DIET OF JUVENILE AND ADULT ATLANTIC MENHADEN IN ESTUARINE AND COASTALHABITATS

The Atlantic menhaden Brevoortia tyrannus is a pelagic, obligate filte r-feeding fish. Although much of the primary literature indicates that amorphous matter is a major component of the stomach contents of Atla ntic menhaden, the diet of these fish is typically perceived to consis t primarily of phytoplankton and zooplankton. This disparity between o bservation and perception results from the belief that amorphous matte r 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 juveni le 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 sma ller amounts of amorphous matter (47%) and higher amounts of phytoplan kton (36%) and zooplankton (18%) than stomachs of fish from estuarine creeks. The higher concentration of plankton in the diet of fish in co astal 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 w as 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 menhade n was not recently ingested living or moribund plankton, but was proba bly material that was produced in the environment by the aggregation o f dissolved organics from various detrital sources, including decaying vascular plants. If vascular plants contribute to the production of a morphous matter, their role in coastal and estuarine energy flow and f ishery production may be underestimated.