8114

• UCREFRP Catalog Number: 8114
• Author: Nesler, T. P.
• Title: Five-year Stocking Plan for Endangered Colorado River Fish Species in Colorado.
• USFW Year: 1998.
• USFW - Doc Type: Denver, CO.
• Copyright Material: NO

fish (based on data in Carlander 1969) was used for the biomass calculation for fathead
minnow. This resulted in an estimate of 539 kg/ha for 3.9 ha total of backwater habitat, or
40kg/mi. Backwater prey species were dominated by flannelmouth sucker, bluehead sucker,
roundtail chub, white sucker, and fathead minnow. A further extrapolation was made to
estimate a mainchannel shoreline biomass for prey species based on an assumptions that this
component would be composed of small-bodied. minnow species, would not be effectively
sampled by the mainchannel electrofishing technique used by Anderson (1997), and would
equal only 10% of the biomass estimate for backwaters. This resulted in an additional prey
fish biomass of 74 kg/mi. Combining the biomass estimates for these three components of
the prey fish community available to adult Colofado pikeminnow resulted in a total prey fish
biomass of 233 kg/mi in the Palisade to Rifle reach. The biomass of a target density of 10
adult Colorado pikeminnow per mile would equal about 11 percent of the available fish prey
base biomass in this reach.

For the Gunnison River, Burdick (1995) showed a similar number of fish collected in
backwaters-22,988 in 1992 and 23,918 in 1993 versus 22,659 in Anderson (1997).
Backwater area sampled by the two studies showed Burdick (1995) sampled 3,391-4,446 m2
in the Gunnison River compared to 6,261 m2 by Anderson (1997) in the Colorado River.

For each river reach, achievement of stocking objective would result in an adult
population of 500 adult fish aged 6-8, or 1,500 fish distributed over 150 river miles of three
reaches in the Upper Colorado River subbasin. This total is in addition to the population of
650 fish estimated for the currently occupied reaches of the Upper Colorado River in
Colorado and Utah. Interim management objectives (1Mb) for Colorado pikeminnow
(Lentsch et al. 1998) indicate at least 3,400 adult fish are necessary as an effective
population size to maintain long term genetic diversity. Using a demographic model
developed by Crowl and Bouwes (1998), simulation results show a mean population or
subpopulation size of 5,500 Colorado pikeminnow (;tl,OOO fish) are necessary to be 95%
confident the population will be maintained over a 100 year period. Similar to razorback
sucker IMOs, the ambiguity of how the population abundance objective is distributed over
available river reaches in the Upper Colorado River confuses comparison of target population
size. If one considers the occupied reach, and each of the three target river reaches
designated in this plan as supporting a subpopulation, then the IMO objective for this
subbasin would be up to 22,000 adult Colorado pikeminnow. The IMO population
objective/reach represents 110 fish/mile and slightly greater than five percent of the fish
community biomass estimated in Anderson (1997).

The IMO objective for Colorado pikeminnow uses a sex ratio of 4.5 males: 1 female,
highest of the four listed fish species in the recovery program; and a 50 % contribution of
adults to succeeding generations to estimate the total population size (5,500) needed to
maintain.an effective breeding population of 1,000 adult fish. The sex ratio information
provided in Lentsch et al. (1998) for Colorado pikeminnow is based on results from studies
that reported sex of fish captured in spawning condition, but were not designed to investigate
spawning population parameters such as total spawning population size. gender composition.

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