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
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.