Effects of Light Intensity on the Catchability of Juvenile Anadromous Alosa Species

Trawl catches indicated diel migratory activities by young-of-the-year alewife Alosa pseudoharengus, American shad A. sapidissima, and blueback herring A. aestivalis in Virginia rivers. Bottom catches were significantly greater during day than at night; conversely, surface catches were significantly greater at night than during day. Spatial separation of alewife and blueback herring stocks, which could reduce feeding competition, also was indicated. Surface pushnet catches of blueback herring were associated with a sky-opacity index, suggesting that this species (or its prey) is negatively phototropic. The influence of light on distributions of juvenile Alosa should be taken into account when sampling location, time, and gear are chosen for stock-assessment programs.