responses to the ISRP queries weren't particularly thorough. They provided interpretation from other investigations not the actual quantitative data summary from the other projects. It is surprising how little is known about the lake, considering it contains several listed salmonids as well as a sockeye pen-rearing program. The need to gather more data from Lake Wenatchee to understand trophic processes justifies further research. This proposal essentially examines whether juvenile sockeye and Chinook rearing in the lake are limited by top-down (predator) or bottom-up (nutrients and plankton) factors. Phase I examines the role of predators -- bull trout and pikeminnow; Phase II examines nutrient limitation and the potential for lake fertilization to boost salmonid productivity. The response to the ISRP's questions suggest that justification of Phase I is adequately supported. It does seem possible that predator populations are consuming enough juvenile sockeye and Chinook to have a significant impact on the populations. Determining whether or not predators are consuming these fish incidentally to supplement their regular prey organisms or are targeting them seems worth exploring. To address Phase I will require measurements of abundance of predators along with information on the components of their stomach contents and their rates of digestion. According to the Response, information is lacking other than observations that "Large congregations of predators, for example, have been observed at net pens and the mouth of rivers during times of hatchery releases and out migrations of naturally produced fish into the lake" (Response page 2). Sponsors plan on using tangle nets to collect predators. The ISRP appreciates that sponsors are designing a system to avoid harm to captured listed species. The tangle nets they plan on using may not be effective, however. They could seriously miss the abundance of predators like northern pikeminnows, and could be counter productive if the collections are not sufficient to estimate the important parameters. The justification for Phase II, the nutrient limitation work, is weaker. To really understand whether Lake Wenatchee will benefit from a fertilization program it will be necessary to conduct a fairly thorough examination of the lake's physical limnology. This will entail understanding the timing and pattern of stratification and the influence of the incoming tributaries on the lake's circulation pathways. A worst-case scenario (and we're not implying it would occur here) would be that added nutrients would be quickly transported in surface layers to the lake's outlet without ever reaching the target phyto- and zooplankton. Unless the circulation patterns are well understood, the effectiveness of lake fertilization can be compromised. In addition to the physical limnology, developing a thorough bioenergetics model requires measurements of the key components of the chemical and biological makeup of the lake. Information is needed to develop the estimates of interactive effects among fish species in the lake. While the proposal makes reference to bioenergetic models in this context, it is not clear that the references cited for spring Chinook and sockeye salmon adequately take into account the zooplankton that might be utilized by other fish species in the lake, for example, juvenile bull trout or northern pikeminnow and others. It is not clear how the zooplankton samples will be expanded to abundance estimates for the lake as a whole or for a volume of water occupied by a given mass of Chinook or sockeye. There needs to be an explicit bioenergetics model and the plans for populating the model with Lake Wenatchee data as part of the work that is recommended for funding. Therefore, the ISRP suggests that Phase I is fundable, with funding for Phase II contingent on Phase I findings and a complete review of what is known about Lake Wenatchee's physical limnology.

State/province recommendation: MS: Subbasin

Review group: MSRT

Recommended budgets: FY07: (n/a) | FY08: (n/a) | FY09: (n/a)

Comment: This project will look at predation and other limiting factors (i.e., nutrients) for sockeye in Lake Wenatchee. This is an on-the-ground action that is wholly contained in the Wenatchee subbasin.

State/province recommendation: Washington

Review group: Washington list

Recommended budgets: FY07: (n/a) | FY08: (n/a) | FY09: (n/a)

Comment: See Washington guidance