Combined Program
The combination of the Dworshak Dam Resident Fish Mitigation Project and the Lake Pend Oreille Fishery Recovery Project will lead to several important efficiencies.  First, one IDFG Fishery Research Biologist will be assigned to monitoring limnology and kokanee population dynamics in both Dworshak Reservoir and Lake Pend Oreille.  This will lead to efficiencies of equipment usage, personnel time, temporary employee supervision, and consistency of analyses.  Second, it will allow another IDFG Fishery Research Biologist to focus effort on monitoring and management of Lake Pend Oreille predator species, including lake trout and walleye.  Facilitating program specialization for these project biologists will also improve deliverables.  Finally, combination of these two programs will improve flexibility to shift resources from one body of water to the other, depending on dynamic management priorities.  Finally, having a combined project will lead to project administrative efficiencies with only one contract to manage versus two.

Project Histories
Dworshak Reservoir
The goal of this project has been to improve resident fisheries in Dworshak Reservoir as partial mitigation for losses from the construction of Dworshak Dam and impacts from ongoing dam operations. After completion in 1971, Dworshak Dam irrevocably blocked the North Fork Clearwater River for access to hundreds of miles of tributaries for anadromous fish production and flooded 86.9 km (54 mi) of riverine habitat for resident fishes. The resident fisheries that were developed in the reservoir were intended to partially mitigate for some of these losses, but reservoir operations continue to negatively impact native and non-native resident fish in Dworshak Reservoir and the North Fork Clearwater ecosystem. In addition, the productivity of this ecosystem has been significantly reduced (oligotrophication) due to natural reservoir senescence, reservoir operations (e.g. drawdowns), and loss of marine derived nutrients. Further, because nitrogen has been the limiting nutrient in Dworshak Reservoir, phytoplankton communities have occasionally become dominated by inedible and potentially toxic cyanobacteria blooms (“blue-green algae”).
To mitigate for oligitrophication and harmful algal blooms (HABs), the Idaho Department of Fish and Game (IDFG) and U.S. Army Corps of Engineers (USACE) initiated a pilot study to test manipulation of nitrogen to phosphorous ratios. Data collection began in 2004 and two nitrogen restoration periods (2007 – 2010, 2012 – 2015) were evaluated for their effectiveness.  The project was in compliance with water quality standards set by the Idaho Department of Environmental Quality and U.S. Environmental Protection Agency. The mean Secchi depth for the restoration period was similar to the mean for the non-restoration period. The prevalence of Dolicospermum, the primary species contributing to HABs, was reduced by 77%. Mean concentrations of nitrogen and phosphorus in the reservoir did not increase following restoration, indicating rapid biological uptake of added nutrients. On average, heterotrophic bacteria densities were 71% higher during the restoration period and picocyanobacteria densities were 64% higher. The mean chlorophyll a concentration was slightly lower for restoration years. Although the mean biovolume of phytoplankton was similar for both periods, the mean biovolume of edible phytoplankton and the proportion of the phytoplankton community that was edible to zooplankton were substantially higher for the restoration period. The mean density of Daphnia large enough to be consumed by kokanee Oncorhynchus nerka was 103% higher for the restoration period as compared to non-restoration period. Kokanee grew faster at a given abundance as a result of increased food availability and there is evidence that the carrying capacity of the reservoir has increased as a result of nutrient restoration. As a result of improved growth and increased abundance, the biomass of kokanee, both in the reservoir and spawning in the watershed above, has increased by 50% and 40%, respectively. An increase in the abundance of kokanee of the same size or slightly larger than pre-restoration is expected to improve catch rates in the recreational fishery and provide more forage for piscivorous fishes and renew some upriver nutrient flux. Dworshak Reservoir appears to be responding to nutrient restoration as anticipated, and greater improvements to the fishery are possible if results are sustained.

Lake Pend Oreille
This project is on-site, partial mitigation for impacts of Albeni Falls Dam. The goal of this project is to recover and sustain the Lake Pend Oreille and Pend Oreille River fishery that was negatively impacted following construction and operation of Albeni Falls Dam. Historically, Lake Pend Oreille was the most popular fishery in Idaho, supporting both a high-yield kokanee fishery and a trophy fishery for bull trout and rainbow trout. Recovery efforts currently focus largely on predator management in order to sustain a robust kokanee population because they are a highly-valued sport fish and the primary prey for native bull trout (ESA listed) and rainbow trout.
Starting in the mid-1960s, the kokanee population rapidly declined in response to consistent full draw-downs of the lake that reduced the quantity and quality of shoreline spawning habitat. Since 1996, winter lake level manipulations designed to improve kokanee spawning success were implemented and evaluated. This strategy was believed to be working effectively for a number of years, but a monitoring project later showed that a benefit to kokanee recruitment was, in fact, not occurring. This same project showed that downwelling currents provide a survival advantage to incubating kokanee eggs. Using this new information, we implemented a spawning habitat enhancement strategy. We mechanically added suitable-sized spawning substrate in a shoreline reach that has downwelling currents present, but formerly had substrate too large for kokanee to use for spawning. This approach, combined with the knowledge that the four foot winter water level strategy has not worked as intended, has provided more flexibility for the hydrosystem.

In addition to historical impacts to kokanee from water level management, another limiting factor emerged in the late 1990s. Nonnative lake trout became abundant as a delayed response to mysid shrimp introduction and posed a predation threat to kokanee. The kokanee population nearly collapsed as a result of exponential growth of, and predation by, the lake trout population. In response, an aggressive lake trout suppression program was initiated in 2006 to reduce their abundance. This program has demonstrated remarkable success, with the lake trout population now at low density and the kokanee population now at high density in response to reduced predation. Additionally, the rebound of kokanee allowed a harvest fishery to be re-opened in 2013 for the first time since 1999. This fishery has been sustained since and is popular with anglers.  Continued activities associated with predator reduction are planned. Our lake trout population model suggests that we are likely to reach our objective of reducing the lake trout population to pre-1999 adult abundance (1,800 fish) in the next 10-20 years, depending on our ability to maximize removals of large fish. After the target is reached, our modeling suggests lake trout suppression activities could be greatly reduced while maintaining the target densities.

Current Limiting Factors and Mitigation

Dworshak Reservoir
Reservoir senescence, nutrient limitation, and nutrient imbalance will continue to limit the resident fisheries in Dworshak Reservoir. In 2017, IDFG and USACE signed a memorandum of understanding (MOU) agreeing to continue cooperation on the Dworshak Reservoir nutrient mitigation program as a long-term management action.  As part of the this MOU, IDFG has agreed to continue to monitor and prepare reports on the limnological and fishery responses to management in Dworshak Reservoir, while USACE has agreed to continue to apply nutrients as a management strategy, as well as to provide logistical support and laboratory contracting to facilitate IDFG monitoring.

Lake Pend Oreille
Given the progress toward evaluating and addressing the primary limiting factors (predation, spawning habitat) for kokanee, we will continue monitoring to evaluate other potential limiting factors that could play an increasing role. A potential new threat is emerging with a recently documented increase in nonnative walleye abundance.  IDFG presented documentation of this increase in abundance, as well as diet analyses demonstrating walleye predation of kokanee, to a panel of experts with experience studying walleye across the fish’s native range.  The panel’s recommendation was to evaluate efficacy of walleye suppression programs as a management tool. They also recommended monitoring walleye distribution patterns, diet, and population characteristics both to facilitate management and evaluate suppression feasibility. Taking a broader view on predator management and evaluation (including rainbow trout, bull trout, lake trout, walleye, and smallmouth bass) will be required to maintain the mitigation gains achieved to this point. Additionally, we will evaluate the effects that zooplankton and mysids have on kokanee survival, especially with respect to altering hatchery stocking practices to improve kokanee survival and growth.  Importantly, we have also recently learned that mysid abundance controls kokanee production, and therefore, the acceptable scope for predation.