Pend Oreille mysis shrimp population shrinks

PONDERAY — A major player in the imperilment of Lake Pend Oreille’s kokanee population is now itself against the ropes.

Mysis shrimp density in the lake is at its lowest point since monitoring of the tiny crustaceans began in the early 1970s, according to the Idaho Department of Fish & Game.

“Mysids have basically collapsed,” Fish & Game fisheries biologist Andy Dux said during the department’s annual State of the Lake presentation on Thursday.

The half-inch shrimp were introduced in the lake to fatten kokanee and lure in more anglers. They were planted in the lake from 1966 to 1969, but the plan backfired badly.

Pend Oreille kokanee did not care for mysis shrimp and the two species had mismatched feeding patterns. Mysis grew abundant and had a hearty appetite for the lake’s zooplankton, a primary food source for kokanee.

Moreover, mysis shrimp increased the survival of juvenile lake trout, contributing to its exponential population growth in the lake.

Mysis shrimp were blamed for the near collapse of the kokanee fishery.

It was closed to harvest in 2000, but reopens this year following years of work to suppress the lake trout population through aggressive bounty and netting programs.

Fish & Game still draws criticism for introducing mysis shrimp, although the department points out there was also considerable pressure from the angling community to introduce them.

“Everybody thought it was a great idea,” said Jim Fredericks, Fish & Game’s Panhandle fisheries manager.

Mysis density peaked in Lake Pend Oreille in 1980, when there were an estimated 1,740 mysids per square meter. Densities fluctuated in following years, but bottomed out last year.

Fish & Game estimates there were a mere 45 mysids per square meter in 2012, which is comparable to densities that were recorded as the mysis population was being established in the lake.

Dux said it’s unclear what’s causing the collapse.

“We’ll continue to evaluate it,” said Dux.

There is evidence of mysid decline in other lakes, but Dux said those declines aren’t as extreme as the decline in Lake Pend Oreille. One possible factor could be two consecutive years of high and delayed runoff, he said.

Dux said the situation provides an opportunity to monitor kokanee and zooplankton response to mysid collapse and eventual recovery.

Fewer mysids are expected to be beneficial to kokanee recovery because it decreases competition for zooplankton and puts a crimp in the diet of juvenile lakers, Dux said.