Drawdown’s effect on trout populations unknown

Assessing the damage to the Bighorn River's young-of-the-year trout following an accidental drawdown Wednesday will be difficult, according to Ken Frazer, regional fisheries manager for Fish, Wildlife and Parks.

"There's no way to do a quick assessment," Frazer said today.

But he added that given the severity of the drawdown, a sudden drop from around 4,000 cubic feet per second to 1,500 cfs, the effects could be substantial to young fish on the upper end of the popular trout fishery.

"My guess is that it was a pretty significant impact on the upper river, especially on small brown trout," he said.

The abrupt decline in the river's flow was blamed on an electrical surge that caused new gate automation equipment at the Yellowtail Afterbay Dam to malfunction, according to the Bureau of Reclamation.

Although FWP does an annual fish survey on the Bighorn in mid-September, Frazer said small fish don't show up during electroshocking when flows are higher, such as they are now. That's because the smaller fish hide in the shallows and backwaters.

"It will be two years before we get a good handle on it," he said, when the smaller fish are large enough to show up in the agency's sampling.

Although the upper stretch of the Montana river sees much of the fishing pressure, the lower section near Mallards Landing - about 24 miles downstream from the dam - may not have fared as badly, Frazer said. The water fluctuation there may have been less intense.

At this time of year, trout fry are only about an inch long. They hide in shallower waters to avoid predation by larger fish. The river level's drop would have made the fish that escaped into the main river more susceptible to predation by their larger relatives.

FWP said that any damage to the fishery was somewhat mitigated by the short duration of the drawdown - less than half an hour - and the fact that temperatures were cool for late July.

Contact Brett French at french@billingsgazette.com or at 657-1387.

Posted in Montana on Friday, July 31, 2009