Interactions among stream fishes: predator-induced habitat shifts and larval survival.

Adult largemouth bass alter habitat use by, and abundances of, other fishes in small streams. Experimental manipulations of bass in natural stream pools (Brier Creek, Oklahoma) showed that responses of other fishes to adult bass were highly dependent on prey size, and that both direct and indirect effects of adult bass influence the distribution and abundance of other stream fishes. Experiments measuring the distributional responses of members of natural pool assemblages to adult bass revealed differences among adult sunfishes, "small" fishes (16-80 mm SL), and larval sunfish and minnows. Adult sunfishes (Lepomis spp.) did not detectably alter their depth distribution in response to adult bass, but changes in abundance of adult Lepomis on the whole-pool scale appeared positively related to changes in the number of bass. Small fishes tended to occupy shallower water when adult bass were present; changes in abundance of small fishes were negatively related to the number of adult bass. Larval minnows and larval Lepomis occupied primarily deep, mid-regions of pools, and were found only in pools which contained, or had contained, adult bass. A second set of experiments was motivated by censuses of small prairie-margin streams which revealed co-occurrence of larval fishes (of both minnow and sunfish species) and adult largemouth bass. Experimental manipulation of bass and Lepomis larvae on the whole-pool scale showed that adult bass enhanced short-term survival of Lepomis larvae. This effect appears to be an indirect result of habitat shifts by small fishes in response to bass; additional experiments indicated that these small fishes are potentially important predators of larvae. The interactions suggested in this study are analogous to those hypothesized for bass and sunfish in lakes by Werner and Hall (1988).