The delta smelt Hypomesus transpaciftcus is endemic to the upper Sacramento-San Joaquin estuary. It is closely associated with the freshwater-saltwater mixing zone except when it spawns in fresh water, primarily during March, April, and May. The delta smelt feeds on zooplankton, principally copepods. Its dominant prey was the native copepod Eurytemora affinis in 1972-1974 but the exotic copepod Pseudodiaptomus forbesi in 1988. Because the delta smelt has a 1 -year life cycle and low fecundity (mean, 1,907 eggs/female), it is particularly sensitive to changes in estuarine conditions. Tow-net and midwater trawl samples taken from 1959 through 1981 throughout the delta smelt's range showed wide year-to-year fluctuations in population densities. Surveys encompassing different areas showed declines in different years between 1980 and 1983. After 1983, however, all studies have shown that the populations remained at very low densities throughout most of the range. The recent decline ofdelta smelt coincides with an increase in the diversion of inflowing water during a period of extended drought. These conditions have restricted the mixing zone to a relatively small area of deep river channels and, presumably, have increased the entrainment ofdelta smelt into water diversions. Restoration of the delta smelt to a sustainable population size is likely to require maintenance of the mixing zone in Suisun Bay and maintenance of net seaward flows in the lower San Joaquin River during the period when larvae are present.

Abstract.-We examined factors affecting fish entrainment at California's State Water Project and Central Valley Project, two of the largest water diversions in the world. Combined, these diversions from the upper San Francisco Estuary support a large component of the municipal and agricultural infrastructure for California. However, precipitous declines in the abundance of several estuarine fish species, notably the threatened delta smelt Hypomesus transpacificus, have generated major concern about entrainment as a possible cause of the declines. We examined a 13-year data set of export pumping operations and environmental characteristics to determine factors affecting entrainment (as indexed by salvage at fish screens) and the potential for manipulation of these factors to improve conditions for fish. Entrainment of three migratory pelagic species-delta smelt, longfin smelt Spirinchus thaleichthys, and striped bass Morone saxatilis-was primarily determined by the seasonal occurrence of particular life stages close to the export facilities. We also found that the direction and magnitude of flows through the estuary and to the export facilities were reasonable predictors of pelagic fish entrainment. Entrainment of resident demersal species (prickly sculpin Cottus asper and white catfish Ameiurus catus) and littoral species (Mississippi silverside Menidia audens and largemouth bass Micropterus salmoides) was not explained by diversion flows, although large numbers of individuals from these species were collected. Our study suggests that entrainment of pelagic species can be effectively reduced by manipulating system hydrodynamics.

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Splittail Pogonichthys macrolepidotus, an endemic cyprinid of the Sacramento-San Joaquin estuary, has been proposed for listing as threatened under the U.S. Endangered Species Act. Almost continuous low outflow conditions in the estuary from 1987 to 1994 led to reduced abundance of young splittails, but adult abundance did not decline consistently except in the downstream portion of the species' range. This range had decreased primarily as a result of historical levee and dam construction but did not appear to have changed substantially in the past 20 years. The distribution of young splittails appears to be relatively plastic on an interannual basis. Evidence of the resilience of the species was seen when high freshwater outflows in extremely wet years (such as 1982, 1983. 1986, and 1995) resulted in high numbers of young splittails. Splittail year-class strength was positively related to freshwater outflow during the spawning season. High outflow inundates the floodplain, which provides spawning, rearing, and foraging habitat. The relatively long life span, high reproductive capacity, and broad environmental tolerances of splittails are contrasted with delta smelt Hypomesus transpacifcus and longfin smelt Spirinchus thaleichthys. other native species of special concern in the system.