8208 - Laserfiche WebLink

Home Browse Search

summer and winter flows be met, but spring flows with sufficient flushing capability need to be provided with routine frequency. Habitat Heterogeneity A component of habitat quality often overlooked in many instream flow models is habitat diversity. Osmundson and Kaeding (1991) found that adult Colorado squawfish in the Grand Valley prefer river segments with a complex morphometry over those that are simple, i.e., squawfish were located in complex river segments more often than availability of those sites would predict if selection was random. Osmundson and Kaeding (1991) hypothesized that selection for complex channel areas (containing one or more islands, large backwaters, or side channels) was due to greater habitat diversity within these areas. A suite of mesohabitats in close proximity to one another allow fish to efficiently exploit a range of habitat types when fulfilling daily requirements (foraging, resting, avoiding predation, etc.). Energy that would otherwise be expended for excess travel among habitat types can instead be conserved for growth or reproduction. Osmundson and Kaeding (1991) stressed the need for high spring flows to create and maintain channel complexity so as to preserve habitat diversity. Here we explore the role of stage in influencing habitat diversity. We make the same assumption that clusters of various habitats are beneficial to squawfish, but rather than compare complex with simple channel segments, or investigate how spring flows affect channel complexity, we take the analysis in a different direction to see how habitat diversity within a complex channel area changes as a function of flow level or stage during base flow conditions. Thus, assuming channel configura- tion stays relatively constant during that portion of the year following spring runoff, is there a range of flows during summer and winter at which habitat diversity is maximized? Numerous indices have been devised to measure species diversity within ecological communities (see Odum 1971). We explored the use of these indices as a means to measure habitat diversity, replacing types of species with types of habitats. Diversity indices are generally of three types: 1) those that measure species richness, i.e., number of different species; 2) those that measure species evenness, i.e., number of individuals of each species; and 3) those that take both richness and evenness into account. The widely used Shannon-Weaver index of overall diversity (Shannon and Weaver 1963) falls under the third type listed above. We attempted to use this index but found that. 1) habitat richness, or number of different habitat types, did not vary within our study sites from one flow level to the next (excepting the addition of the gravel pit pond at site 3 at flows of 4,426 cfs or higher); and 2) maximizing habitat evenness, i.e., the flow at which proportions of each habitat type are most nearly equal (either in terms of area or number of mapped polygons) was somewhat contrary to our objective of maximizing the area of preferred habitat types. Diversity, using the Shannon-Weaver index, was highest at 4,426 cfs. Density turned out to be the measure best describing how an individual fish might exploit neighboring habitat types with minimal energy expenditure. Density can be measured as either the number of mapped polygons (discrete habitat units) per unit area of water or as polygons per unit length of river. For polygons per hectare of water, we summed the total number of polygons across 46