Charter-boat captain Bruce Cash's clients are hooking more porgies, striped bass and fluke in Long Island Sound, and less floating garbage. Environmentalist John Atkin is hearing reports of harbor seals and purple sea urchins for the first time in years.

State official Karen Chytalo's phone rings less often with complaints about sewage spills and algae blooms, and lobsterman John German's traps are brimming with a record harvest.

As officials meet today to approve a long-awaited plan to make deep cuts in nitrogen pollution in Long Island Sound, there are signs - but not much hard data - that the long-suffering estuary is making a slow comeback.

"It's hard to draw firm conclusions, but there are some encouraging signs that we are reversing the trend" of ever-worsening water quality, said Mark Tedesco of the U.S. Environmental Protection Agency, who serves as technical director of the Long Island Sound Study.

Since 1985, the $22-million study has methodically assessed the Sound's environmental problems and developed a series of cleanup measures culminating in the 15-year, $650-million plan that is expected to be approved today by the EPA and the environmental commissioners of New York and Connecticut.

The plan calls for cutting the amount of nitrogen discharged into Long Island Sound from sewage plants and fertilizer-laden stormwater runoff. The goal is to cut the nitrogen by 58.5 percent. This would be enough to virtually eliminate the algae blooms that soak up underwater oxygen and regularly transform parts of the western Sound into a dead sea during the late summer, the EPA asserts.

Implementing the 58.5-percent cut will require unspecified tax hikes in communities near the Sound, although much of the cleanup money will come from state environmental-bond-act funds and possibly federal sources. New York City, the source of two-thirds of the human-induced nitrogen entering the Sound, says the EPA's cost estimate is too low and the city's share alone may top $1.5 billion. New York City resisted the plan for years before signing on two weeks ago, after the states and EPA agreed to reassess the plan every five years.

With so much money at stake, the plan's architects say they wish they could more confidently predict how the cleanup will affect the Sound. They concede that the plan is in part a leap of faith, despite 14 years of research by the Long Island Sound Study and the development of a $3-million computer model designed to predict the impact of various cleanup scenarios on the Sound. In the past decade, governments have spent tens of millions of dollars to restore wetlands, improve urban harbors and take small steps to reduce nitrogen discharges. Despite the many positive signs identified by fishermen and environmentalists, the experts acknowledge that they aren't certain that the Sound's water-quality has really improved.

"The unsatisfying answer is that it's really been too short a time frame to draw conclusions about trends, particularly because we haven't yet made a big dent in the nutrient load entering the Sound," said the EPA's Tedesco. "That's why the new agreement is so important."

Experts unconnected with the cleanup agree. "We've taken some meaningful but still relatively small steps, and the response of the Sound is really not going to be ascertainable thus far," said Bill Wise, who heads the Living Marine Resources Institute at the State University at Stony Brook.

For instance, measurements of dissolved oxygen in Long Island Sound over the last decade suggest that the late-summer oxygen shortage in the bottom waters of the western Sound may be easing slightly, although the yearly variations seem to have a lot more to do with rainfall and water temperature than the recent small reductions in nitrogen discharges from sewage plants. In 1990, the Long Island Sound Study approved a freeze on nitrogen discharges at existing levels, and in 1994 passed a plan to cut discharges by about 13 percent by 2006. The new plan expands the cut to 58.5 percent by 2014.

A few remaining critics say the lack of solid evidence of progress and the uncertainty of future benefits support their case that the proposal is ill-conceived. "We don't know whether this plan will provide real benefits," said Richard Cogen, who represents the four small sewer districts - Great Neck, Great Neck Village, Oyster Bay and Port Washington - that are the only vocal opponents of the plan left among the 78 communities lining the Sound, now that New York City has decided to support it.

The scientific uncertainty is troubling not only because of the huge cost of the proposed cleanup, but also because removing so much nitrogen from the Sound might lead to some unexpected and possibly negative effects. One worst-case scenario is that the reduction in nitrogen could somehow alter the Sound ecosystem in a way that causes jellyfish to supplant finfish at the top of the food chain.

"We feel it's very unlikely that those types of circumstances will happen," said Chytalo, director of marine monitoring and assessment for the state Department of Environmental Conservation.

Progress has been difficult to measure scientifically in the Sound for three reasons, experts say.

First, weather can dramatically change the picture from year to year. Rainfall patterns determine how much nitrogen-laden runoff reaches the Sound, and a cold winter followed by a hot summer can make the situation worse by creating a temperature "barrier" that blocks oxygen from penetrating the warmer shallow water and reaching the cooler bottom waters where many fish live.

Second, nitrogen discharges into the Sound haven't been cut very much so far. New York has done virtually no nitrogen-removal work, although several long-delayed New York City pilot projects are under construction, and Connecticut's projects have been relatively small.

Finally, there's a built-in lag time before any nitrogen-control effort will be fully felt in the Sound because its sediments can hold nutrients for five years or even longer before they dissipate, delaying the impact of nitrogen reductions.

With so many uncertainties, Sound-watchers have had to fall back on less verifiable signs of improvement in the waterway.

The state conservation department, for example, operates an extensive water-monitoring network, but one of the clearest signs of improvement is much more basic: the phone rings less often with reports of algae blooms and oil slicks, Chytalo said.

At the Coalition to Save Hempstead Harbor, volunteers have been painstakingly measuring water quality in the harbor every summer since 1992 without any clear trend emerging, said the group's Carol DiPaolo. But on her weekly trips into the harbor to take samples, DiPaolo has noticed "tremendous improvements" in populations of birds, oysters and blue-claw crabs, among other species.

Similarly Atkin, who heads Save the Sound Inc., an environmental group with offices in Stamford and Glen Cove, takes heart in growing reports of sightings of formerly scarce species such as harbor seals, purple sea urchins, alewives and herring.

And fishermen say that the consensus among charter captains and commercial fishermen alike is that pollution and fish stocks are gradually improving.

"You can see much further down into the water column now," said German of Brookhaven hamlet, president of the Long Island Sound Lobstermen's Association and a 33-year veteran of the Sound fishery.

Cash, who runs a charter boat out of Port Jefferson, agreed. "You're seeing less and less oil spills and floating junk in the water," he said. Tougher fishing restrictions and improving conditions are allowing some important species to rebound, according to Cash.

To scientists like Stony Brook's Wise, those anecdotal reports are suggestive but not convincing. But he does allow that the Sound at least doesn't seem to be getting worse.

"I do think there isn't any compelling evidence that the overall environmental quality of the Sound is declining from what it was, say, ten years ago," he said. "It's too soon to say much more than that."