WSP02325 - Laserfiche WebLink

Home Browse Search

382 j, KORMAN, S, M, \\lELE AND M, TORIZZO where q is the catchability coefficient (Hilborn and ,Walters, 1992), Catchability can be defined as the proportion of the stock caught by one unit of effort, or more explicitly as the ratio of the area swept by the sampling gear (a) to the area over which the fish are distributed (A). that is, q = a/A, The majority of sampling in Grand Canyon for juvenile fish is conducted in relatively sh3110w and low-\'elocity habitat, and the area swept at each site (n) is rela- tively const;mt among trips, The area over which fish are distributed (A), however, will presumably increase with increases in the amount of suitable habitat. If we assume that reductions in discharge from GeD increase the amount of low-velocity shallow habitat and, hence, increase the amount of suitable habitat, we would expect a/A (= q) to decrease, In this situation. CPE would decline proportionally with the increase in A even though stock size remained unchanged, The use of a numerical habitat model to estimate the degree of change in suitable habitat area is warranted, even if the objective is simply to ex.amine the potential for changes in discharge to introduce bias and variation in CPE data, In this paper. we use a two-dimensional hydrodynamic model (Wiele e/ 0/" 1996) to examine how operation of GCD has affected the availability of suitable habitat and the dispersal of young native fish, We apply the model to seven reaches in the critical humpback chub rearing area immediately downstream from the LCR to develop a series of relations between the availability of suitable habitat and discharge, The effect of impoundment and his- torical operating regimes on habitat is evaluated by comparing monthly statistics of suitable habitat availability computed from the habitat-discharge relations and hourly discharge data, We also quantify the amount of suitable habitat that persists over a typical 24-hour period. and examine how it changes in response [Q historical differences in the amount of variation in hourly discharge. We compute changes in suitable area that occurred during fish sam- pling trips conducted in 1993 to examine the potential for discharge-driven changes in suitable habitat area to intro- duce bias and variation into CPE data, Finally, we use a particle-tracking algorithm to examine how discharge and assumptions about swimming behaviour affect the ability of young fish to remain in critical rearing areas down- stream from the LCR, We discuss our results in the conte~t o[ current hYP'ltheses abaut humpback. chub juvenile survival in the Colorado River and operational flow regimes from GCD. STUDY SITE Hydrodynamics and habitat availability were modelled in seven study reaches in the mainstem Colorado River below the confluence of the LCR (Figure I), The spatial distribution of reaches that we modelled has considerable overlap with the range of the LCRlmainstem humpback chub aggregation (river km 91-110; Valdez and Ryel, 1995), Our study reaches cover 3,6 km of this habitat (199<) between river km 99 and 107 (Table I), The complex flow patterns in the study reaches result from the irregular bedrock channel, large-scale blocks along the channel sides, and debris fans (Figure 2), Debris flows and floods from streams in side canyons (Howard and Dolan. 1981; Schmidt. 1987; Webb el a/" 1989; Schmidt and Graf. 1990; Schmidt and Graf. 1990; Melis elal,. 1994; Schmidt and Rubin. 1995) form debris fans that partially constrict the channel. and recirculation zones are generated in the lee of the channel constrictions, Deep holes typically Occur in the bedrock channel downstream from debris fans, The spacing between debris fans is controlled largely by bedrock structure (Dolan el a/,. 1978), Table I. Length and welted areas al two different discharges for model reaches Welted area (m' x 10]) Reach Length (m) 498 328 258 253 448 890 993 3668 84m]!, 907 m]!s Rt R2 R3 R4 R5 ALe PAL 39 34 26 25 68 83 III 386 57 41 32 41 78 117 165 531 Total Copyright (Q 2004 John Wil~y & Sons. Ud. River R~s. Applic. 20: 379-400 (2004)