Short-term predation responses of adult bay anchovies Anchoa mitchilli to estuarine zooplankton availability

Abstract

Feeding activity and prey selection of adult bay anchovies Anchoa mitchilli in the highsalinity North Inlet Estuary, South Carollna (USA), were examined during 2 summer days and nights in 1985. Stomach contents were compared to 2 size fractions of zooplankton (153 and 365 pm mesh) collected at the same time (every 2 h) and place (near the bottom) in a tidal creek. Adult bay anchovies (40 to 60 mm) consumed a wide variety of zooplankton, but selected items according to prey size, density, and type. Most prey items were between about 0.4 and 3.25 mm in length, although some copepods (less than 0.3 mm) and decapod shrimps (up to about 12 mm) were eaten. Among the available potential prey taxa, crab (primarily Uca) megalopae were by far the most important. Uca megalopae comprised almost 60 O/O of the diet based on numbers and volume and were favored even when their densities were much lower than other s~milar sized animals. First stage pinnothend crabs and hydromedusae were completely avoided. There were hstinct tidal and diel rhythms in feeding patterns and prey selection. Generally, large prey (e.g. Uca megalopae, shrimp zoeae, amphipods) dominated the &et around high tide especially at night, and small items (e.g. copepods, Uca zoeae, barnacle cyprids) were most important around daytime low tide when larger prey items were less abundant in the water column. Rapid changes in feeding were observed when preferred prey became avadable, especially around dusk. Adult anchovies fed throughout the night and immedately responded to the sudden avallabihty of crustacean larvae originating from nocturnal hatching events.