Tissue Residues and Hazards of Waterborne Pesticides for Federally Listed and Candidate Fishes of the Sacramento-San Joaquin River Delta

Aerial View of the Sacramento-San Joaquin River DeltaThe Sacramento-San Joaquin Delta is formed at the confluence of the south-flowing Sacramento River and the north-flowing San Joaquin River.  The estuary encompasses 1,600 square miles, drains over 40 percent of the State of California, and provides habitat and stop-over ground to numerous species of fish and wildlife.  Two-thirds of salmon that migrate into California pass through the Delta, as do nearly half the migrating waterfowl and shorebirds.  The estuary, due to the world's largest manmade plumbing job, provides 7.2 million acre-feet of water a year for export, irrigates 4.5 million acres of farmland, and provides drinking water for 20 million Californians (SFEP 1992). Sacramento Winter-Run ChinookThe Delta provides habitat to many species of aquatic wildlife, including the federally-listed, threatened Delta smelt (Hypomesus transpacificus) and Sacramento winter-run chinook (Oncorhynchus tschawytscha) and the proposed-threatened longfin smelt (Spirinchus thaleichthys) and Sacramento splittail (Pogonichthys macrolepidotus). Other aquatic species dependent upon the watershed, also suffering severe population declines, include the recently listed California red-legged frog (Rana aurora draytonii), the tiger salamander (Ambystoma tigrinum), giant garter snake (Thamnophis gigas), and the western pond turtle (Clemmys marmorata). Many fisheries are in a rapid decline in the Delta, smelt populations are estimated to have declined approximately 90percent in the last 20 years.  Of the original 29 indigenous fish species in the Delta, 12 have either been eliminated entirely, or are currently threatened with extinction (SFEP 1993).  Population declines are attributed Tiger Salamanderto a combination of factors including increasing water diversions for export, loss of habitat, increased competition and predation from introduced species, and impaired water quality. This report summarizes the results of three studies conducted by the U.S. Fish and Wildlife Service between 1994 and 1995.  Biologists surveyed water and fish for metals, trace elements, and organics from the Sacramento and San Joaquin Rivers, to evaluate potential metal and trace element loading, and performed toxic identification evaluations (TIEs) on water from the back sloughs of the Delta.  The studies were scoping in nature, designed to screen for potential problems and define the direction and focus of future investigations.

SilversideMethods: Salvage Delta smelt  were analyzed for selenium and mercury, and composite egg samples were analyzed for selenium.  Individual and composite smelt and silverside samples were analyzed for aluminum, arsenic, boron, barium, beryllium, cadmium, chromium, copper, iron, mercury, magnesium, manganese, molybdenum, nickel, lead, selenium, strontium, vanadium, zinc, and scanned for organic contaminants.  Grab water samples were also collected at these two sites, preserved with nitric acid, and scanned for metal and trace element concentrations.


  • Delta SmeltWhole-body mercury concentrations are elevated in both Delta smelt in the Sacramento River and inland silversides in the San Joaquin River.  Further research is needed to determine potential impacts to these and predatory fish populations from these body burdens.
  • Copper concentrations are over 30 times higher than normal published background concentrations in Delta smelt in the Sacramento River.  Zinc is 10 times higher than normal background for rainbow trout.
  • Naphthalene concentrations may be elevated in Delta smelt. The source and potential impacts to smelt reproduction need to be evaluated.
  • Although detectable concentrations of pesticides were not found in this study, it is probably only a reflection of the timing and weather conditions associated with this particular sample collection regime. Pesticide residues remain a potential risk to aquatic Bay/Delta communities at other times of the year.

Satellite View of the DeltaLearn More by Reading the Full Report: Bennet, J., Hofius, J., Johnson, C., and Maurer, T., Tissue Residues and Hazards of Waterborne Pesticides for Federally Listed and Candidate Fishes of the Sacramento-San Joaquin River Delta, California: 1993-1995, USFWS, Div. of Env. Contaminants, Sacramento, CA. 2001.

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