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JULY 8, 2010

Progress report on nutrient addition to Dworshak Lake

By Charlie Pottenger

   A well attended meeting conducted by representatives of the Corps Of Engineers and Idaho Fish & Game (IDFG) reported the results to-date of the now controversial nutrient addition program aimed at helping improved size and numbers of kokanee salmon in Dworshak Lake. The detailed report explained the program and the preliminary findings to an audience with highly polarized emotions ranging from very opposed to extremely enthusiastic observers.

   For the past three and a half years Idaho Fish & Game in cooperation with the U.S. Army Corps of Engineers has been adding ammonium nitrate fertilizer to the Dworshak impoundment in an effort to experimentally determine whether or not they might promote increased numbers and size of kokanee salmon to improve sport fishing prospects. Similar activities in British Columbia such as Kootenay Lake and Arrow Lake have shown favorable results and have been continued by managers there.

   Studies of nutrient levels in Dworshak, primarily indicated by analyses for nitrogen and phosphorous levels, since the lake was filled showed a drop of 96% from 0.050 t0 0.003 milligrams per liter by the year 2000. Biologists have seen similar results in many large impoundments and, when Dworshak kokanee numbers and size diminished, became interested in determining if nutrient addition might help the fishery.

   The study which began adding the nutrients from a specially equipped barge in 2007 will continue until the five-year experiment is complete. Fertilizer applications involve a carefully controlled application of 2608 gallons of ammonium nitrate solution to the surface water zone of the lake. This is a very small dose rate and the comparison given to help give a comparative reference was that this is equivalent to a teaspoon in 46,000 gallons of water.

   The study goals are to determine if careful nutrient addition will help restore the lost ecosystem productivity; improve the nitrogen to phosphorous ratio, improve the food-web efficiency and reduce outbreaks of blue-green algae. With special interest in kokanee, the goal is to improve size at a given population of fish and increase the biomass (total weight of kokanee in the lake). It would be expected to benefit other species and to help recycle or replenish nutrients to the water which were lost when the size of spawning kokanee dropped.

   Results to date show there is more edible plankton algae and more useable zooplankton, which are food for kokanee including large daphnia, a type of fresh water shrimp. The kokanee are averaging about one inch longer now than in pre-study populations and they have a higher weight to length ratio (they are chunkier). These kokanee are more desirable and catchable.

   Audience member voicing disappointment with the study were concerned with both the process used to authorize the experiment and concerns that the nutrients might harm both the lake fishery and adversely impact the steelhead and salmon hatcheries below the dam. In addition they voiced their belief that the study was contributing to blue-green algae blooms.

   Their questions were well received by both the representatives of the Corps and IDFG who gave reasoned explanations reporting their conversations with consulting biologists which suggest that the nutrient addition should not promote blue-green algae. They reported the biological theory that the added nutrients should assist the growth of the more desirable edible algae which helps the fish food chain. It is accurate to say these issues were not resolved at the meeting.

   Support for the nutrient program was also strong as participants applauded the studies goals and stated their desire for more and bigger fish in Dworshak as a major lure to increased use of the lake by tourist recreationists and local people.

   The future direction of this experimental program will be determined after the five year trial is completed. The results will be presented and a full process to assess proper avenues is followed before the next steps are taken.