9565

• UCREFRP Catalog Number: 9565
• Author: Bestgen, K. R. and e. al.
• Title: Population Status of Colorado Pikeminnow in the Green River Basin, Utah and Colorado
• USFW Year: 2007
• USFW - Doc Type: Transactions of the American Fisheries Society
• Copyright Material: NO

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negative coefficient for the quadratic term forced the
relationship to decline for large fish. We hypothesize
that relatively small or very large fish are more difficult
to capture because of habitat use or behavioral
differences. Small fish can occupy relatively shallow
water or small backwaters, where accessibility by
electrofishing boats is limited (Tyus and Haines 1991).
Large fish may occupy particularly deep water where
electrofishing is simply inefficient or may be powerful
enough to evade the electrofishing field when it is
detected, While it is possible that there were more
small fish in the system than our abundance estimates
suggest, it is unrealistic to speculate that large numbers
of very large and old individuals went undetected by
our sampling efforts. Thus, we believe that low capture
probabilities for large adults have caused negligible
bias of abundance estimates for adult Colorado pike-
minnow.

Transition Rates

Transition probabilities reflected a general upstream
movement pattern for relatively small Colorado pike-
minnow from the lower Green River and the
Desolation-Gray Canyon reaches. This is consistent
with the size structure information presented here and
elsewhere, which shows relatively high abundances of
small Colorado pikeminnow in those reaches, includ-
ing recruits, compared with other reaches (Tyus 1991).
This pattem was also consistent with that in the
Colorado River, where relatively small-bodied Colo-
rado pikeminnow reared in lower sections of the river
and eventually moved upstream as subadults or adults
(Osmundson et al. 1998). The prevailing wisdom
suggests that recruit-size Colorado pikeminnow and
smaller juveniles in downstream portions of the Green
River basin would also generally move into upstream
areas and establish home ranges (e.g., Irving and
Modde 2000). Net movement was neutral or negative
to . upstream reaches such as the Yampa and White
rivers, however. Perhaps drought or other conditions
forced fish to move from relatively small upstream
reaches to larger ones such as the middle Green River,
a reach where the net annual Wi was positive.
Additional empirical analysis of the growth and
movements of Colorado pikeminnow based on tag
recaptures would also be useful to further understand-
ing of the ecology of Colorado pikeminnow in the
Green River basin, as was demonstrated by Osmund-
son et al. (1997, 1998).

Applications

This study has demonstrated that the use of
coordinated multiple-pass, capture-recapture sampling
to obtain reliable abundance estimates for fish in large

BESTGEN ET AL.

drainage basins is logistically possible and can
successful even when some reaches are in remote
white-water canyons. Future efforts to obtain abun-
dance estimates to address recovery goals for Colorado
pikeminnow should follow a similar approach. Sam-
pling for Colorado pikeminnow should also continue to
focus on high-density areas that support the majority of
the Green River population. In river reaches where
relatively few Colorado pikeminnow reside, increased
sampling efficiency is needed to improve the precision
of abundance estimates.
This study illustrates the complexity of the abun-
dance dynamics of big-river fishes like Colorado
pikeminnow that are long-lived and provides further
insights into the multitude of factors that affect them. In
our study, the abundance of adult Colorado pike-
minnow declined because of reduced survival and
recruitment. However, it was difficult to ascertain
whether the reductions were from anthropogenic or
natural causes or some combination of the two.
Recruitment of young to the adult life stage requires
several years, and recruit abundance may be affected
by flow regulation from upstream Flaming Gorge Dam,
habitat quality and availability, stochastic events such
as floods or drought, the interacting effects of abiotic
factors and predation, and other factors (Haines and
Tyus 1990; Tyus and Haines 1991; Bestgen 1996;
Bestgen et al. 2006a). Several of those factors may be
responsible for the changes in the abundance of adults
since 1991, which showed a large apparent increase in
population size up to 2000 but then declined. The
reductions in the abundance and survival of adult and
recruit -size Colorado pikeminnow in the Green River
basin may also be related to low, drought-related base
flows that began about the same time as this
investigation or to the increased distribution and
abundance of nonnative predaceous fishes. Thus,
although it is difficult to determine what specific
factors have caused Colorado pikeminnow populations
to fluctuate since the early 1990s, a better understand-
ing of the mechanisms underlying their abundance and
survival is needed to prevent future declines.
The results presented here also suggest that the
demographic parameters for endangered Colorado
pikeminnow do not meet recovery goal criteria for
downlisting or delisting at the present time (U.S. Fish
and Wildlife Service 2002). The Green River basin
criteria require relatively stable abundance over a 12-
year or longer period, with recruitment sufficient to
maintain a minimum of 2,600 adults. The recovery
goals also require, among other things, the provision of
adequate habitat and the reduction of other threats
(U.S. Fish and Wildlife Service 2002; Valdez and
Muth 2005). This work has begun with the reregulation

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