Spatial and Temporal Variation in Fishes of Riffle Habitats: A Comparison of Analytical Approaches for the Roanoke River

Abstract

-Spatial and temporal variation in fishes in riffles of the upper Roanoke River system (Virginia) were analyzed in three ways: a two-way (space-time) analysis of variance (ANOVA) of species abundance; a comparison of spatially adjacent and temporally consecutive fish assemblages by Morisita's index of similarity; and comparison of all space x time samples by cluster analysis. At six sites on the river mainstream or large tributaries, fish in riffles were collected by kickset sampling in five surveys from May to November 1978. For species with significant differences in abundance across space or time, variance components were combined into ratios that show relative importance of temporal, fixed and ephemeral effects. The ANOVA was carried out on individual species, but Morisita's index and the cluster analysis were calculated for the entire assemblage of riffle fishes in each sample. Four of the nine most common species (three benthic darters and one sucker) exhibited significant spatial and/or temporal variance, with more of the variance accounted for by spatial than by temporal differences. For two of the four species, the spatial variance was primarily ephemeral, but for the other two spatial variance was persistent (fixed). Results of the cluster analysis reinforced for whole assemblages the patterns detected for individual species, in that more within-site than within-time similarity existed. Comparison of adjacent and temporally consecutive sites by Morisita's index produced results differing somewhat from those of the first two analyses, in that spatial and temporal differences in assemblages appeared essentially equal. Each of the analytical approaches provided a somewhat different view of sources of spatial and temporal variation in riffle fishes of this stream system, but the overall conclusion was that spatial variation generally exceeded temporal variation. The results suggested that to adequately survey fish of riffles in this or similar systems it may be better to increase the total number of sampling sites, than to increase the number of collections per site.