The Distribution of Neomysis americana in the Estuary of the Delaware River1

Observations on the planktonic shrimp, Neomysis americana (Smith), in the Delaware River estuary were carried out over a period of two years. It was found that Neomysis was far more abundant in the deep water of the estuary than in the near‐surface or shallow water. Moreover, it was more abundant within the estuary than in the coastal waters at the mouth of Delaware Bay. Measurements of the length of the animal showed increase in size from the ocean toward the river end of the estuary. These features are interpreted in light of the probable circulation of the estuary to show that the animals became older during their “up‐estuary” drift in deep water. Furthermore, they tended to accumulate within the estuary due to their avoidance of the out‐flowing surface waters. A principal source for Neomysis in the coastal waters outside the Bay is thus suggested. During spring and summer, however, reproduction within the Bay contributed to the stock of animals there. The up‐estuary extent of Neomysis was related to the 4‰ isohaline, and the partial exclusion from surface waters was attributed to their avoidance of excessive light.