Identifying Bull Trout Migration in the Upper Boise River Basin

Science Center Objects

In 1999, the U.S. Fish and Wildlife Service (FWS) listed the bull trout (Salvelinus confluentus) as a threatened species. In 2010, the FWS designated critical habitat for the bull trout, including the upper Boise River basin. The Bureau of Reclamation (Reclamation) operates three reservoirs on the upper Boise River for irrigation and flood control. Reclamation and FWS have a need to understand if these reservoir operations may be contributing to the decline in the bull trout population. In particular, the effects of regulated streamflow may be altering bull trout migration and successful spawning and may be reducing their growth and/or longevity.

We implanted 18 bull trout with acoustic tags and tracked them between April-August 2012 using mobile and fixed receivers. We analyzed fish movement and streamflow data to examine the effects of reservoir operations on bull trout migration and to address, on a regional scale, how climate change is affecting the threatened bull trout and other freshwater biota.

Recently, the USGS and Bureau of Reclamation began developing a decision- support tool using telemetry data to help managers optimize reservoir operation and protect migrating bull trout.

Individual bull trout horizontal travel ranged from 0.1 to 11.8 km.

There were no bull trout detected in the deepest part of Arrowrock Reservoir near the dam. This is evidence that entrainment through Arrowrock Dam would be unlikely during spring to summer, when discharge releases would be highest.

Although colder water temperatures are available in the South Fork Boise River throughout the summer, the predominant movement patterns were in the Middle Fork arm of Arrowrock Reservoir.

Detected bull trout were over deep water habitat ranging from 8.0 to 42.6 m. Actual fish depths were shallower, ranging from 0.0 to 24.5 m. The last acoustic-tagged bull trout was detected on June 1, 2012.

From the first week of August through the latter part of September, little if any suitable habitat remained for bull trout in Arrowrock Reservoir, with most temperatures exceeding 15°C at all locations where water-quality profiles were measured.