Geologic Analyses for Evaluating Watershed Heterogeneity: Implications for Otolith Chemistry Studies

Studies using otolith chemistry to distinguish fish stocks in fresh waters have suggested that spatial heterogeneity in basin geology determines the scale of stock discrimination possible with this approach. However, no studies have illustrated an association between spatial variation in fish otolith chemistry and watershed geology. We consider this relationship in the context of a recent study describing within- and between-river variation in trace element chemistry of otoliths from YOY smallmouth bass (Micropterus dolomieu) from the Maury and James rivers (Virginia). Cluster analysis of multivariate geologic data for discrete river segment basins illustrates a phenomenological association between geologic heterogeneity and our ability to discriminate spatial groupings of fish from their otolith chemistry. This analysis provides two significant results: 1) a starting point for considering the mechanistic relationship between watershed geology and fish otolith chemistry; and, 2) a framework for assessing basin heterogeneity prior to designing studies that use otolith chemistry to distinguish fish stocks in river-tributary networks. The latter can be used a priori to determine the efficacy of otolith chemistry comparisons and to guide sample collection over large spatial areas.

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