The effects of rainbow trout ( Oncorhynchus mykiss ) on the use of spatial resources and be - havior of rosyside dace

Rainbow trout (Oncorhynchus mykiss) and rosyside dace (Clinostomusfitnduloides) exhibit substantial overlap in microhabitat use in Coweeta Creek, North Carolina, USA. We conducted a replicated experiment in an artificial stream to assess the effects of both the presence of rainbow trout and dace density on: 1) microhabitat use, 2 ) agonistic behavior, 3) social behavior and 4) feeding rates of dace. Our results demonstrated that microhabitat use of dace was highly variable both among competition treatments within a replicate and for the same competition treatment among replicates. Moreover, the pattern of the differences between competition treatments was not consistent among replicates. As a result, we detected a significant effect of experimental replicate on dace microhabitat use but no significant competition effect. Trout had little effect on the behavior of dace and intraspecific aggression was much more common than interspecific aggression. There were strong inter-individual differences in the agonistic behavior of dace, and 94 of the IT7 aggressive displacements observed were initiated by one or two highly aggressive individuals per replicate. We detected few significant acclimation or time of day effects. The significant differences in microhabitat use and behavior observed within replicates appeared to be a consequence of differential initial settlement patterns of dace or the presence of one or two highly aggressive dace in treatments. Hence, we do not believe that interspecific competition with rainbow trout has a strong effect on microhabitat use by dace in Coweeta Creek. ' Authors' address: Wamell School of Forest Resources, University of Georgia, Athens, GA 30602 USA. e-mail: [email protected] To whom correspondence should be addressed. Present address: Departamento de Ecologia Evolutiva, Museo Nacional de Ciencias Naturales, Consejo Superior de Investigaciones Cientificas, c/Jose Gutitrrez Abascal 2, Madrid 28006, Spain. e-mail: [email protected] 334 Pedro A. Rincon and Gary D. Grossman