WSP05885

• File Number: 8240.200.38.A
• Description: Coordinated Reservoir Operations
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/1998
• Author: CWCB
• Title: 1998 Coordinated Reservoir Operations Bypasses for Endangered Fish - Annual Summary of Operations for 1998 to Benefit the Endangered Fishes of the Upper Colorado River Basin
• Water Supply Pro - Doc Type: Publication

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range were caught that were probably hatched in the early 1990's. In addition, the new 1998
population estimate (using Program CAPTURE) of the lower-reach group indicated that this part of
the population has remained steady at about 300 individuals (N = 328; 95% c.1. = 193-635).
However, in the upper reach, the catch rate was substantially higher than in 1994 when the last such
survey was done: in 1994 the catch rate was 0.69 fish/net; in 1998, it was 0.97 fish/net, or about a
40% increase (see FigA). Compared to the catch rate in 1991 (0.32fish/net), the 1998 results
indicate a 200% (triple) increase in densities of adults in the upper reach. The new population
estimate suggests that catch rates may over-estimate the increase: the 1998 upper reach point
estimate is 43 5 fish; thus, about a 31 % increase over the 1994 estimate (3 32fish) and a 100 %
increase over the 1991 estimate (205 fish).

However, the most interesting aspect of this year's results was the poor condition of adults. Everyone
handling the fish noted that they looked skinny and 'snake-like'. When we calculated relative
condition and partitioned results by length-class and reach it was readily apparent that fish of all
sizes and throughout most of the river were in poor shape. The pattern of declining condition with
length in the lower reach and increasing condition with length in the upper reach, as noted in the
earlier study, still held true. However, except for the largest and smallest fish, fish of most
length-classes had lost considerable weight.

Based on the 1998 data, a preliminary interpretation is that the carrying capacity for the occupied
portion of the Colorado River has been reached. Habitat capacity is defined as the level above which
emigration occurs (Mesick 1988; Bartholow et at. 1994). In the lower reach, this was probably
reached during the 1991-1994 study when fish lost condition and emigrated to the upper reach.
Numbers there have apparently remained stable since then despite more fish recruiting to the adult
population. In the upper reach, adults had evidently not exceeded their food supply by 1994.
However, based on body condition, by 1998 they had.

Other evidence along these lines is-the history of use of the Redlands fish ladder on the lower
Gunnison River. In its first year of operation (1996), only one squawfish used the ladder; however, in
1997, 18 used the ladder, and in 1998, 23 fish used the ladder. Emigrationfrom the Colorado to the
Gunnison river clearly picked up in 1997 and may indicate when carrying capacity in the Colorado
River was reached. Hopefully, additional data collection in 1999 and 2000 will shed more light on
this subject. "

Benefits of Coordinated Reservoir operations are discussed in the standardized monitoring
efforts. In addition, on February 13, 1999, the FWS provided a report on releases from Ruedi
Reservoir, which included discussion of the benefits of peak flows. This report is included in
Appendix B.

IX. Issues to Address

. Runoff and Flow Prediction Imorovements

At present there are 23 SNOTEL sites located in the Upper Colorado River Basin. The sites
range in elevation from 8,700 feet above MSL to 11,400 feet above MSL. These sites provide a
fairly good indication of what the snow pack is. However, additional information on how melting
ofthe snow pack translates into stream flows would be helpful. The rough rule of thumb is that
when the snow water equivalent at the SNOTEL sites reaches 10 inches, the peak runoff (or very
close to it) occurs. Refinement of this approach and better estimates ofthe melt rate would be .
beneficial.

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