An Evaluation of Fish Community Structure and Habitat Potential for CO Squawfish and Razorback Sucker

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The direct influence of flow on the fish community was studied by comparing density, size and age structure of two river reaches with flow condition over the two -year study period. Admittedly, two years of data is insufficient to statistically determine relationships between flows and the fish community. However, implications made from only two years of fish data were strengthened by the fact that the fish sampling done in March 1994 was conducted following a very high flow year and the March 1995 fish sample immediately followed a very low flow year. The minimum flow prior to fish sampling in 1994 was 1430 cfs, and it was 1200 cfs prior to sampling in 1995. Density and biomass estimates were very similar between these two years, which suggested that the observed minimum stream flows had not differentially impacted survivorship of any species or older life stage in the fish community. A relationship was indicated for peak flow conditions and numbers of young - of -year fish. In early June 1993 flow peaked at 22,000 cfs (fish were sampled the following March), while peak flow in 1994 was only 11,000 cfs. Fish larvae and fry numbers were much higher in March 1995, following the low flow year, compared to fish of this size collected in March 1994. It was postulated that roundtail chub and white sucker nursery habitat conditions in the main channel were improved in the 1994 summer compared to 1993, which could explain the increase in number of small fish in March 1995 compared to March 1994. However, since neither Colorado squawfish or razorback suckers are currently part of this fish community, no empirical data was available for studying effects of maxima or minima flows on the habitats of these species. In spite of this, the author speculated that the minimum flow, which apparently was adequate to protect habitats of the native sucker and chub (1,200 cfs), would also probably provide adequate protection for adult Colorado squawfish and razorback sucker habitats. Insufficient data was collected to determine the impacts of peak flows on habitat availability for endangered, other native and nonnative fishes. It is believed that razorback sucker exploit low velocity backwater and flooded habitats during runoff for their nursery and rearing habitats and therefore were vulnerable when these habitats were severely impacted by channelization activities and reduced peaked flows. Remaining floodplain ponds and backwaters were found to be dominated by nonnative species. Further reductions in peak flow, without compensatory channel restructuring, could likely result in further habitat degradation and concomitant increases in nonnative fish numbers thereby complicating recovery efforts. Availability of spawning and nursery habitat for Colorado squawfish was not considered limiting. Adults residing above the diversion dams may chose to move to downstream spawning sites. Even if squawfish spawn above the dams, e.g. in Debeque Canyon, larvae would probably display a strong tendency for downstream dispersal to nursery areas. It was suggested that experimental stocking of small razorback sucker and Colorado squawfish could be useful for studying effects of peak flow on habitat availability for fish of this life stage. Key words: Colorado River, fishery assessment, habitat assessment, endangered fish, species composition, flow recommendations, endangered fish stocking viii