Scientists and agencies have struggled for decades to restore oyster reefs around the Bay, often with few signs of success.

In part, that's because no one ever agreed on what "success" would look like.

To some, it might be a towering reef that stands above waves at low tide, like those seen by Capt. John Smith more than 400 years ago. To others, a successful project might be one that manages to hang on by catching a good spat set every few years, even though the oysters never survive to market size.

That's about to change, at least on paper. State and federal fishery managers in December signed off on new quantitative methods to determine whether oyster reefs - and eventually entire tributaries - can be considered recovered.

What does success look like?

An oyster reef needs to have live oysters covering a minimum of 30 percent of the reef, and within that area a minimum of 15 adult oysters per square meter - or their equivalent weight in small oysters. Preferably, oysters would number at least 50 per meter.

An entire tributary could be considered successful if those reef-scale goals were met on 50-100 percent of the river's restorable oyster bottom.

Fifteen 3-inch oysters per meter might not be what John Smith was looking at when he described oysters that "lay thick as stones" in the Chesapeake, but that number was derived from the minimal populations that appeared on self-sustaining bars in the past.

"Most of the criticism from the scientific community and others has been that those oyster numbers are low," said Mark Luckenbach, a scientist with the Virginia Institute of Marine Science who worked on the protocol. "But compared to most of what's been done, it's very high."

Oysters were once a dominant feature of the Chesapeake, with large reefs piled so high that they were a hazard to navigation.

That's changed dramatically after more than a century of overharvesting, habitat destruction and the devastating impact of oyster diseases. In today's Bay, oyster restoration projects often thrive for a few years, then die off.

Scientists think that part of the reason for past failures is that projects were too small - the equivalent of pinpricks in large rivers, easily overwhelmed by disease or an adverse weather event.

A new restoration goal set by federal agencies seeks to change that dynamic by undertaking large-scale restoration projects. The hope is that oysters will not only survive, but provide a springboard to achieve a recovered river system, where robust oyster populations help filter the water while their reefs provide habitat for an array of fish, clams and other creatures.

A 2010 federal Chesapeake strategy called for restoring such populations in 20 Bay tributaries by 2025. The strategy also recognized that clear guidelines were needed to define success. That gave rise to the new protocols, or "oyster metrics" as they're being called, which were developed by a team of scientists from agencies and academia over the past year.

"Clearly, this is needed in order to know when we are 'there' for this goal," said Stephanie Westby, of the National Oceanic and Atmospheric Administration Restoration Center, who chaired the metrics team. "But it is also important for oyster restoration generally. If we are going to restore 1 acre of reef, what does that mean?"

That need was emphasized by a 2009 effort in which a team of scientists reviewed 1,000 Bay oyster projects that spanned two decades to identify lessons learned. Instead, they found that most projects were built with no clear objective and were poorly monitored, making it difficult to draw any conclusions.

It was evident that some kind of common standard was needed. But the metrics team found that defining success was no easy task. Oyster bars in high-salinity areas may produce hundreds, even thousands, of tiny oyster spat per meter almost every year, but almost all die before they grow to 3 inches. Meanwhile, oysters in low-salinity areas may live for years and become large, but will rarely reproduce.

"One of the big challenges was to craft something that works across those very, very different scenarios," Luckenbach said.

The solution they reached is that a reef must have at least 15 oysters (or 15 grams of dried oyster meat) per cubic meter, with the preferred goal being 50 oysters or grams. One gram of dried oyster meat is roughly the equivalent of a plump, 3-inch (market size) oyster. That means the goal can be met with a few large oysters, or lots of small oysters. But it also requires that two year classes of oysters be present as a sign of continued reproduction. Also, those oyster densities need to be found on at least 30 percent of the reef for the reef to be considered a success.

Luckenbach said those numbers were challenging, but doable. "We didn't want to set things so low that they seemed like a joke," he said. At the same time, "it's not something where you throw up your hands right at the beginning and say it's impossible and will never be achieved."

The reefs also have to meet the goals six years after establishment to be counted toward success. But the metrics report offers some wiggle room to help projects secure a foothold: It allows more oysters or shells to be placed after the reef is constructed, but during the 6-year period, if needed to help maintain the reef.

Some panel members acknowledged that what constitutes "success" under the guidelines might be different than what many would consider restored.

"At the deepest level, we skirted that issue of when does one get to put a stamp on it that says 'restored,' because I'm not sure we can do it all in that time frame, at least in a true ecological sense," Luckenbach said. "But at some level this has to be a practical bookkeeping sense for the agencies involved."

Bringing back individual reefs is the easy part, though. The goal of bringing back self-sustaining oyster populations at the tributary scale - so they provide ecological functions such as water-filtering and reef habitat - is even more challenging.

"Exactly what will be necessary to achieve those functional goals is unknown," the metrics team report states. "Simply stated, it has not been done previously."

The metrics report drew upon clues from a variety of sources to try to help craft its tributary goals.

It used estimates from an upcoming U.S. Army Corps of Engineers Chesapeake Bay oyster plan that suggests that 20-40 percent of the hard bottom historically occupied by oysters in a tributary needs to be restored to trigger an overall tributary change.

But huge areas of oyster habitat in the Bay have been lost over the years as oyster populations diminished; many places that were once reefs are now muddy bottoms.

"We are not living in history. We have what we have there today, which is a mix of functioning habitat - hard bottom that can support reef restoration - and large expanses of mud that are not suitable for restoration," said Angela Sowers, a Corps biologist who served on the oyster metrics team.

Because of past habitat losses, the metrics team agreed that a disproportionate amount of the hard bottom that remains in a tributary - probably 50-100 percent of remaining potential reef habitat in most tributaries - must be restored and meet the reef goals before the tributary can be chalked up as a success.

That may be problematic for some areas, team members acknowledged. Some tributaries may no longer have enough hard substrate left to cover 20-40 percent of the historic area, scientists say. Also, the strategy calls for the restored reefs to be in sanctuaries, limiting the tributaries that are candidates for large-scale restoration.

As with reefs, tributaries have to meet their goals for six years to be counted as a success.

Right now, it's not known if any restored reefs, or tributaries, measure up to the new goals. This summer, the Army Corps of Engineers Norfolk District will use the metrics to assess the Lynnhaven River in Virginia - which some think might be the first tributary to meet the "restored" goal. They'll also use the protocols to assess large reefs in the Great Wicomico that were the subject of restoration efforts.

"We are fairly confident at this point that the reefs are meeting those metrics," said Susan Conner, of the Corps' Norfolk District, who worked on the metrics team. "But we do have future phases of those rivers (the Great Wicomico and Lynnhaven) that are yet to be done to call the whole tributary successful."

Besides being an assessment tool, the metrics can be used to identify future work and design restoration projects. By assessing conditions in a given tributary, agencies can determine how much of an area needs to be restored before it could be considered a success.

They're being used for that right now in Harris Creek, a small tributary near the mouth of the Choptank River that will soon be the site of Maryland's largest oyster restoration effort.

Biologists have been systematically surveying all of the creek's oyster bars. When the data are analyzed, they will know which bars already meet the goals. "If places have enough oysters, we can cross them off, it is already functional," said Mike Naylor, director of the shellfish program for the Maryland Department of Natural Resources.

Data will also identify bars where oyster populations simply need to be enhanced, as well as habitats where few, if any, oysters remain that may need construction of shell habitat and oyster plantings. That can help them fine-tune estimates of how many more reefs are needed to restore to meet the tributary goal.

Even in relatively small tributaries where most of the work will likely be targeted, such as Harris Creek, the Little Choptank and Piankatank - all of which are on the drawing board for restoration - that will be a daunting amount of restoration work costing millions of dollars per tributary. Being able to use the metrics to tweak the designs has the potential to save a lot of money.

Because oyster "success" has not been defined before, scientists working on the metrics team say their measures will likely need to be fine-tuned as new information becomes available. "It seems likely there will be places that combinations of all these criteria work, and there will be a lot of places where they don't," Luckenbach said.

But Westby said the metrics report outlined the steps needed to achieve what's been elusive after two decades of Bay oyster projects - a tributary that is restored from an oyster's point of view.

"What we laid out in the report, what we are calling operational goals are how many oysters do you need on a reef? How many reefs on a tributary?" she said. "The idea is that if you do enough of these operational goals long enough, you get toward that higher change: the improved water quality, the fish utilization. We want the whole kit and caboodle."

The report, "Restoration Goals, Quantitative Metrics and Assessment Protocols for Evaluatng Success on Restored Oyster Reef Sanctuaries," is available on the NOAA Chesapeake Bay Office website, http://chesapeakebay.noaa.gov/.