No threatened bull trout that were radio tagged were detected in the deepest part of Arrowrock Reservoir, suggesting that passage through the dam is unlikely when farmers are tapping into it for irrigation, a USGS study states.
The new study released Thursday showed bull trout stay in the cooler waters of the lake in the winter, fall and spring and are gone by June 1. Then they migrate up the Boise River forks above to spawn.
Scientists with the U.S. Geological Survey tracked the bull trout through the reservoir using acoustic telemetry, in cooperation with the U.S. Bureau of Reclamation, which manages the reservoir. The study is one of several the Bureau of Reclamation is conducting with its partners to protect Arrowrock’s bull trout population, a news release said.
In the USGS study, 18 bull trout were captured and implanted with acoustic tags that transmitted data about each fish’s location, body temperature, and the depth at which the fish was swimming. Scientists tracked the fish from April through August 2012 in three parts of Arrowrock Reservoir: the Middle Fork Boise River arm, the South Fork Boise River arm, and the main body of the reservoir. The scientists also recorded water temperatures and dissolved oxygen concentrations in the reservoir, important factors for fish survival.
Biologists learned that bull trout traveled an average distance of 3.23 miles within the reservoir in areas that were on average three degrees lower than surface waters. From the first week of August through the end of September, little if any suitable habitat remained for bull trout within the reservoir, because it was too hot.