Cape Cod's population of its namesake fish dipped by 25 percent between 2001 and 2004, according to preliminary findings by federal scientists, indicating that the once-abundant cod has yet to rebound despite years of government protection.

A group of scientists from the National Oceanic and Atmospheric Administration's Fisheries Service and the New England Fishery Management Council reported Monday at a public workshop that fishermen continue to take too many adult cod and that not enough juvenile fish are surviving to replenish the population's ranks.

Andrew A. Rosenberg, a professor of natural resources policy and management at the University of New Hampshire who oversaw New England groundfish stocks under President Bill Clinton, said the new numbers show the government needs to impose further restrictions on fishing cod.

"They're still fishing them, and when you kill them, they tend to die," Rosenberg said.

But Pat Fiorelli, a spokeswoman for the New England Fishery Management Council, said officials did not expect the government's new rebuilding plan to show results for several more months. The average New England fisherman can take groundfish only 53 days a year now, she added, down from 88 days in 1996.

"Just because overfishing is happening doesn't mean the stock is not rebuilding," Fiorelli said. "We've had a hard time reducing overfishing without having severe [economic] impacts on local communities."

Often called New England's "founding fish," cod once abounded off the Massachusetts coast. But the population crashed in the mid-1990s because of overfishing and has yet to recover, even as local haddock and scallops have rebounded.

Conservationists, who have unsuccessfully sought more stringent fishing restrictions in federal court, said the declining numbers mean federal authorities should protect nursery habitat as well as adult fish.

"Stopping overfishing is not enough," said Chris Zeman, a Northeast fisheries policy expert for the advocacy group Oceana, who advises the region's fishery council. "You also have to protect baby cod and make sure they're surviving."

Susan Buchanan, a spokeswoman for NOAA's Fisheries Service, said under federal law regulators have to allow overfishing at times to minimize a rebuilding plan's impact on local commercial fishermen. She added that this week's scientific findings are preliminary and said agency officials will comment once the final results come out later this month.