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,- 99 i , 98 This brief and incomplete account of trout behavior and ecology indicates the importance of water velocity, and the complexity of the relationship of fish ecology to streamflows. Unfortunately, the inter- relations among and between species add additional variables that make interpretations even more difficult. so that habitat(s) occupied during spring and winter cannot be defined precisely. Some information is available on the water depth and velocity used for spawning by some warm-water and cool-water fish (Bovee, 1974; table 7). Several species listed in table 7 have been introduced and have become Other Fish established in the Colorado River, such as the white sucker (Catostomus Few data are available on streamflow requirements of most fish species inhabiting the larger main tributaries of the Colorado River. Ongoing research by the Utah Cooperative Fishery Research Unit may pro- vide data on the spawning habitats of Colorado squawfish, humpback sucker, flanneLmouth sucker (Catostomus latipinnis) and bluehead sucker (Catostomus discolobus), In general, the suckers appear to require a _gravel substrate and a moderate water flow for spawning. Except during spawning, they may be found in eddies or at the interface between the eddy and the main river current. Adult Colorado squawfish and round- tail chub are generally found in pools, riffles, or eddies, Ripe round- tail chubs were collected in shallow pools and eddies over rubble or boulder stream bottoms covered with silt (Vanicek and Kramer, 1969). However, because it has not been possible to effectively sample fish in fast current in the main rivers, uncertainty exists about habitat use, In addition, passive gear such as gill nets and trammel nets have been used to collect fish in these rivers. Because these nets depend on fish movement for capture, and because the fish may have been moving between habitats when captured, tbere is no way of knowing what part of the stream the fish were inhabiting. Also, high runoff during the spring, and ice during the winter, preclude effec~ive sampling of these species commersoni), largemouth bass (Micropterus sal.oides), and walleye (Stizostedion vitreum). The creek chub was reported as native to Utah (Richardson, 1976) but is now found in Colorado and Wyoming as well, Inferences can be drawn from table 7 concerning spawning requirements of related species in the Upper Colorado River, but further generalizations beyond this are risky. In tbe Lower Colorado River, backwater marsh babitat was never ex- tensive but was extremely important to various species of wildlife (Ohmart, Deason, and Freeland, 1975). This habitat is also important to ~ . . ~ ~ ( the endemic fish in tbe Upper Colorado River, Vanicek and Kramer (1969) < , 4 ~ - reported that young Colorado squawfish and chubs were commonly taken in quiet water or sballow pools over silt, sand, and occasionally gravel ~ , , , , 1 ~ 4 bottoms. These backwater habitats may become filled with accumulations of silt and organic materials if the streamflow is reduced, and the natu- ral flushing action of tbe spring runoff is necessary to keep these habi- tats open or to create new eddies or backwaters. Reduction of the annual peak flows by storage reservoirs on the Upper Colorado River may reduce these critical nursery areas for tbe endemic fish species. Another problem concerning the streamflow of the Colorado River is , :1 ~ ~ ~ i the continual reduction of the flow by depletions. Most (62 percent)