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Idaho Fish and Game

Cold Winter Water Doesn't Slow Dworshak Bull Trout

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By Danielle Schiff - Idaho Department of Fish and Game As winter arrives, most wildlife species settle in for the long cold spell by trying to conserve as much energy as possible. Saving energy is also the norm for most of Idaho's native trout species as they find a deep run or pool to make their new winter home. Surprisingly, conserving energy during the winter months is not what bull trout in the North Fork of the Clearwater River and Dworshak Reservoir do. For the last four years, the Idaho Department of Fish and Game has been studying bull trout in the Clearwater Region, particularly in Dworshak Reservoir and the North Fork Clearwater River basin. The overall goal of the project is to acquire life history and population information to substantiate a delisting of bull trout in the North Fork Clearwater River. Bull trout, frequently called Dolly Varden, have declined both in abundance and across their historic range. As a result, they have made national headlines after being listed as a threatened species by the U.S Fish and Wildlife Service under the Endangered Species Act. The study is designed to determine the extent bull trout use the reservoir and what areas are used in the basin at different times of the year. Bull trout are captured and have a small radio transmitter implanted in their abdomen. The transmitters allow us to locate the fish throughout the basin as they migrate during the year. Over the last four years biologists have captured and radio-tagged well over 500 bull trout and discovered some amazing things. The fish have been found to migrate upwards of 100 miles within a summer to spawning areas in headwater tributaries, some as far as the Idaho-Montana border. It is quite the feat for a 14-inch fish to swim upstream 100 miles in a couple months to spawn and then, within two weeks, make the long journey back to Dworshak Reservoir. Research shows that the reservoir is where bull trout spend the majority of the year from October to June, however, the reservoir is where biologists have the least amount of information. Researchers knew when the trout were in the reservoir but the fish would simply disappear off radar while they were there. The radio transmitters used at first were not effective in tracking bull trout because of the depth the fish inhabited in the reservoir. But with technological advancements in telemetry equipment in 2003, researchers began using a different type of transmitter which has allowed them to document bull trout occupying the entire reservoir from the dam to the upper reaches during the winter and early spring. As mentioned earlier, the most interesting discovery is that bull trout in the reservoir do not behave like other trout or other bull trout outside of Dworshak. Most bull trout that winter in a river or large stream move to a single run or pool and remain there throughout the winter. But in Dworshak, we've documented bull trout movements of two to three miles a day. These extensive movements have raised more questions than they have answered. The research has continued with a new element added to address the questions of why do the trout move around so much, and how is this beneficial to their survival. In the fall of 2004, research crews began implanting transmitters that would detect both the depth and temperature bull trout sought out while in the reservoir. Biologists want to know if the trout were seeking preferred temperatures or were they seeking out kokanee, their presumed prey. Researchers hope this research will provide the information needed to allow Idaho anglers to once again harvest bull trout in the North Fork Clearwater River. Danielle Schiff, fisheries research biologist for IDFG, has been studying bull trout in the Clearwater Region the last four years.