Diet of Juvenile and Adult Atlantic Menhaden in Estuarine and Coastal Habitats

Abstract

The Atlantic menhaden Brevoortia tyrannus is a pelagic, obligate filter-feeding fish. Although much of the primary literature indicates that amorphous matter is a major component of the stomach contents of Atlantic menhaden, the diet of these fish is typically perceived to consist primarily of phytoplankton and zooplankton. This disparity between observation and perception results from the belief that amorphous matter in the stomachs is the remnants of plankton that were ingested alive or that it is detritus of low nutritional value and only incidentally ingested. In this study, we found that the stomach contents of juvenile and adult Atlantic menhaden in estuarine creeks were 81% amorphous matter, 17% phytoplankton, and 1% zooplankton, by dry weight. Sampling in two estuarine creeks in North Carolina revealed no seasonal change in diet. Stomachs of fish collected from coastal waters contained smaller amounts of amorphous matter (47%) and higher amounts of phytoplankton (36%) and zooplankton (18%) than stomachs of fish from estuarine creeks. The higher concentration of plankton in the diet of fish in coastal locations probably reflects the lower concentration of suspended amorphous matter and higher relative abundance of plankton typical of that environment. In stomachs that contained largely amorphous matter, the concentrations of ash (70%, dry weight) and humic compounds (38%, ash-free dry weight [AFDW]) were high and the carbohydrate content was moderately low (14%, AFDW). On the basis of reported concentrations of these compounds in plankton and amorphous aggregates, we conclude that the amorphous matter we found in the stomachs of Atlantic menhaden was not recently ingested living or moribund plankton, but was probably material that was produced in the environment by the aggregation of dissolved organics from various detrital sources, including decaying vascular plants. If vascular plants contribute to the production of amorphous matter, their role in coastal and estuarine energy flow and fishery production may be underestimated.