WSP07697

• File Number: 8240.200.10.B
• Description: UCRBRIP - Riverine Fish Flow Investigations
• Basin: Colorado Mainstem
• Date: 7/1/2001
• Author: CDOW
• Title: Riverine Fish Flow Investigations 2001
• Water Supply Pro - Doc Type: Publication

strongly influenced by predation and competition, which means size structure, as well as

species composition, is not regulated by habitat availability and therefore is not what would be

expected if nonnative predators were absent.

No small roundtail chub were collected in the Yampa River in 2000 and size ranged

from 33 to 44 em at Sevens (Figure A13) and from 41 to 48 em at Duffy (Figure AI4). Only

one chub was collected at Lily Park in 2000 at 40 em (Figure AI5). On the Dolores River

there were only small chub and sizes ranged from 2 cm to 30 em (Figure AI6). On the

Colorado River both large and small chubs were present at Com Lake (Figure A 17) and at

Clifton (Figure AI8).
Size structure of roundtail chub has the same pattern as described for the native

suckers, large fish in the Yampa, small fish in the Dolores and all sizes in the Colorado River.

The fact that all three native species have similar tendencies in size structure indicates similar

factors are responsible. The lack of a large predator population and the fact there is a high

percent of native fish in the population in the Colorado River indicates size distribution

reflects the potential that is a function of habitat availability. Length frequency for native fish

on the Colorado River is probably nearest to pristine or ideal for the native species, bluehead

and flannel mouth sucker and roundtail chub. However the Dolores River has a definite

absence oflarger individuals of all native fish. This is probably not a function of predation or

competition, since predators would remove small not large fish. The lack oflarge fish is

probably a function of habitat availability, indicating a lack of deep runs and rimes in this

river. At this time it is not hypothesized whether size structure on the Dolores is pristine or is

a recent result of habitat or flow modification which has reduced potential. The author felt

that sedimentation in rimes was high and invertebrate numbers were relatively low. Native

suckers have a normal shape to their length frequency larger fish at Lily Park site on the

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