Adult rainbow trout Oncorhynchus mykiss were observed in a 20-m reach of river to determine habitat use (at four flow levels) and preference (at two flow levels) under daily fluctuations in discharge from a hydropower peaking operation. Maximum increase in discharge was threefold (from 1.6 to 5.1 m 3 /s), which is small compared with that of some hydropower peaking operations. Available habitat (based on velocity and depth) was different under low and high flows. At the low discharge level, nearly twice as much of the lowest velocity-class (0.00-0.15 m/s) and eight times more of the shallowest depth-class (0.0-0.2 m) were available. Distributions of adult habitat use revealed that fish focal point and water column velocities increased with increasing discharge. At the highest discharge levels, more fish were found in the deepest water, and the fish assumed positions closer to the streambed, than at the three lower discharges. Fish were usually associated with boulders at all discharges. Habitat preference shifted to deeper and faster water as discharge increased. Two types of individuals were identified on the basis of habitat use under various discharge levels. Pattern-1 individuals displayed strong site fidelity and used higher focal point velocities at higher discharges. Pattern-2 individuals were generally more mobile than pattern-1 individuals and showed no relationship between discharge and focal point velocity. Repeated observations made on individually marked fish indicated that description of habitat use and preference in terms of microhabitat may yield a false interpretation of optimal habitat for the population as a whole. It is likely that few individuals in a population of territorial fish occupy the optimal habitat. Interpreting the most frequently used microhabitat as optimal habitat for a population is probably incorrect and could result in erroneous predictions of available habitat based on instream flow assessment models.

We sampled fishes at 15 stream sites in the Ozark Highlands ecoregion of Arkansas and examined the effects of sampling effort on an index of biotic integrity (IBI). We found that sampling effort affected component metric scores, IBI scores, and qualitative site classifications. The scores of taxonomic richness metrics increased with increasing effort, while relative abundance metrics were inconsistently affected by sampling effort. The scores of all metrics stabilized with increasing effort. Although some deviation from the sampling effort designated for the IBI appeared acceptable, we found that even a sampling effort deviation of 10 mean stream widths from the designated effort (19.6%) caused one qualitative site classification to change. Our results showed that differences in sampling effort can affect IBI scores and result in erroneous stream assessments. They also indicated that an accurate assessment of regional stream conditions relies on incorporating the specific sampling effort designated for the Ozark Highlands IBI.

SynopsisWe estimated the rate of energy consumed and expended by 43 juvenile smallmouth bass, Micropterus dolomieu, videotaped in the North Anna River during 1991. Habitat data were also collected in the areas where each individual was videotaped. The net rate of energy gained (Jemin") increased as water depths decreased and mean water-column velocities increased. This relationship remained statistically significant even when we assumed that consumption was as little as 40% of our original estimates, and that respiratory costs were three times higher than we suspected. The net rate of energy gained by juvenile smallmouth bass was only affected by the characteristics of the area that they searched and not the foraging tactics they employed.

No abstract

We sampled 15 stream sites in the Ozark Highlands ecoregion of Arkansas and examined the effect of increased backpack electrofishing effort on the richness and relative abundance estimates of fish species. Each site was 75 mean stream widths (MSWs) long and was divided into 15 consecutive segments that were each 5 MSWs long. For each site the percent of empirical and theoretical species richness and the percent of relative abundance similarity to the entire fish assemblage were calculated by adding consecutive segments using an approach that resulted in 15 accumulation curves per assemblage character for each stream site. On average, a distance of 53.8 MSWs (SD ϭ 7.4) was needed to sample 95% of empirical species richness at a stream site, which was equal to an area of 2,722.0 m 2 (SD ϭ 1,967.0). For sampling 95% of theoretical species richness, an average of 101.8 MSWs (SD ϭ 34.5), or 5,055.7 m 2 (SD ϭ 3,667.4), was needed. Obtaining 95% relative abundance similarity required an average sampling effort equivalent to 24.0 MSWs (SD ϭ 8.9), or 1,269.7 m 2 (SD ϭ 932.1). Mean stream width explained more variance in the reach lengths and areas needed for estimates of species richness and relative abundance than did riffleϪpool sequence length or watershed size. Our results should offer insight into species richness and relative abundance accumulation rates when using a one-pass backpack electrofishing sample in Ozark Highland streams of Arkansas.