Macroscale Assessment of American Shad Spawning and Nursery Habitat in the Mattaponi and Pamunkey Rivers, Virginia

Variation in habitat suitability can alter the growth and mortality of early life stages of fishes but is often difficult to measure, quantify, and apply to the entire system. We designed and tested habitat suitability index (HSI) models incorporating both proximate riverine parameters and surrounding landscape features as determinates of optimal spawning and nursery areas for American shad Alosa sapidissima. American shad eggs and larvae were collected in the Mattaponi and Pamunkey rivers, Virginia, during 1997–1999 as direct evidence of nursery habitat use and indirect evidence of spawning reaches. Hydrographic, physical habitat, shoreline, and land use features were examined for associations with the presence of eggs and larvae. Principal components analyses and logistic regressions indicated the importance of hydrographic parameters (current velocity, dissolved oxygen, and depth), physical habitat features (sediment type and woody debris), forested shoreline, and land use features to the presence of eggs. Larvae were more dispersed than eggs were, and distinct habitat associations for larvae could not be discerned. This corresponds to the hypothesis that sites are selected by spawners, and larvae (more so than eggs) are subjected to net downstream transport. Morphological features indicate the presence of three distinct regions along the Mattaponi and Pamunkey rivers. The presence of eggs is typically associated with upper and midriver regions, whereas larvae are dispersed among the three regions. The combination of remote sensing and on-site data collection and analyses used in this study may be an effective way to rapidly assess essential fish habitat when data are limited, allowing the linkage of fish population data with habitat evaluations. As more data become available and HSI models are refined, habitat ratings may be modified for a more precise delineation of specific reaches of critical fish habitat.