9566

• UCREFRP Catalog Number: 9566
• Author: Miller, W. J., D. E. Rees and J. A. Ptacek.
• Title: Final Report - Elkhead Reservoir Escapement Recovery Program Project No.
• USFW Year: 2005.
• USFW - Doc Type: 118, Investigation of nonnative fish escapement from Elkhead Reservoir.
• Copyright Material: NO

The escapement sampling in 2003 and 2004 confirmed that fish leave Elkhead Reservoir during
periods of high flows associated with runoff. The majority of the fish captured during netting
were less than 150 mm in total length during 2003 and less than 100 mm total length in 2004.
The rate of escapement (and size of fish) appeared to be positively related to the level of
discharge (Figure 12). Most of the fish that were captured each year were acquired during
periods of relatively high outflow from the reservoir. In 2003 (a year with higher sustained
runoff from snowmelt) the rate of escapement was consistently greater than escapement found in
2004 (Table 4). During each year the rate of escapement was also greatest during weeks with
higher outflow.
Escapement was dominated by black crappie and bluegill with crappie dominating the early
season catch and bluegill becoming dominant in subsequent weeks. Species composition results
from other fish sampling investigations within critical habitat on the Yampa River have
demonstrated captures of black crappie to be sporadic and relatively minor, and those of bluegill
to be almost nonexistent despite the numbers of these species evident in outflow escapement
(Anderson 2003, 2004; Bestgen and Hawkins 2004, Nesler 1995). There may be a seasonal
influence on escapement related to species composition, but this was difficult to detect given the
short duration of this study. An outflow from the reservoir that persisted through the month of
June would be necessary to provide accurate information regarding the seasonal influence on
species composition.
The results of this study suggested that several other physical and biological processes may also
influence escapement. Much of the data indicated a diel pattern to escapement with the majority
of the fish moving through the spillway near sunrise, sunset or after dark. This may be related to
fish becoming more active with the onset of darkness thus increasing the potential for
entrainment in the outflow. Wave action at the spillway may also result in an increase in the rate
of escapement. Very few large individual fish were captured during the study. The swimming
ability of larger size classes may provide an advantage in the avoidance of entrainment (Bell
1991). This ability may not yet be realized in younger fish of smaller size classes.
No tagged nonnative gamefish relocated to Elkhead Reservoir were documented in the
escapement, however, one smallmouth bass in the same size class as those stocked but not
tagged in Elkhead Reservoir in 2003 and 2004 was captured in the nets. Since other adult
nonnative gamefish were captured it is possible that relocated fish could escape from the
reservoir. The escapement of relocated fish is most likely low based on the low numbers of bass
captured in the escapement study.
Screen Options
Screen options for Elkhead Reservoir range from fixed screens to kevlar nets in front of the
spillway. Several options were considered in this study based on prior feasibility evaluations of
structures to control escapement (Miller and Laiho, 1997). Structures that may work at Elkhead
to control escapement include traditional fixed screens in front of the spillway. This would be a
steel wedgewire to screen all flow from the spillway and outlet works. A second option is a
kevlar net placed in front of the spillway and work in conjunction with screens on the low level
outlet works. A third option is to screen the low level outlet works or operate the release from
the reservoir to minimize spill.
FINAL REPORT -Elkhead Reservoir Escapement, Project No.: 118 Page 17
Miller Ecological Consultants, Inc. May 20, 2005