9588

• UCREFRP Catalog Number: 9588
• Author: Bestgen, K. R. and e. al.
• Title: Population Status of Colorado Pikeminnow in the Green River Basin, Utah and Colorado.
• USFW Year: 2005.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

none were recaptured among years (two individuals within the same year). Non-existent captures
among years would cause the survival rate function to decline to near zero for the large fish
rather than increase as it did for the 1991 to 1999 data. Non-existent recaptures of large
individuals suggested that they were either very difficult to capture because they learned to avoid
the capture gear, used habitat where capture gear was not effective, or that some individuals died.
Lower survival rates of Colorado pikeminnow in the Green River Basin in the period 2000
to 2003, particularly for larger-bodied fish, may be due to several factors including negative
effects of handling (including tagging) or electrofishing, stress-induced mortality due to low
streamflow levels in drought years 2002 and 2003, expanding populations of introduced
predaceous fishes such as northern pike, or a combination of these and other yet undiscovered
factors. Although population declines appeared consistent with increased mortality due to effects
of electrofishing or handling, we did not find support for that hypothesis. The number of
Colorado pikeminnow mortalities that we observed as a direct result of capture during this study
were small (N = 6 over the entire study), so high levels of electrofishing-induced or handling-
induced mortality were not evident. Colorado pikeminnow recaptured in multiple passes within a
year generally appeared in good condition, with no externals signs of damage (e.g., bruising),
handling-induced disease (e.g., fungus), or other ill effects. We have no data to address effects of
delayed mortality on Colorado pikeminnow, except that electrofishing-captured fish implanted
with radio transmitters for use in telemetry studies generally have high survival (Tyus 1990;
Kitcheyan and Montagne 2005).
In support of observations of healthy fish captured and recaptured in the field, independent
evaluations did not suggest electrofishing was harmful to Colorado pikeminnow. Controlled
laboratory studies (Meismer 1999) found minimal soft-tissue and vertebral damage from the
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