Four general categories of instream flow methods were evaluated to determine their biases relative to each other. The categories included (1) the Termant method, (2) wetted perimeter curves, (3) habitat retention models, and (4) physical habitat simulation (PHABSIM) models.The Tennant method (30% of average flow) was one of the least biased methods, but it does not include biological data and is incapable of identifying trade-offs. No wetted perimeter methods were significantly unbiased, and methods relying on subjective identification of inflection points were biased upwards. Two habitat retention methods were significantly unbiased. These methods included (1) the mean recommendation of all riffles in a study reach where all three criteria are met, and (2) the recommendation for the single most "critical" riffle in a study reach where two of the three criteria are met. No PHABSIM models were unbiased. The IFG-4 model was biased upwards for small streams and low for large streams.

An assessment was made of the biological validity of weighted usable area (WUA) from the physical habitat simulation (PHABSIM) model based on standing crops of trout (Salvelinus and Salmo spp.) measured in Wyoming streams and standing crops predicted by the habitat quality index (HQI). Tests were made in trout streams for (1) validity of the HQI, (2) relationships between WUA and measured standing crops in different streams, and (3) relationships between WUA and the HQI within streams. Significant correlation (r = 0.934; P < 0.05) was found between HQI scores and trout standing crop during the low-flow period. No significant correlation was found for WUA and the measured standing crop among different streams; correlation coefficients for all tests were either near zero or moderately negative. Significant positive correlations (P < 0.05) did exist for 7 of the 60 within-stream analyses of WUA versus HQI; 19 other positive correlations were strong (r > 0.90), but statistical significance was limited by the number of data points at each site. Although positive correlations were expected for all 60 cases, 18 tests showed a negative correlation, 3 of which were significant. Analyses indicated that trout species, stream size, and stream gradient influence the validity of the within-stream relationship between WUA and the trout standing crop predicted by the HQI. Among test streams with steeper gradients and where velocity exerted the greatest influence on the HQI score, a positive correlation was observed in all cases, regardless of stream size or dominant species. When an attribute other than velocity has the greatest influence on trout density with change in discharge, WUA estimates may be invalid. This observation indicates that a relationship between WUA and trout standing crop may exist, but the nature of the relationship is likely to be unique for each stream. A variety of methods is available to establishinstream flow recommendations for fisheries American Fisheries Society 19:35-43. (Colorado State University, Fort Collins.) Orth, D., and O. Maughan. 1982. Evaluation of the incremental methodology for recommending instream flows for fisheries. Transactions of the American Fisheries Society 111:413-445. Shitveil, C., and R. Dungey. 1983. Micro-habitats chosen by brown trout for feeding and spawning in rivers. Transactions of the American Fisheries Society 112:355-367. Shitveil, C., and D. Morantz. 1983. Assessment of the instream flow incremental methodology for Atlantic salmon in Nova Scotia. Transactions of the Canadian Electrical Association, Engineering and Operating Division 22:83-H-108. Stalnaker, C. 1979. The use of habitat structure preferenda for establishing flow regimes necessary for maintenance of fish habitat. Pages 321-337 in V. Ward and V. Stanford, editors. The ecology of regulated streams. Plenum, New York. U.S. Department of the Interior. 1979. Instream flow guidelines. Bureau of Land Management Washington Office Instruction Memorandum (WO-78-534), . 1981. Response of fish and fish-food organis...

We constructed energetic models of habitat use for 82–322 g rainbow trout (Oncorhynchus mykiss) in a large regulated river, and 8–28 g Colorado River cutthroat trout (O. clarki pleuriticus) in a small headwater stream, to determine if observed summer habitat use by these species could be attributed to net energy acquisition, and to develop habitat suitability criteria based on net energy gain. Metabolic models of energy expenditure were derived from literature sources, but measurements of energy availability were site‐specific. From the energy models, we assigned a suitability value of 1.0 to the entire range of velocities where positive net energy gains were predicted, and a suitability value of zero to velocities where negative net energy gains were predicted. Predicted net energy gain velocities were compared with observed velocities used by each species. For rainbow trout, the energetic model predicted energetically profitable velocities ranging from 5 to 45 cm s−1. Predicted velocities were similar to velocities used by rainbow trout. This indicated that rainbow trout, as a group, were using energetically profitable stream locations, but some rainbow trout used non‐profitable velocities. For Colorado River cutthroat trout, the energetic model predicted energetically profitable velocities ranging from 5 to 45 cm s−1; however, Colorado River cutthroat trout used significantly lower velocities than predicted. The dissimilarity between velocities predicted and used by Colorado River cutthroat trout may be attributed to their inability to utilize energetically profitable velocities available in the stream because of depth restrictions The results suggest that the predictive abilities of energetic models vary between streams because of differences in depth and velocity availability. © 1997 John Wiley & Sons, Ltd.

Abstract:Fisheries managers have often suggested that survival of trout during the winter is a major factor affecting population densities in many stream ecosystems in the Rocky Mountains. In Wyoming, trout population reductions from fall to spring in excess of 90% have been documented in some reservoir tailwaters. Though biologists have surmised that these reductions were the result of either mortality or emigration from some river sections, the specific mechanisms have not been defined and the factors leading to the trout loss are unknown. This is a review of four studies that were conducted or funded between 1991 and 1998 by the Wyoming Game and Fish Department to understand the extent of overwinter losses, identify some of the mechanisms leading to those conditions and develop management strategies to help avoid those impacts. Winter studies were conducted on tailwater fisheries in the Green, North Platte, Bighorn and Shoshone rivers to document trout population dynamics, assess physical habitat availability, evaluate trout movement and habitat selection, and understand the relationships between food availability and bioenergetic relationships. Results indicate that winter trout losses are extreme in some years, that trout movement and habitat selection are affected by supercooled flows, and that mortality is probably not directly due to starvation. The combination of physiological impairment with frequently altered habitat availability probably leads to indirect mortality from predators and other factors.

There is little information on the winter features of salmonid habitats associated with constructed instream structures to provide guidance when planning habitat improvement projects. We assessed winter habitat features for trout of the genera Oncorhynchus and Salvelinus in pools associated with two types of instream structures constructed on a low-gradient reach of a mountain stream in western Wyoming with a mean wetted width of 6.4 m. Pool habitat was affected by temporal variability in ice formations from fall into winter. As surface ice and snow accumulated with the progression of winter, variation in ice formations was less frequent and winter habitat conditions became more stable. However, groundwater inflow that maintained water temperatures at 0.2-0.6ЊC in a portion of the study reach appeared to contribute to incomplete surface ice cover and variation in ice formations in pools through most of the winter. Hanging dams and anchor ice dams were the primary ice features that affected winter habitat in pools associated with constructed instream structures. Trout were observed in these pools in the fall but tended to abandon pools with variation in ice formations as winter progressed. The potential impacts of groundwater inflow and winter ice processes on trout habitat in pools associated with instream structures should be considered when planning habitat improvement projects.