We employed field and laboratory data to test the hypothesis that microhabitat use by rainbow trout (Oncorhynchus mykiss), rosyside dace (Clinostomus funduloides), longnose dace (Rhinichthys cataractae) and mottled sculpin (Cottus bairdi) was affected by energetic constraints .Both rainbow trout and rosyside dace occupied water-column microhabitats in Coweeta Creek, North Carolina, U .S .A. These species were over-represented at velocities requiring lower rates of oxygen consumption, as predicted by an energetic cost hypothesis . Morphological and behavioral adaptations of the two benthic species (mottled sculpin and longnose dace) enabled them to hold position over a wide range of velocities without significant changes in oxygen consumption . Focal point velocities of longnose dace were not significantly different from those available, suggesting a lack of velocity preference . Mottled sculpin, however, consistently were over-represented at low velocities, even though oxygen consumption data showed that energetic costs were not decreased by occupying these microhabitats .Our results indicate that microhabitat use by rainbow trout and rosyside dace is consistent with predictions of an energetic cost hypothesis, whereas the morphological adaptations of longnose dace and mottled sculpin minimize the constraints of energetic costs in microhabitat selection .

We monitored fish populations at three sites in a southern Appalachian stream system during a 40-mo study. Assemblages at two sites, consisting of four and five resident species, were persistent in terms of relative species abundances and production. This persistence was related to low variation in abundance of the two numerically dominant fishes, mottled sculpins (Cottus bairdi) and longnose dace (Rhinichthys cataractae). Less abundant residents, rosyside dace (Clinostomus funduloides), rainbow trout (Salmo gairdneri), and greenside darters (Etheostoma blennioides), displayed lower levels of persistence. We also observed pronounced annual variation in either young-of-the-year recruitment and/or subsequent year-class strength of four resident species, possibly resulting from a severe drought during the third year of study. Abundance and production of rosyside dace and trout were greatly affected by fluctuations in recruitment and year-class strength, suggesting that environmental variability strongly influenced these populations. Density-dependent mechanisms may have regulated sculpin and longnose dace populations. The three-species asssemblage at a third site also was persistent as a result of relatively stable sculpin and longnose dace populations, whereas the rainbow trout population fluctuated among years. We observed significant intersite differences in relative year-class strength in a given year and in population structures of sculpins and longnose dace, suggesting that spatial variation in habitat characteristics affected assemblage dynamics and responses to environmental fluctuations.

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