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10 . Executive Summary soo 1233 700 600 .c .!!!500 u.. .... ~400 ClI .c g300 z 200 o Shoreline . Offshore Final Report aggregations of humpback chub in Grand Canyon, which were river reaches with abundant debris fans and channel expansion zones, These complexes were large recirculation zones that entrained and entrapped drifting food, and provided low- velocity vortices in which the fish could feed and rest. Adult humpback chub in Grand Canyon used large recirculating eddies to a greater degree than adults have displayed in other populations (Valdez et al. 1990). Adults in other populations appear to use a wider range of habitats, possibly because more normal warm spring, summer, and fall temperatures enable the fish o 100 o g g ~ ~ 2 ~ ~ ~ ~ 8 g g g ~ 2 ~ ~ ~ ~ N N N M M M M ~ ~ ~ Total Length (10 mm increments) Fig. 9. Length-frequency distribution of humpback chub captured in shoreline habitats (with electrofishing, seines, minnow traps) and in offshore habitats (with gill nets, trammel nets) for 1991-93. Hence, although shoreline habitats of the type selected by subadults appeared to be suitable and abundant in Grand Canyon, we hypothesize that use was reduced by the limited swimming ability of the young fish at cold temperatures, and by the destabilizing effect of fluctuating dam releases. Greater use of vegetated shorelines was attributed to reduced velocities, high cover value, and relatively high food production among tamarisk, willows, sedges, and other riperian vegetative types. This vegetative cover, which was absent under predam conditions, except during high runoff or flood flows, may be used by the fish in the absence of high constant turbidity as cover from predators and to avoid high light levels. Nearshore velocities along cobble bars and bedrock exceeded the range of maximum cruising speed ofYOY humpback chub (range, 30-100 mm TL) at 140C (Fig. 10); sand beaches lacked cover and despite low velocities, were not used by YOY during the day or in clear water, Adult humpback chub used eddy complexes disproportionate to their composition of surface area; 88% of adults captured and 74% of radio contacts were from eddies which averaged only 21% of surface river area (Fig. 11). Disproportionate use of eddy complexes coincided with the location of the three largest 0.5 0 0.5 1.5 2.5 0.5 1.5 2.5 Subreach 1 (Meters from Shore) Subreach 2 0.7 B. Velocity 0.6 0.5 - .!!! .5. 0.4 ~ '8 0.3 ~ 0.2 0.1 - 1.5 2.5 0.5 1.5 Subreach 1 (Meters from Shore) Subreach 2 2.5 Depth - Bedrock -Cobble - Debris Fan -Sand - Talus - Vegetation 2 1.5 Fig. 10. Average depth (A) and velocity (B) at three distances from shore (0.5, 1.5 and 2.5 m) for six shoreline types in Subreach 1 (RM 61.9-65.4) and Subreach 2 (RM 65.4-73.4). The ranges in cruising speed for YOY and juveniles are shaded areas.