Diel differences in abundance and diversity of fish species and blue crab (Callinectes sapidus) in the sandy beach shore zone of lower delaware bay

Abstract

Diel sampling was conducted during June-September 2013, in the shore zone and adjacent nearshore zone of Delaware Bay, Delaware, to identify day and night changes in nekton density, species richness, and the species assemblage. Mean species richness and total nekton density in the shore zone were higher at night. A detailed examination of abundant species revealed that bay anchovy (Anchoa mitchilli), weakfish (Cynoscion regalis), spot (Leiostomus xanthurus), and blue crab (Callinectes sapidus) occurred in higher densities in the shore zone at night than during the day. Bluefish (Pomatomus saltatrix) occurred in higher densities during the day. Additionally, small (<20 mm in fork length) Atlantic silverside (Menidia menidia) and bay anchovy, were observed in abundance in the shore zone only during daytime. Day and night differences in predator-prey dynamics were likely a primary driver of diel differences in nekton abundances observed in the shore zone. No diel differences in species richness, nekton density, or species assemblage were found in the adjacent nearshore area. Daytime sampling predominates in studies of shore-zone nekton and our results show the value of also sampling at night. Future research should include samples throughout the diel cycle and include tagging and movement data to allow a better understanding of diel dynamics of nekton along sandy beach shores.