Dworshak Reservoir Kokanee Update

By Eric Stark Summer is right around the corner, and for many, that means kokanee and bass fishing on Dworshak Reservoir. But, anglers don't have to wait until summer to catch some of the best action around. Kokanee fishing can be good year round. In fact, in some ways, it can be easier this time of year. During winter and spring, kokanee tend to congregate in the lower end of the reservoir closer to the dam, which means anglers don't have to drive or boat too far. As summer progresses, the kokanee move towards the more remote, upper end of the reservoir. Fishing pressure is virtually nonexistent then, so anglers often have the whole reservoir to themselves. Some of the best smallmouth bass fishing can also be had right now as bass feed in the warm water near shore prior to spawning. As water temperatures increase, catch rates should increase as more zooplankton becomes available for kokanee and bass become more active. Kokanee are planktivores, which means they feed almost exclusively on zooplankton. There is a limited amount of zooplankton in any body of water, so when there are many fish, there is less food available per fish. The result is that they grow slower and don't get as big. We have determined from years of population monitoring, that kokanee densities between 12 and 20 adult kokanee per acre results in optimal catch rates along with good sizes of fish. However, keeping densities and abundance of kokanee stable within this range is very difficult. Unfortunately, losses of fish into and through the turbines and reservoir outlets of the dam, called entrainment, have caused kokanee populations to vary widely from year to year. In fact, entrainment loss is the main factor limiting kokanee populations in the reservoir. In 1996, over 1.3 million kokanee were lost from the reservoir during a period of high discharge. This meant that about 95 percent of all the kokanee in the reservoir were lost. Therefore, controlling entrainment losses is a critical problem and is one of the current focuses of our research on Dworshak Reservoir. During preliminary investigations into potential ways to keep fish from becoming entrained through the dam, scientific literature suggested underwater strobe lights might provide the most applicable and promising way to effectively keep kokanee away from the intake areas of Dworshak Dam. Strobe lights were temporarily placed on a floating research barge near the dam in late 2000, and on-site testing began in early 2001. Repeated nighttime testing in 2001 resulted in an 88 percent reduction in fish densities directly in front of a single discharging turbine. Testing in 2002 achieved a 66 percent reduction in fish densities in front of operating reservoir outlets, which have much higher water velocities. Continued testing during January and February 2004 achieved an 87 percent reduction in fish densities despite higher discharges and two simultaneously discharging turbines, nearly as effective as the 2001 tests. The last of our strobe-light testing will be completed this summer in order to determine their effectiveness under the highest discharge conditions with all turbines operating and poorer water clarities. With the help of an engineering firm, we are now preparing preliminary design plans, which will guide the permanent installation of underwater strobe lights on Dworshak Dam. Once strobes are implemented and operated on a continual basis, we should observe benefits to more than just kokanee in the reservoir. Avoiding entrainment could also minimize the losses of rainbow trout and bull trout. An improved kokanee population also provides forage for the reservoir's smallmouth bass. And lastly, having over 300,000 kokanee migrate up tributary streams and die each fall could also add significant nutrients to these stream systems, thereby potentially enhancing the reservoir productivity. For more detailed information on Dworshak Reservoir research go to http://www2.state.id.us/fishgame/common/technical/fisheries.cfm Eric Stark has worked as a Senior Fisheries Research Biologist with IDFG on Dworshak Reservoir for over five years.