Local conservationists say the decline of fish in area rivers is a warning for the region’s watersheds. Grass-roots associations seeking to protect watersheds along the Taunton, Neponset, Jones, Eel, North, and South rivers are looking to safe yield — a standard never implemented since the passage of the Watershed Management Act 25 years ago — as a way of controlling demands for more water and allowing stressed rivers and streams to recover.

Last year a published study by the US Geological Survey and state environmental agencies of the relationship between stream flow in Massachusetts rivers and the populations of fish such as brook trout and redfin pickerel concluded that a relatively small difference in water levels in flowing streams can make a big difference to fish. A drop of 1 percent in median stream flow in August, typically the annual low-water mark, can reduce fish numbers by 55 percent, the study concluded.

South-of-Boston rivers, streams, and tributaries, along with the ecosystems they support, have suffered for years from low water, especially during summer dry spells when demand for water by public water suppliers increases, according to local environmental organizations and expert studies. Low water levels in flowing streams raise the water temperature, promote weed growth, choke fish, and harm other creatures adapted to their environment.

“The goal is to develop an integrated safe yield and stream flow strategy,’’ she said.

A spokeswoman for the Executive Office of Energy and Environmental Affairs said last week that the state’s Sustainable Water Management Advisory Committee is working on scientific issues that will help formulate safe yield figures. Recommendations from that body could be expected “in the coming months,’’ spokeswoman Catherine Williams said.

And if state regulators implement safe yield as intended by the 1986 Water Management Act, Shelley said, “they face an immediate compliance problem with a number of watersheds that are over allocation. They’re reluctant to create that problem for themselves.’’

The dilemma for state regulators, however, is that low safe yield caps for watersheds mean less water for public water suppliers.

“The current status is the state continues to be reluctant to really implement the safe yield protective mechanism put in place by the Legislature,’’ said Peter Shelley, senior counsel for the Conservation Law Foundation.

State environmental regulators promised to calculate safe withdrawal amounts — a concept known as “safe yield’’ — by last October but then informally extended the deadline to June. But as June approaches, conservationists say no specific proposals are under discussion. They say they hope that once realistic safe yield numbers are determined for the state’s 27 watersheds, stricter limits on how much water public water suppliers can take out of the ground will follow.

While south-of-Boston rivers suffer stress from low water and fish populations decline, the state is stalling on efforts to protect watershed basins by determining the amount of water that can be safely withdrawn, local environmentalists say.

“Many streams and wetlands in this watershed, especially those located within headwaters, are currently stressed or impaired as a result of excessive withdrawal of water,’’ the Taunton River Watershed Alliance wrote last year to state officials. The group asked the state not to approve requests for more water until safe yields for the river and its tributaries are determined.

The Taunton River flows through Halifax, Middleborough, Raynham, and Lakeville; its watershed encompasses all or parts of 24 south-of-Boston communities.

Two years ago, after environmentalists resigned from an advisory panel to protest a proposed safe yield policy that downplayed protecting the environment, the Department of Environmental Protection went back to the drawing board. It promised to develop a new policy and targeted last October as the date to produce it.

The complex task of deciding on a scientific basis for safe yield was given to the Sustainable Water Management Advisory Committee, a body with conservationists among its members but led by state officials. When the October target went by, officials extended the deadline to June.

But in recent months, meetings have been held without any discussion of safe yield, said water conservation expert Paul Lauenstein of Sharon.

“As a citizen I would like to see a transparent process and an open debate to come up with the recommendation,’’ said Lauenstein, a member of his town’s water management advisory board. “Are they deciding the safe yield issue behind closed doors?’’

But Williams said the committee’s technical discussions are on track and will lead to a safe yield policy.

“The priority of the Sustainable Water Management Advisory Committee and its technical subcommittee has been to provide technical advice on both the development of a new safe yield and the development of innovative stream flow criteria that will complement safe yield,’’ Williams said.

Lauenstein said the state can develop a safe yield formula that will protect the environment by relying on the scientific study by US Geological Survey and state agencies such as the Division of Fish & Wildlife of how much water fish in flowing rivers need to survive.

But however determined, safe yield numbers are unlikely to please everybody, since while environmentalists want lower numbers for conservation, water suppliers want higher numbers for population growth and economic development.

The rub, Lauenstein said, is that cities and towns have typically been given as much water as they want. “We treat [water permits] as an absolute right of the cities and towns that have them, regardless of scientific evidence that they may be incompatible with healthy aquatic ecosystems, and turn a blind eye to the rights of unborn generations.’’

Robert Knox can be reached at rc.knox2@gmail.com.

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