WHEN THE TALK AROUND THE CHESAPEAKE turns to blue crabs — and the talk turns that way every spring — there are two questions that matter most: When will blue crabs start moving up the Bay? And how many will be coming?

John Barnette was talking about blue crabs this spring, saying he can predict when the crabs are coming each year. He's a waterman who goes crabbing on the Wicomico River with bank traps, a little-known technique found only on a few Eastern Shore rivers. Getting ready for crabs, he spent his work days wading out in the river, planting poles, stringing wire netting, and setting up tall cages designed to trap peeler crabs, blue crabs that are scuttling along the shore looking for a safe place to molt. As he worked, Barnette kept his weather eye on one species of local vegetation.

"We go by certain plants, when they bloom," says Barnette, a lean and thoughtful man who's a sharp observer of life along his river. His preferred crab predictor is the snowball bush. "When the snowball bush blooms," says Barnette, "you got soft crabs."

When Willard Van Engle would talk about blue crabs, he would say he could predict two years ahead of time when a big year for blue crabs was coming. This was shortly after the end of World War II, and Van Engle was a well-known crab biologist with the Virginia Institute of Marine Science who claimed he had discovered the perfect predictor. Water flow down the James River, he said, told you how many blue crabs would be coming up the Chesapeake.

River flow was his snowball bush. High river flow in the James, he said, would lead to a low blue crab harvest two years later. And vice versa: low river flow would bring a high harvest total. This was an inverse correlation, and it seemed to make sense: blue crabs spawned down near the mouth of the Chesapeake, and a rainy, wet year could wash crab larvae right out of the Bay into the ocean where they would be lost to the system. A dry year would do the opposite: it would keep crab larvae near home and turn the lower Bay into a big nursery for new crabs.

A reliable forecast for blue crab harvests has long been a Holy Grail for scientists in the Chesapeake, where crab harvests have a history of bouncing up and down, seemingly at random, sometimes doubling the previous harvest, sometimes halving it. So crab biologists spent a lot of time talking about Van Engle's prediction — until it fell apart.

"Van Engle kept track of that relationship every year, and every year that he added data, it got weaker and weaker," said Gene Cronin, a noted biologist who was talking blue crabs at a symposium some 40 years later. "It was a false correlation, a coincidence. You have to watch out for those things."

When Chuck Epifanio and Steve Sulkin would talk about blue crabs, and they spent years talking about them, they decided Van Engle's blue crab predictions might be right sometimes — but usually for the wrong reasons. This was the early 1970s when they were young biologists, new Ph.D. graduates up from Duke University now working at marine labs in Delaware and Maryland. They wanted to bring some fresh thinking to the old mysteries about the up-and-down harvests of blue crabs.

The best predictor, they said, wasn't the river flow coming down the James River. According to the new guys, the real predictor was probably the currents and wind fields at work out on the ocean. But it would take a while for the old guys to believe them.

For their blue crab talks, Epifanio and Sulkin would meet at a restaurant that was once a menhaden fishing boat. Epifanio was a new hire at the Lewes lab run by the University of Delaware, and Sulkin a new hire at the Horn Point Laboratory down near Cambridge, Maryland, some 60 miles away. Once a month Epifanio would leave his office and start driving south. Sulkin would start driving north.

Mid point was Seaford, Delaware, a small town with only two good places to eat, and one of them was the Flagship Restaurant, an eatery operating out of a large wooden boat. Built for menhaden fishing, it once spent time patrolling for German subs but now sat beached along the banks of the Nanticoke River. According to one of the locals, the Flagship was where you went for birthdays and anniversaries. That's where Epifanio and Sulkin went to talk science, lab politics, and Duke basketball. The new faculty members called their monthly meetings "mini-sabbaticals."

Atlantic blue crab

Callinectes sapidus

Credit: Iain McGaw and Carl Reiber

Distribution: Atlantic Ocean and Gulf of Mexico, from Nova Scotia to northern Argentina; it is most abundant from Texas to Massachusetts. Half of the U.S. harvest comes from the Chesapeake Bay.

Key distinguishing markings: Brilliant blue color on their front claws (tips are red on females) with an olive or bluish-green carapace. They have a pair of paddle-shaped legs that are excellent for swimming.

Size: Adults can grow up to 9 inches, or 23 centimeters (carapace width).

Source: Maryland Department of Natural Resources.

Over beers and crab cakes, meeting once a month, year after year, Epifanio and Sulkin used these mini-sabbaticals to begin pulling apart the classical theory about the blue crab life cycle. The heart of the older theory was what Sulkin called "the in-Bay hypothesis." Blue crab larvae were hatched in the summer waters of the southern Bay, all the way down near the entrance capes, and most of them stayed in the area except when heavy river flow washed some of them out of the estuary. "The in-Bay hypothesis," Sulkin said at the time, "is the intuitive, reasonable, and widely accepted hypothesis."

Shortly after Epifanio and Sulkin began meeting, Hurricane Agnes arrived over the Chesapeake watershed and helped jump start their rethinking of the old paradigm. Curving inland from the Atlantic, Agnes came ashore as a tropical storm and immediately began unleashing heavy rains that sent historic floods surging down all the Bay's great rivers. It was late June 1972 and female crabs were smack in the middle of their spawning season, releasing eggs and larvae into the waters of the southern Bay.

Those huge floods of river water would surely wash most of those crab eggs and larvae, billions and billions of them, out into the coastal ocean, and all those new, wannabe crabs would be lost to the Chesapeake forever. Two years later, blue crab harvests would probably slump.

That was the expectation for scientists working within the old paradigm. And they were half right: most of those newly hatched crab larvae were indeed swept out to sea by the flood waters of Agnes. But they were also half wrong: two years later, blue crab harvests went up slightly. How did those crab larvae get back into the Bay?

Other leaks in the theory came from other scientists. An oceanographer at the Lewes lab came to Epifanio with some tiny, strange-looking species that were turning up in his nets. Epifanio grew out the samples in culture, and when he did he discovered they were blue crab larvae well into their second life stage. What were these larvae doing so far out on the ocean? Epifanio started driving south. Here was food to chew over at a lot of lunchtime mini-sabbaticals. "I went to Steve," says Epifanio, "and said, well what do you think is going on?"

The young biologists began building an alternate paradigm, an "offshore hypothesis" that made some startling claims. Perhaps a lot of blue crab larvae were swept out to sea every year — not just during an Agnes year or a big river flow year. Good evidence came both from lab studies and from early research cruises that went looking for larvae. In lab studies, first-stage larvae seemed programmed to move upwards; in field studies most of them were netted in seaward-flowing surface waters, especially at night. Epifanio and Sulkin came to think that nearly all blue crab larvae leave the Bay every year. The big nursery for feeding new blue crabs into the Chesapeake would not be found in the southern Bay but in waters off the coast, waters that also held crab larvae from Delaware Bay and North Carolina estuaries.

The upstarts from Duke felt some strong pushback. "To the established Chesapeake Bay blue crab aristocracy," says Epifanio, "it was anathema to think there might be larvae from a dirty place like Delaware Bay contaminating the Chesapeake stock."

Epifanio can now look back on those years with some detachment. The biologist eventually published over 120 articles, many of them on the mysteries of blue crab population shifts, and he now holds an endowed chair at the University of Delaware. Sulkin soon became a lab director, first at the Horn Point Laboratory, and later at the Shannon Point Marine Center in Washington State. "When Steve and I started this, we were the young rebels," says Epifanio. "There was a lot of resistance to the idea. It was honest resistance from great scientists, guys like Van Engle and Gene Cronin," he says. "But they did not like the idea."

Elizabeth North did like the idea behind the offshore hypothesis, but she knew it left some big questions unanswered. The biggest goes like this: how do all those blue crab larvae floating out there in coastal waters ever find their way back into Delaware and Chesapeake Bays? How does their offshore odyssey end?

North was a young fisheries oceanographer who was getting her first chance to serve as chief scientist leading a large-scale research cruise. This was September 2005, and the boat was the 120-foot RV Cape Henlopen, scheduled for a night departure out of Lewes, Delaware, Epifanio's home base. When he dropped by the dock to say bon voyage, North had a request. "I asked Epi to bless the boat," she said, joking with her crew as they unpacked sampling gear. "He said, 'I bless the boat.'"

And well he should. Nearly 30 years after his mini-sabbaticals in the Flagship Restaurant, Epifanio has seen his thinking become the flagship paradigm setting the research agenda for a new generation of scientists, many of whom are oceanographers rather than biologists. In fleshing out his offshore hypothesis in the 1980s, Epifanio teamed up with the late Richard Garvine, one of the rare oceanographers willing to work with biologists. "Physical people basically didn't like to talk to biologists," says Epifanio. Working with Garvine changed everything, says Epifanio. "We became much more sophisticated in our understanding of coastal circulation."

To get her cruise started that night, North and her crew were working under lights on the back deck, setting up nets and electronics and communications. With her hair tied back in a bun, she hustled between the work deck an the stern and the ship's lab. In a sweatshirt and loose pants, she could pass for one of her graduate students.

The cruise plan called for 10 days of data gathering. Five days and five nights around the mouth of Delaware Bay. Then five more days and nights down at the mouth of Chesapeake Bay. The sampling would start that night if they could get the gear ready in time.

Before casting off, Captain Bill Byam called the science group together for his speech: small craft warnings, winds from the northeast, a four-to-seven-foot swell, Dramamine available in the galley, breakfast at 5:30 a.m. No mention of a hurricane called Ophelia already meandering off the coast of Florida.

This North-led cruise was the latest in a series of cruises launched over several decades to test various theories, hunches, and guesses about the wanderings of blue crab larvae in coastal waters. Epifanio's offshore scenario has, in effect, sent blue crab scientists off to sea on their own three-decade odyssey, a search that's still trying to explain to everyone's satisfaction how blue crab larvae come home.

If you want to solve the puzzle of blue crab migrations, sometimes it helps to think like a crab (or like you think a crab thinks). "If I were in charge of crab larvae," says Bill Boicourt, "I would ask myself how would I maximize my chances of getting back into the estuary?" Like North and Roman, Boicourt works at the Horn Point Laboratory of the University of Maryland Center for Environmental Science (UMCES). One of the lead scientists for the Henlopen cruise, he's one of those physical oceanographers who's willing to talk with a biologist or think like a blue crab.

Smart crabs seem to understand the basic physics of how water masses behave in an estuary, something smart oceanographers figured out only 60 years ago. The Chesapeake Bay, it turns out, is driven by a two-layer flow in which two streams of water are moving in opposite directions. Along the surface flows a stream of freshwater that drains out of all the rivers in the watershed. Along the bottom runs a second stream of denser, saltier water that surges in from the ocean. The river water slides south toward Norfolk, headed out to the ocean. The bottom-water inflow surges north toward Baltimore, headed up the estuary. Twice a day their progress is interrupted by two flood tides and two ebb tides.

Blue crabs apparently figured out all that physics eons ago, or evolution figured it out for them, and they learned how to go with whatever flow is likely to help them. Most crab larvae, for example, take the opportunity to ride the surface waters out to sea. Why head for the ocean? Because there aren't as many predators out there. And because larvae need saltier water to help them make it through their early molt stages, especially the big switch from the zoea stage to the megalopa stage. That's the creature that will try to get back into the estuary.

Atlantic Blue Crab Life Cycle

When female blue crabs release their eggs, they hatch into zoeae. These strange-looking beings (gnatlike? shrimplike?) can float and swim, mainly to move up and down in the water column. They need high salinity water to grow and molt. more . . .

If you're a crab megalopa trying to come home from offshore, you face some trickier choices. If you swim down, maybe you ride back into the estuary with the bottom-water inflow; if you swim up, maybe you ride in with a big wind event like a nor'easter, or maybe with a flood tide, or maybe a nighttime flood tide, or a full-moon flood tide, or a quarter-moon flood tide.

That's a lot of maybes for smart crabs to try. And so far smart scientists have found evidence that supports each idea — and other evidence that undercuts it. The physics of offshore waters, it turns out, is more complicated than estuarine circulation, also more variable and less understood, despite the best efforts of oceanographers like Boicourt who spent years mapping the circulation of mid-Atlantic coastal waters.

All that offshore science helps him think like an offshore crab larvae. "I would say,'Let's play the market and hedge our bets,' " says Boicourt, in his crab mode. " 'Let's try a number of different mechanisms.' " Flood tides, spring tides, hurricanes, nor'easters, bottom-water inflow. Whatever works.

That, of course, leaves a lot of options for scientists to test and debate.

The net rose out of the night sea, looking like a black, snakelike monster and trailing four long scraggly tails. It's called the MOCNESS (multiple opening and closing net, with an environmental sensing system), meaning it can open and close its nets at multiple depths. Its tails are actually plankton nets, and the whole contraption does resemble its namesake, the Loch Ness Monster. Elizabeth North and Dave Kimmel, a plankton ecologist at Horn Point Laboratory, wrestled the contraption onto the deck of the Cape Henlopen and started unhooking the PVC canisters from the end of each net.

The canisters were designed to catch crab larvae, but they were crammed with jellyfish. North and Kimmel started pouring the gloppy contents into a bucket. They rinsed out the canister and poured the bucket through a screen and into small glass fruit jars. The payoff should be samples holding crab larvae from four different levels of the water column.

To track blue crab larvae, scientists first try cutting the offshore world into little data pieces: the number of larvae at each depth, temperature, salinity, oxygen, data on flood tides and ebb tides, winds, and currents. Then over months and years they try putting the world together again back in the lab and on their computers. Graduate students will find and count any larvae lodged in the gummy mess. North and her collaborators will correlate those larvae numbers with all those other numbers. The result, everyone hopes, may help answer some questions: Where were the crab larvae? Were they moving up or down? Were they moving towards the estuary? Or away from it?

Researcher Elizabeth North (above) grew up in Annapolis catching crabs along the Severn River.  As a fisheries oceanographer she catches blue crab larvae in the coastal waters between Delaware Bay and Chesapeake Bay. For part of her research, North created a modeling project that used particles to estimate where larvae end up after they're spawned at the mouth of the two bays. Her model cranked in all the known data from 2005 and 2006 that could affect larval movements: freshwater flow, sea temperatures, humidity, wind fields, tidal current velocities. Her key findings: only a small percentage of megalopae make it back to their home estuaries (green boxes); in good years, weather patterns could bring in 10 times more larvae than in other years; crabs from Delaware Bay may, indeed, make it into Chesapeake Bay but not vice versa. Map courtesy of Elizabeth North.

Crab larvae, oceanographers once predicted, would usually be moving away from an estuary, a prediction that would blow a big hole in Epifanio's offshore scenario. When larvae leave Chesapeake or Delaware Bay, they are riding seaward-flowing water, and that water generally takes a right turn as it exits each estuary and flows south. It joins an expansive, south-flowing current that can be 60 miles wide. If nothing alters this broad flow, the oceanographers warned, then most Chesapeake larvae should end up in North Carolina.

One of the breakthroughs that would keep the new blue crab theory afloat began with data from a single instrument, a meter tracking current flow.

When Bill Boicourt first started talking about the current meter at mooring 408A, some scientists dismissed his data as a curiosity. The meter at mooring 408A bobbed in the waves some 17 miles offshore, just south of the mouth of the Chesapeake Bay. During certain months of the year, it recorded consistent evidence of a current moving north.

The clinching evidence for Boicourt's discovery, however, came from the logs of old lightships that were once anchored all along the Atlantic coast. Vessels like the Chesapeake Lightship or the Diamond Shoals Lightship marked entrances to rivers and estuaries or warned of dangers like reefs and shoals. On these ships that were going nowhere, crewmen would try to estimate the speed and direction of currents that seemed to be going somewhere. They would toss out drift poles, each with one end weighted, and then take bearings as the poles drifted with the current.

Instead of one current meter, Boicourt now had 40 years of drift logs dating from 1930 back into the 19th century. In the logs: clear evidence that a current reversal would often rise in July and August and flow north from Cape Hatteras toward the Chesapeake. "It was," he says,"the most exciting data I had ever seen on this issue."

Blue crab larvae, in theory at least, now had a way to stay near home, and blue crab scientists had another good reason to hang onto the offshore hypothesis.

Currents play a big role in keeping crab megalopae near their estuary of origin, but the larvae themselves may also play a big role in their own homecoming. Another breakthrough came when biologist Richard Forward discovered that offshore larvae seem to smell the water from their home estuary. And when they do, they take action.

Working at the Duke University Marine Lab, Forward originally set out to study mud crabs only to stumble into blue crab research when he found they were more plentiful. "I was so ignorant I didn't even know what a blue crab larva looked like," says Forward. "We got these things in our samples, and we said, 'Goodness! What species is this?'"

Fresh thinking often comes from outsider scientists, and Forward quickly came up with a simple, brilliant experiment. He created lab tanks with ocean water and lab tanks with estuarine water and then watched what larvae did. In ocean water the larvae swam towards the surface. In estuarine water, they swam down towards the bottom. "We can say their behavior changes," says Forward, "when they get into water near the estuary."

Those changes could help larvae enter estuaries. Megalopae could ride the bottom-water inflow that surges in from the ocean, or perhaps float in with nighttime flood tides. The chemical cues that kick off this behavior: yellow-looking humic acids that leak off the land whenever rainwater washes into rivers. Carried offshore by the outgoing plume, these cues off the land seem to be calling any would-be crabs to come home.

Hoping to find some blue crab larvae, scientists Mike Roman (left) and Tom Wazniak open the collectors at the end of long plankton nets. It's a long way from home for Roman who grew up partly in Illinois and even went back there for college. "Farmers want to be fishermen," he says, "and fishermen want to be farmers." He became a biological oceanographer who specializes in the ecology of zooplankton, the seeds of fishery productivity. Credit: Michael W. Fincham.

Working the day shift on the RV Cape Henlopen, Mike Roman was looking down at the net cable cutting through green, sun-spackled water. He's a wiry, weathered, low-key man who's stood on a lot of boat decks over the last 30 years. On several of those cruises, he found evidence that those chemical come-hithers could be carried far out to sea.

Working with Boicourt, he once measured outgoing freshwater plumes 24 miles off the mouth of the Chesapeake. Chemical cues carried that far out could draw in a lot of offshore megalopae, those tiny pre-crab life forms that look vaguely like a lobster and vaguely like a visiting extraterrestrial. It's like salmon smelling their home river, says Roman. "Larvae change their behavior and it helps ET get home."

When the big black net lands back on deck, Roman launched yet again into the endless routines of biological oceanography: haul in the net, unhook the canisters, pour the stuff through a sieve, rinse out the canister, close up the fruit jar. "See all the green stuff?" he asked a graduate student. "It's coastal stuff."

The Henlopen is right over the estuarine plume. The water in those fruit jars should hold plenty of chemical cues, perhaps plenty of larvae, perhaps a couple of answers. A north-flowing current gets these pre-crabs close, a chemical cue gets them ready. But what gets them across the threshold and back into the Bay?

A nighttime flood tide is one option, says Forward. Megalopae can rise up, ride one in, then wait on the bottom for the next one. He calls this ploy "leapfrogging up the estuary." Another option is a hurricane. "Hurricanes are dynamite," says Forward. "You want to get larvae coming into the estuary? Bring a hurricane up the coast. The winds blow and all of a sudden you get lots of larvae."

It's a hurricane that ends North's cruise. Hurricane Ophelia wanders up the Atlantic, downgraded and upgraded between tropical storm and hurricane. The last upgrade is the cruise killer. Winds and high waves off the mouth of the Chesapeake. The net monster wouldn't work. There's not enough Dramamine on board. The Henlopen heads home.

It will be another year before North leads another crab cruise. And it will be another four years before all the larvae in all those fruit jars are identified and counted and crunched and correlated with flood tides and ebb tides, day and night cycles, winds and currents.

So how do crab larvae come home? According to Epifanio, it's an unlikely odyssey full of twists and U-turns. Crab larvae leave the Chesapeake, borne south along the coast — only to make a U-turn in July and August when they encounter a north-flowing current. The next twist comes from wind events like nor'easters in September and October. They drive water and larvae south — only to have the rotation of the earth shift everything westward, creating a right turn of sorts. The shift is called the Eckman Effect, and it piles up water and larvae near the mouth of estuaries. Megalopae can now smell the estuary, and they react, moving down in the water column, getting ready to hitch a ride home.

"It took us a couple decades to nail all that down," says Epifanio. But it may not be completely nailed down yet. In her final cruise report, North finds little evidence that megalopae are riding in on flood tides. Her graduate student does cite data that megalopae do change their behavior near estuaries, swimming down during the day and up at night.

According to North, crab larvae can slip into an estuary during favorable wind events or episodes of low freshwater outflow and advantageous tides. Or they can be carried in by the bottom-water inflow.

And what drives the winds that drive the larvae? Could it be large-scale climate patterns like the Bermuda-Azores High? Or the North Atlantic Oscillation that shifts air masses between Iceland and the Azores? Or the Atlantic Multidecadal Oscillation in water temperatures?

"We don't understand that link between the large-scale weather patterns and what is happening at the mouth of estuaries to transport the blue crabs in," says North. "It is the forefront of where this kind of research is going."

The offshore odyssey of blue crab science seems headed even farther afield out over the Atlantic. And North is clearly excited by the prospect.

The old lightships with their drift logs are gone from the coast, replaced by flashing buoys or towers tall enough to be oil rigs. The Flagship Restaurant is closed. The old fishing boat that housed it stands abandoned, windows broken, its dining room vandalized. If you have $750,000, you can buy it. The exterior is chipped and peeling, but you can still see the blue crab that someone once painted on the hull.

The offshore hypothesis that was born in that boat has sprung some leaks, but it still remains afloat after three decades. Blue crabs still come and go every year in wildly varying numbers. But a big picture overview of their wanderings is emerging.

The frame of the puzzle is clearly outlined, and scientists are filling in more of the pieces in the middle, albeit with frequent disagreement. But a science-based snowball bush, a simple way to predict how many crabs are coming in next year, may still be a Holy Grail beyond their grasp.

That doesn't bother Boicourt, the oceanographer who supported the new paradigm from the start. "I'm not grumpy about the inconsistencies. I'm excited about the uncertainties, the unknowns," says Boicourt. He believes new technology will tell us where the crab larvae are. And new scientists will flesh out the theory. "We'll have young scientists fired up to figure it out."