Distribution and transport of bay anchovy (Anchoa mitchilli) eggs and larvae in Chesapeake Bay

Mechanisms and processes that influence small-scale depth distribution and dispersal of bay anchovy (Anchoa mitchilli) early-life stages are linked to physical and biological conditions and to larval developmental stage. A combination of fixed-station sampling, an axial abundance survey, and environmental monitoring data was used to determine how wind, currents, time of day, physics, developmental stage, and prey and predator abundances interacted to affect the distribution and potential transport of eggs and larvae. Wind-forced circulation patterns altered the depth-specific physical conditions at a fixed station and significantly influenced organism distributions and potential transport. The pycnocline was an important physical feature that structured the depth distribution of the planktonic community: most bay anchovy early-life stages (77%), ctenophores (72%), copepod nauplii (O76%), and Acartia tonsa copepodites (69%) occurred above it. In contrast, 90% of sciaenid eggs, tentatively weakfish (Cynoscion regalis), were found below the pycnocline in waters where dissolved oxygen concentrations were !2.0 mg l 1. The dayenight cycle also influenced organism abundances and distributions. Observed diel periodicity in concentrations of bay anchovy and sciaenid eggs, and of bay anchovy larvae O6 mm, probably were consequences of nighttime spawning (eggs) and net evasion during the day (larvae). Diel periodicity in bay anchovy swimbladder inflation also was observed, indicating that larvae apparently migrate to surface waters at dusk to fill their swimbladders. Overall results suggest that wind-forced circulation patterns, below-pycnocline dissolved oxygen conditions, and diel changes in vertical distribution of larvae and their copepod prey have important implications for potential transport of bay anchovy early-life stages.