Habitat Suitability and Fish Production: A Model Evaluation for Rock Bass in Two Virginia Streams

Abstract

A habitat suitability index (HSI) model for rock bass Ambloplites rupestris was developed from available suitability curves and field tested by comparing HSI and habitat units with seasonal standing stock, biomass, and production of rock bass in two fourth-order streams in Virginia. Consistent ranking between HSI and mean standing stocks for the study year was observed only for 250-m stream sections, the largest spatial unit of comparison. In contrast, changes in HSI explained no more than 22% of the variability in mean standing stocks in 50-m subsections of Back Creek, and were not significantly correlated with standing stocks in Little Walker Creek. Factors that apparently contributed to low correlations and ranking success included geographically homogeneous habitat and seasonal fish movements, as well as potentially important variables such as depth and cover that are not features of the tested HSI models. Annual discharge and fish distribution data further suggested that physical habitat was more limiting to populations of rock bass in the fall, not during the summer low-flow period as previously assumed. Unmeasured temporal variability in the environment may have been equally important in limiting presumed carrying capacities in study streams. Annual production for rock bass was nearly identical in both streams and averaged 1.04 g˙m−2˙year−1. This similarity in annual production was consistent with the habitat homogeneity implied by the narrow range of observed HSIs. Validation of our HSI model and similar short-term assessments of habitat quality would be more likely if study sections included the home range or annual habitat requirements of all life stages of the target species. Additional quantitative data on species-habitat relationships and systematic evaluation of individual suitability indices should also improve testing of aggregate HSI models.