COUGAR — Kokanee numbers in Yale Reservoir and its tributaries remain relatively low with the reason yet unknown.
The annual report by PacifiCorp on the 2011 kokanee spawning in Cougar Creek and other locations estimated about 45,000 of the landlocked sockeye, well below average.
Yale is the middle of the three reservoirs on the North Fork of the Lewis River. The impoundment is about 3,000 acres and has a naturally spawning population of kokanee in Cougar Creek, a large tributary.
Yale’s kokanee population historically waxes and wanes, especially due to big events like the eruption of Mount St. Helens or flooding.
Normally, the population bounces back in two to three years as the female kokanee grow larger and with more eggs, presumably due to the lack of competition for zooplankton, a key food source.
“The spawner escapement numbers for the last seven years do not seem to be following this trend, as estimates remain low,’’ said Jeremiah Doyle, a biologist for PacifiCorp.
The Portland-based utility operates the three dams on the North Fork of the Lewis and does much of the biological data gathering.
The average spawner count in Cougar Creek, dating back to 1978, is 65,000. The 10-year average is 35,000.
The 2011 estimate of 45,000 is a big improvement over the 18,500 in 2010.
But in 2011, Yale’s kokanee were smaller than normal.
The average size for a Yale female spawner is 11.4 inches. In 2010, the females averaged 11.9 inches. That average was down to 10 inches in 2011.
Doyle said when the kokanee are small it may indicate that food availability in the reservoir was limited at some point in their life.
“There are a number of factors that may be limiting kokanee escapement including but not limited to predation, low reservoir productivity from water turbidity, lack of access to quality spawning habitat in Cougar Creek due to numerous logjams, harvest, disease and competition,’’ he said.
It is hard to quantify each factor.
“However, it is clear that kokanee escapement is much lower in recent years when compared to historical records and the exact cause is yet to be determined,’’ Doyle said.
Yale’s kokanee do have some new spawning habitat.
The utility reestablished flows in the “bypass reach’’ of the Lewis River, the old channel between Swift Dam and powerhouse No. 2.
Biologists estimate about 1,200 kokanee spawned in the in the Lewis River channel and about 600 in Ole Creek.
If greater numbers of kokanee continue to use these areas, they will be regularly surveyed like Cougar Creek, he said.