BALTIMORE -- Increased acid rain may be contributing to the decline of some of the Chesapeake Bay's most valuable fish species, such as rockfish, shad and herring, according to a new study.

Researchers found that the average rainfall can leave more than half the streams in Southern and Western Maryland with levels of acid that could endanger fish larvae. Maryland rainfall is 10 times as acidic as natural rainfall.

Preliminary results of the state stream survey indicated at least 10 percent of Maryland's rivers have acid levels high enough to kill fish larvae in laboratory conditions and 33 percent could become acidified.

The study began last spring when researchers collected samples from 554 brooks, streams and creeks as part of the most extensive study of Maryland's streams ever carried out, said Michael Bowman, a Department of Natural Resources administrator.

The results of the $220,000 survey surprised Michael Hirshfield, director of the agency's Power Plant Research Program, which commissioned the survey from International Science and Technology environmental consultants of Reston.

"The levels of pH we have seen in the streams are low enough to be concerned about the survival of larval fish," Hirshfield said. The lower the pH level, the more acidic; the higher the pH level, the more alkaline, with 7 a neutral reading.

Scientists found that 74 percent of the miles of streams on the lower Eastern Shore, from the Choptank River south, and on the Western Shore, south of Annapolis, do not have the natural chemistry to neutralize acid from rain or soil runoff.

About 34 percent of the miles of streams in those areas are at or below a pH of 6, a level experts say is likely to kill fish larvae in the bay.

In Western Maryland, 12 percent of the miles of streams tested were below 6 pH and 53 percent were found susceptible to acidification.

"My personal view is that there is no single smoking gun you can point to in the decline of the fisheries of the Chesapeake Bay," said Ronald Klauda, a senior fisheries biologist at the Johns Hopkins University Applied Physics Laboratory. "There are a number of causes. I think that the acid deposition cause is probably one that operates infrequently."