Habitat Use by Adult Rainbow Trout under Moderate Artificial Fluctuations in Flow

Abstract

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.

Comment: Testing the Independence of Microhabitat Preferences and Flow (Part 2)

“…Intermediate Downloaded by [Southern Illinois University] at 16:08 20 December 2014 depths are optimal, and both shallow and deep habitats are marginal. As flow increases, fish that had been forced to occupy shallow water may shift to deep water, as they did in Pert and Erman's (1994) study. If the fish distribution data are aggregated within preference ranges, shallow and deep fish would be lumped together in the lowpreference category, and their joint proportion of the population might not change between low and high flows.…”

“…As an illustrative example, we use the frequency distributions of preference for depth and velocity shown by adult rainbow trout (nonanadromous O. mykiss) at low and high flows (Pert and Erman 1994). Preferences shifted to deeper and faster water when flow increased (Figure 3).…”

Comment: Testing the Independence of Microhabitat Preferences and Flow (Part 1)

“…They noted Shirvell's (1990) report that juvenile steelhead Oncorhynchus mykiss used areas of stream with different water velocities at different flows, which suggested that the microhabitat preferences of juvenile steelhead can be a function of flow. Pert and Erman (1994) have found this true for adult rainbow trout, the nonanadromous form of O. mykiss. Because PHABSIM depends upon the assumption that microhabitat preferences are independent of flow, Beecher et al recognized this as a serious problem and tried to test the assumption; however, their test is not persuasive.…”

Development and evaluation of bioenergetic-based habitat suitability criteria for trout

“…Because habitat in the Green River is homogeneous, and stream areas with high velocities ( b 0Á60 cm s À1 ) predominate (Banks et al 1974), energetically pro®table areas are limited, which may contribute to competition for these areas (Bachman, 1984;Fausch, 1984). Rainbow trout that occupied energetically non-pro®table velocities may have been transient individuals without established territories (Pert and Erman, 1994). Nonetheless, the energetic model developed for rainbow trout in the Green River provided accurate predictions at the population level.…”