The Relation between Habitat Availability and the Short-Term Carrying Capacity of a Stream Reach for Smallmouth Bass

Abstract

We used the behavioral carrying capacity (BCC) technique to test the assumption of the instream flow incremental methodology (IFIM) that a positive linear relation exists between available fish habitat (weighted usable area, WUA) and fish biomass for populations of smallmouth bass Micropterus dolomieu at carrying capacity. The BCC experiments were performed at various streamflows, and WUA was measured at each flow. In each BCC experiment, a section of a stream was overstocked with juvenile smallmouth bass, excess fish were trapped as they emigrated, and the remaining resident fish were collected after a 10-d period. We did not find a positive relation between WUA and BCC, and we found a negative relation between stream discharge and BCC. High BCC values recorded during low-flow experiments resulted from less upstream and downstream movement by juvenile smallmouth bass. Such behavioral changes may prevent smallmouth bass populations from being limited by habitat availability during low flows. The low-velocity nature of many warmwater streams is quite different from the high-velocity trout streams where IFIM was developed. In warmwater streams, the microhabitat variables typically used in IFIM studies (depth, velocity, substrate, and cover) may be inadequate for predicting population responses of pool-dwelling fishes to streamflow alteration.