"Big fish," yelled one Idaho Department of Fish and Game biologist as another dipped a long-handled net into the fast waters of the Big Wood River to scoop out a hefty, seemingly lifeless rainbow.
Nearby, a half-dozen other wader-clad, net-toting Fish and Game biologists carefully moved backwards downstream while scanning the current for other drifting fish.
In one 100-yard stretch near Gimlet, south of Ketchum, the biologists collected about 30 fish ranging in size from a finger to a football.
The process, known as electroshocking, sounds worse than it really is—according to biologists the fish are not harmed—and is considered the best method to gauge fish populations, age and health in a given stream.
"It's so variable. It changes every year, which is why we do these population estimates," said Rob Ryan, regional fisheries biologist for Idaho Fish and Game.
For the last two weeks, Fish and Game has been conducting population counts along three sections of the Big Wood River—near Gimlet, north of Ketchum and in Hailey.
"We're seeing about an average number of fish and sizes," Ryan said. "But it's hard to know for sure until we have all of the information."
Using long electrodes, which are attached to a portable generator in a canoe, the biologists send a pulsed current through the river that causes all fish in the immediate surrounding waters to experience involuntary muscle spasms.
The seemingly paralyzed fish are scooped up by net and transported to an oxygenated barrel of water in the canoe. Those fish that are not netted regain muscle use as soon as they are beyond the reach of the electrode current, which is about the length of a fly fishing cast.
Most fish—about 75 percent on average—avoid the electrical current and nets altogether.
After about 100 yards or so, the canoe is beached and a data collecting process commences.
Placed in a bucket containing a solution of fresh water and clove oil, which is a calming aid, the fish are measured and clipped—a tiny piece of the tail no bigger than a snowflake is removed. The clipping lets biologists determine if the fish has been netted before, which is one component of the formula to determine overall populations.
A few scales are also scraped off the fish near the upper part of the tail—a harmless procedure to determine age.
"Scales grow in ring patterns, like trees," Ryan said.
Once the data is collected and recorded, the fish are placed in buckets with fresh water in preparation for their release back into the stream.
A small number of fish are bagged for additional research.
Once all fish are returned to the river, the entire process is repeated in the next downstream stretch. Ryan said there are typically about 1,200 to 1,500 trout living in any given one-kilometer stretch of the Big Wood River.
However, he noted that some stretches are more bountiful than others.
"In one hole, you could pull out 20 fish," he said.
A comprehensive report on fish populations in the Big Wood River will be released by Fish and Game this winter.
Idaho Department of Fish and Game biologists net trout during a fish count in the Big Wood River Wednesday. Photo by David N. Seelig
