9299

• UCREFRP Catalog Number: 9299
• Author: Anderson, R. and J. Mumma.
• Title: Aspinall Studies
• USFW Year: 1999.
• USFW - Doc Type: Annual assessment of Colorado Pikeminnow Larval Production in the Gunnison and Colorado Rivers, Colorado 1992-1996.
• Copyright Material: NO

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Doug Osmundson, pers. comm.). However catch rates and estimates for total transport
abundance were lower at Westwater than at Loma in all five years. This may indicate that
Colorado pikeminnow larval mortality exceeded production between Loma and
Westwater. This could also indicate that many larvae left the drift and shifted to active
rather than passive drift behavior. This shift may occur in larvae over 11 mm, which were
not collected in the drift. Another possibility is that it could be an artifact of data
manipulation, since seasonal densities are difficult to standardize due to the large number
of zero-fish days.

Seasonal drift densities or total abundance for Colorado pikeminnow larvae at
Loma were not correlated with Westwater indicating that the larval population sampled at
Loma was different at Westwater. With current speeds of only 2 ft/sec, passively drifting
larvae could move between Loma and Westwater (26 miles) in less than 24 hours. If that
is indeed what happened, stronger correlations for drift density for these two sites would
be expected. The weak correlation for drift densities between Lorna and Westwater does
not support assumptions that upstream (Lorna) drift densities strongly influence abundance
at Westwater or even that Lorna and Westwater are sampling the same larval population,
though offset temporally.

Drift densities at Loma and Moab, separated by over 100 miles, were more
strongly correlated than Lorna-Westwater or Westwater-Moab. The highest drift densities
at both sites occurred in 1995 and combined with results from other sites suggests
increased larval pikeminnow production is associated with high runoff flows. However
the year with the lowest drift density at Loma (1996) was a high year for pikeminnow in
the drift at Moab. Since the intermediate flow (1996) year did not produce intermediate
larval densities at either Loma or Westwater, some non-related flow variable appears to
have affected larval production in those reaches that year.

Two examples of a non-related flow variables that could impact larval production
are the number of spawning adults in a given year or the location of spawning sites varies
between years. Therefore in 1996, increased spawning efforts downstream of Westwater
coupled with decreased spawning effort upstream ofLoma would be consistent with the
sampling results. However there is no data available on variability of spawning efforts
between years and this study assumed that the number of spawning adults and spawning
locations in a river reach tend to vary in a similar manner relative to other reaches.
Whatever the cause, the 1996 data clearly shows that larval densities in Moab were not or
were only weakly influenced by larval production upstream at Loma. The fact that there
were similar annual trends in density in the other four years suggests larval production and
possibly spawning efforts varied in a similar manner for those two reaches and probably
due to flows or other environmental conditions.

The highest catch rates were at Moab, except in 1994 when Loma was highest.
Colorado pikeminnow larval drift density was poor at all sites in 1994, and a poor year in
regard to flow conditions conducive to transport larvae via a passive drift. Total
estimated abundance was higher at Moab than at Loma or Westwater for all five years,

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