The Distribution and Abundance of Neomysis mercedis in Relation to the Entrapment Zone in the Western Sacramento-San Joaquin Delta

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Abstract
The distribution and abundance of the opossum shrimp, N. mercedis, were studied in a portion of the upper San Francisco Bay estuary in 1976, the 4th 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 were related to the location of the entrapment zone. In years of low flow, such as 1976, the entrapment zone was in the narrow channels of the upper estuary, thereby reducing habitat availability and 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 (.ltoreq. 3 mm long) tended to be higher in the water column during daylight than did mature mysids (.gtoreq. 7 mm long), leading to a geographic separation of these life stages. Estuarine hydraulics rather than salinity tolerance determined the distribution of N. mercedis in the study area.