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.
The abundance of young striped bass Morone saxatilis in the Sacramento-San Joaquin Estuary has suffered an unsteady but persistent decline from population levels that were high in the middle 1960s. The decline was particularly severe in 1977 and abundance of young striped bass has been low every subsequent year. The adult striped bass population also has fallen during the past 20 years, but the exact period over which the decline occurred and the rate of decline are not clear.
The adult population is now about one-quarter of its former size and there is little sign of recovery.We believe the Sacramento-San Joaquin striped bass population and the fishery that it supports are in serious danger. The cause is most likely one or more of four factors. (1) The adult population is now so low that egg production may be inadequate. (2) The plankton food supply of young striped bass in the western Sacramento-San Joaquin Delta and Suisun Bay has been greatly reduced each spring. Diversion of water from the delta for agricultural purposes is a prime suspect for the decrease in food production. (3) Large numbers of young fish are lost by entrainment in water diversions. (4) The population is stressed by toxic substances such as petrochemicals and pesticides. Additional studies are underway to help determine the principal cause(s) of the striped bass decline.Striped bass Morone saxatilis were introduced into the Sacramento-San Joaquin Estuary in 1879. Their abundance increased dramatically, enabling sport and commercial fisheries to develop before 1900. The commercial fishery was closed in 1935 due to pressure from sport fishermen (Stevens 1980). The population has never been dominated by rare strong year classes and until recently has been relatively stable. Now, however, the adult population is one-quarter of what it was 20 years ago, and the production of young over the past 8 years has been one-third to one-half of the expected values. These meager year classes of young probably will further depress the adult stock as they are recruited into the fishery. This paper summarizes current thinking regarding potential causes of the declines of both young and adult striped bass.
Annual abundance indices for young fall-run chinook salmon (Oncorhynchus tshawytscha), American shad (Alosa sapidissima), and longfin smelt (Spirinchus tbaleichthys) increased directly with river flow rates during the spawning and nursery periods. Annual abundance of young delta smelt (Hypomesus transpacificus) did not vary with river flow. Several factors associated with flow could explain the relationships described for chinook salmon, American shad, and longfin smelt. The one factor common to all affected species was that dispersal of young increases when flows increase, which probably results in decreased density-dependent mortality.
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