For the past 30 years, George Abbe has been baiting crab pots with menhaden and dropping them into the Bay near Calvert Cliffs in Maryland and carefully recording what he has caught.

Abbe, a researcher with the Academy of Natural Sciences’ Estuarine Research Center, has seen great years, and bad years, and lots of in-between years as he has examined more than 115,000 crabs caught in nearly 20,000 pots fished.

But one trend has stayed the same: The number of male crabs that he has caught has declined, and their size is getting smaller.

“Twenty years ago, it was not unusual to catch a good number of 6-inch crabs,” Abbe said. “Now we see far fewer 6-inch crabs than we used to. We used to see quite a few 7-inch males, and we don’t see them at all anymore.”

The reason, according to Abbe: “They get caught before they have a chance to get bigger.”

This is what scientists call “growth overfishing.” It is different from what people typically think of as overfishing, where so many are harvested that it harms reproductive potential — something known as “recruitment overfishing.”

Growth overfishing refers to taking too many of a targeted species at too small a size. It can reduce the value of what’s caught because, if left to grow larger, they would weigh more and be more valuable.

Some scientists say this problem could become the catalyst for a change in how blue crabs are managed that could make the Bay’s most valuable commercial species even more profitable for fishermen.

“We need to change the question from ‘How many crabs can we take?’ to ‘How many should we take?’” said Tom Miller, a scientist at the University of Maryland’s Chesapeake Biological Laboratory.

Right now, data such as Abbe’s indicate there is so much fishing pressure that most crabs appear to be caught shortly after they reach the legal size of 5 inches. If they were allowed to get slightly larger, they would molt out of their shell one more time, which would let them to grow to more than 6 inches.

“So,” Miller said, “you could potentially leave them in the Bay, take fewer of them, but those crabs would be bigger so the net weight of the crabs you would be taking out is more, and the net value of crabs would be more.”

Doing that would require that fishing regulations be changed in ways that promote the optimal growth of crabs, rather than the maximum catch, primarily by reducing the amount of fishing pressure. That could mean actions such as larger minimum sizes for harvested crabs and allowing fewer crab pots in the Bay.

It’s not without risks. For example, if crabs stay in the Bay longer, some fraction of them will die naturally and never be harvested.

But at the same time, curbing growth overfishing could reduce the risk of recruitment overfishing — something that is also a concern in the Bay. That’s because larger crabs produce more eggs and sperm than smaller individuals.

“There’s an exponential relationship between fecundity and female crab size and similarly, I believe, in sperm output in males,” said Rom Lipcius, a crab researcher at the Virginia Institute of Marine Science.

“So you could have a major effect on the reproductive output of the population by growth overfishing,” Lipcius said. “That, in conjunction with reductions in the spawning stock could drive you pretty close to recruitment overfishing.”

If there is growth overfishing, then present management is failing to meet the goal of the Bay Program’s 1997 Blue Crab Fishery Management Plan, which calls for actions that not only conserve the stock, but also “optimizes the long-term use of the resource.”

Miller recently completed a report for the Bay Program’s Living Resources Subcommittee as part of its effort to set “targets” that indicate healthy levels of various resources in the Bay. The report recommended that targets ultimately be aimed at controlling growth overfishing instead of recruitment overfishing.

Mike Fogarty, another scientist at the Chesapeake Biological Laboratory, said it is critical — for both watermen and the crabs — that management moves from debates about the number of crabs that can be taken to avoid overfishing toward discussion about maximizing the fishery value.

“If the discussion always get stuck on determining the recruitment overfishing level, and that danger point only, then you never get to a discussion of where things ought to be in terms of maximum benefit from the resource,” Fogarty said. “We need something to shoot for rather than something to stay away from.”

But growth overfishing could require a substantial reduction in the total amount of pressure placed on the crabs — perhaps as much as a 30 percent reduction, according to Miller’s estimate. That would require such a large change in management that it is unlikely to take place until more detailed analyses are completed.

Robert O’Reilly, of the Virginia Marine Resources Commission, cautioned against drawing firm conclusions about decreases in crab size from Baywide data. Increases in catches of small crabs in some areas with intense fishing pressure may mask the fact that there are still plenty of large crabs in other areas, he noted.

For example, Abbe’s data shows that crabs are larger in the Patuxent River, where there is less harvest pressure, than at his Calvert Cliffs site.

Also, while it’s generally true that larger hard crabs are more valuable than smaller hard crabs, soft and peeler crabs — which are mostly small, immature females — are far more valuable, per pound, than others. So the optimal value of a crabs is not always tied to its size.

“I think people need to define what they truly mean by growth overfishing, and what the possibilities are for trying to tackle that,” O’Reilly said.

Because the blue crab fishery is so complex, Fogarty and Miller both said that a move toward managing to prevent growth overfishing would have to be coupled with a detailed economic review of various parts of the fishery.

But Fogarty said the issues should not be insurmountable. For example, he said, in some cases the minimum legal size for catching crabs could be adjusted. Other methods of reducing fishing pressure could include reducing the amount of gear that can be used to catch crabs, establishing new sanctuaries or additional season limitations.

“That kind of decision has important social ramifications,” Fogarty said. “It’s one that would have to be addressed pretty carefully, I would think. But it’s also true that it’s a finite resource, and it can only take a finite amount of pressure, so if you don’t cut back something, some way … it becomes a very difficult situation.”

Bill Goldsborough, senior scientist with the Chesapeake Bay Foundation, said a transition toward a fishery aimed at curbing growth overfishing could be phased in over years, to prevent dramatic short-term impacts on watermen.

“If we are able to come up with something like that, in the end, the crabber will be much better off than he is now,” Goldsborough said. “Crabbers really are the primary victims of the current dysfunctional fishery. They’re the ones that stand to gain the most and who we ought to be looking out for. And in the process of doing that, if we are able to restore a higher quality fishery, we will reduce the risk of a biological problem.”