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The distribution and abundance of the opossum shrimp, Neomysis mercedis, was studied in a portion of the upper San Francisco Bay estuary in 1976, the fourth-driest year on record. For much of 1976 the entrapment zone, an area of zero net horizontal flow, was located in the study area. Variations in the annual abundance of N. mercedis can be related to the location of the entrapment zone. In years of low flow, such as 1976, the entrapment zone is in the narrow channels of the upper estuary, thereby reducing habitat availability and, thus, the abundance of N. mercedis. Mysid population dynamics were highly correlated with reproduction, and fecundity with length, although the latter relationship changed seasonally. Juvenile mysids (43 mm long) tend to be higher in the water column during daylight than do mature mysids (•7 mm long) leading to a geographic separation of these life stages. Estuarine hydraulics rather than salinity tolerance determines the distribution of N. mercedis in the study area. Neomysis mercedis is a relatively small mysid which can attain concentrations in excess of 2,000 m -a in the San Francisco Bay-Delta estuary. It is an integral part of the estuarine food web, especially as food for young striped bass Morone saxatilis (Heubach et al. 1963; Stevens 1966a), American shad Alosa sapidissima (Stevens 1966b), white sturgeon Acipenser transmontanus (Ganssle 1966), and the caridean shrimp of the Delta Crangonfranciscorum and Palaemon macrodactylus (Siegfried et al. 1978). Increased water diversion above the estuary will result in flows similar to those in 1976, the fourth-driest year on record. Information on the distribution and abundance of N. mercedis in 1976 may provide insight into future patterns expected following water diversion. Earlier investigation identified salinity and temperature as the major environmental fhctors determining mysid distribution and abundance (Heubach 1969). Light and current were also recognized as affecting distribution and abundance. Those environmental parameters t Previous studies in the San Francisco Bay Estuary have referred to N. mercedis Holmes as N. awatchensis

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