Trout survival study

Idaho Department of Fish and Game biologists are starting to study survival rates of lake trout that are caught and released by anglers on Priest Lake. The study will help IDFG biologists better understand whether mortality of caught and released fish is high enough in Priest Lake that it is an issue with which anglers and IDFG need to be concerned.

Lake trout are typically found near the bottom of relatively deep lakes, often in 100-200 feet of water. When brought to the surface from such depths by anglers, they sometimes get a fishy version of the bends. The pressure change causes a condition in the fish known as barotrauma in the scientific world.

As do many species of fish, lake trout have an internal air bladder that helps them regulate buoyancy. When a fish swims from deep to shallow water, it naturally expels air from the bladder, just as a scuba diver releases air from a dive vest. Deepwater dwelling fish can have difficulty expelling air when pulled from depth by anglers, causing over-inflated gas bladders.

While extensive research has been conducted on the effects of barotrauma on survival of caught and released marine species, little has been done on lake trout.

Rob Ryan, IDFG Regional Fishery Biologist, has a good understanding of the number of lake trout harvested from Priest Lake. Based on angler creel surveys conducted on Priest Lake in 2014-2015, fishermen release about a third of the lake trout they land. What is less certain is the number of fish that die after being released.

From a population management standpoint, the fish released that do not survive result in additional mortality that is over and above the number harvested. If the number released that die turns out to be a small percentage, then it really is not a concern. In contrast, if we learn that a high percentage of released fish do not survive, we need to consider that when we think about how we are trying to manage lake trout.

With the help of volunteer anglers, IDFG will be capturing up to 200 lake trout in early May. The fish will be released into a large pen anchored in Indian Creek Bay. Each fish will be individually tagged to enable the biologists to record the depth of capture. Once released, the size of the pen will allow the fish to return back to a depth of 100 feet. After about a week, the pen will be pulled and the fate of the fish will be evaluated.

The study is part of a much larger effort that began in 2013 to better understand and develop a long-term management plan for the Priest Lake fishery.

Thanks to a large-scale population assessment in 2013 using commercial-scale netting equipment, biologists now have a much better estimate of the number of lake trout in the system, as well as their size and age structure. In addition, an angler creel survey conducted over the past year provided an estimate of how many lake trout, kokanee and other species are caught and harvested or released in a given year.

IDFG representatives have scheduled an informational public meeting in Priest River on May 27 to share the results of all of recent Priest Lake fishery studies. The meeting will be from 7-9 p.m. at the Priest River Senior Center.

Individuals with disabilities may request meeting accommodations by contacting the Idaho Department of Fish and Game directly at (208) 769-1414; or, through the Idaho Relay Service at (800) 377-2529 (TDD).

Jim Fredericks is the regional fishery manager for the Idaho Department of Fish and Game.