Brown Trout Removal in the Cache La Poudre River, Colorado: Managing Whirling Disease Resistant Rainbow Trout Introductions in Brown Trout Dominated Streams

Whirling disease (WD) resistant rainbow trout have been reintroduced to several rivers in Colorado, with the intention of creating self-sustaining, WD resistant rainbow trout populations.  Despite several stocking events, survival and recruitment are low, and self-sustaining populations have not yet established themselves.  One explanation is that large brown trout populations, which replaced rainbow trout after WD was introduced, prevent the re-establishment of rainbow trout due to increased predation and competition.  To evaluate whether brown trout prevent re-establishment, we introduced WD resistant rainbow trout into two sections of the Poudre River.  In one section we removed brown trout (removal section; 0.6 mile), and in the other, located five miles downstream, we did not (control section; 0.8 mile).  Removal occurred in August 2010.  All brown trout removed were returned to the river below the narrows, fifteen miles downstream, thought to be a barrier to upstream movement.  Brown trout upstream and downstream of the removal section were Radio Frequency Identification (RFID) PIT tagged during the removal operations.  Brown trout upstream and downstream of, as well as within, the control section were also RFID PIT tagged.  Two flat-bed antennas were installed at the upstream and downstream ends of both the removal and control sections (8 antennas) and were used to monitor movement of the PIT tagged fish.  After removal, two thousand PIT tagged, WD resistant rainbow trout, one thousand each of the Hofer x Colorado River Rainbow (HxC) and Hofer x Harrison (HxH) strains, were stocked into both the removal and control sections.  Passive detection of the PIT tagged fish at the antenna locations, and adult population estimates, have been used to monitor the survival, retention, and movement of the rainbow trout in both sections.  Current results indicate that removing brown trout had a short-term positive effect on retention and survival of introduced rainbow trout.  In October 2010, an estimated 854 (43%) introduced rainbow trout still resided in the removal section, in comparison to 504 (25%) in the control section.