Laserfiche WebLink
<br />SUBADUL T HUMPBACK CHUB IN THE COLORADO RIVER <br /> <br />273 <br /> <br />90 data points: three variables (depth, velocity and cover), three distances from shore at lO-m intervals <br />along 100 m of shoreline. With these data, mean transect depth, velocity and total cover were derived for <br />reaches 1 and 2 and for all six shoreline types. A total of 69 longitudinal transects among all shoreline <br />types were measured: 40 in Reach 1, and 29 in Reach 2 (Table II). Discharge at the time transects were <br />obtained from the gauge station located above the confluence of the LCR. <br />Fish sampling. We used electrofishing catch rates to estimate relative densities of subadult humpback <br />chub ( < 200 mm TL) within all three geomorphic reaches and each of the six shoreline types (Table III). <br />Electrofishing was conducted from Achilles SU-16 research boats equipped with Mark XX@ Complex <br />Pulse Systems, as described in Valdez and Ryel (1995). The time required to sample each shoreline was <br />recorded, and catch per unit effort (CPE) was expressed as the number of fish caught per 10 h of fishing. <br />AI: 1 time:area fish-sampling ratio was assumed and thus the catch data were used as a measure of <br />relative density. Time of day and turbidity were recorded for each sample (Valdez and Ryel, 1995). Only <br />samples from high turbidity, nighttime or crepuscular periods were used to reduce confounding effects of <br />light on catch rates (Valdez and Hugentobler, 1993). <br />Flow duration curves. We used mean daily discharge data from the Lee's Ferry gauge to derive pre- and <br />post-dam flow duration curves. Flow duration curves characterize the temporal flow regime by showing <br />the temporal flow distribution as a discharge occurring over a percent of time for a period of record <br />(Leopold et al., 1964). Data from 1922 and 1960 were used to construct the pre-dam curve and from 1965 <br />and 1994 to construct the post-dam curve. To construct these curves, discharge was ranked from highest <br />to lowest and plotted against cumulative percent time. <br /> <br />Analyses <br />Physical differences among geomorphic reaches and shoreline types. Our first objective was to quantify <br />physical differences among reaches and shoreline types. To determine how reaches were geomorphically <br />different, the availability of shoreline types, width-to-depth ratio and total riffle area of the three reaches <br />were examined. To determine if depth, velocity and cover varied between reaches (blocks) and among <br />shoreline types (treatments) and to determine if shoreline conditions depended on reach (interaction), a <br />generalized randomized block multiple analysis of variance was used (GRB MANOV A) (Neter and <br />Wasserman, 1974). Because we were specifically interested in physical differences among only these <br />reaches and our inferences would be limited to this system, we considered the reach effect as fixed. This <br />assumption allowed us to use the residual error term as the mean square error when calculating F-ratios <br />(Neter and Wasserman, 1974). Mean transect depth and velocity were 10glO-transformed to correct for <br />heteroscedasticity in the MANOV A (Zar, 1984). We used an a priori a value of 0.1 (10% chance of type <br />I error) for all statistical analyses due to low sample size and probable low power. <br />Relationships between subadult humpback chub and geomorphology. To determine if subadult humpback <br />chub were associated with reach and shoreline differences (objective 2), associations of fish with specific <br />depth, velocity and cover conditions were examined and relative densities of fish among reaches and <br />cf shoreline types were compared. Initially, we wanted to determine if the longitudinal distribution of <br />subadult humpback chub could be explained by something other than habitat selection (e.g. passive <br />dispersion). We therefore examined how densities varied throughout the three reaches for each shoreline <br />type. Downstream distributions of subadult humpback chub densities within each shoreline type were <br />fitted with a LOWESS line-of-best-fit (SYSTAT, Inc., 1992). A monotonic decline in density would be <br />consistent with the distribution expected from passive dispersion. A flat line with large differences in <br />magnitude of densities among shoreline types would imply local habitat selection was occurring. <br />Discriminant functions analysis (DF A) was used to determine if the presence of fish within sample units <br />was associated with differences in mean depth, velocity and cover. For this analysis, only fish samples that <br />were spatially concurrent with a longitudinal transect (n = 173) and thus had associated habitat informa- <br />tion were used. <br />To determine if variation in fish densities was associated with differences among reaches or shoreline <br />types, a GRB ANOV A was conducted with reaches as blocks and shoreline type as the treatment. An <br />interaction term between reach and shoreline type was included in this model. Block effects were also <br /> <br />~ 1998 John Wiley & Sons, Ltd. <br /> <br />Regul. Rivers: Res. Mgmf. 14: 267-284 (1998) <br />