The county, alarmed over a significant drop in fish, shellfish and other marine life in Marina del Rey over the past four years, is launching a study into whether a highly toxic chemical contained in boat paint is to blame.
Supervisor Deane Dana asked the County Board of Supervisors on Tuesday to expand the county's water-quality testing program to study the chemical tributyltin, or TBT.
In seeking the additional testing, Dana cited a recent county-funded study by marine biologists at USC, which concluded that the decline in the number of mussels and fish larvae in the marina, and a similar drop in the number of newborn and young fish, could be caused by high concentrations of TBT.
Dana said the county Department of Beaches and Harbors is concerned about reports of "environmental degradation caused by TBT in other harbors" where studies are also under way.
County officials said they will consider regulating the compound if scientists can show evidence that it is causing the problems.
Tributyltin is a bactericide and fungicide added to marine paints to protect boats from marine growths.
The USC study dismissed several other possible causes for the troubles in the marina.
The report said the declines were not caused by DDT that has been trapped in bottom sediments since that chemical was dumped off the coast in the 1960s or by dirty street runoff or by raw sewage spills from the city's overburdened sewage system.
It also ruled out as a cause the lingering effects of El Nino, the ocean current that disrupted weather patterns worldwide and brought huge storms to Los Angeles in 1982 and 1983. Scientists say the decline in marine life began soon after the El Nino storms.
County officials said a recent study by the state Water Resources Control Board found that among 90 harbors along the California coast, most had concentrations of TBT higher than the safety standard of 20 parts per trillion set by the French and British governments. The United States has no safety standard for TBT.
In the state's study, researchers found that TBT levels in several areas of Marina del Rey were greater than 140 parts per trillion, a level scientists say is toxic for many shellfish.
In one section of the marina, just off the swimming area in Basin D, researchers measured TBT at 470 parts per trillion. However, scientists never have found evidence that TBT is toxic to humans.