The Biology of Abundance

To Catch a Bay Scallop

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Exotics in the Chesapeake

An Endless Invasion?
Green Crabs, New England Intruders, Move West

By Michael W. Fincham

On Martha's Vineyard, the new scallop season opened with high skies, bouncing sunlight and blustery winds but only modest hopes that a good harvest would be had this winter out of the harbors, bays and tidewater ponds of this picturesque island.

So Mike Picciandra spent this windy first morning fishing for scallops - and in the afternoon he went fishing for green crabs. Picciandra has a daily limit on scallops: as soon as he catches three bushels he has to quit. On a good day he may get $8 a pound for his scallops.

Most scallop fishermen think of the green crab as a local pest, but few realize it is actually a transplant from European seas.

On green crabs, however, he has no season, no limits - just as green crabs have no season, no daily limit on scallops: they keep eating scallops all day, every day. And that's one reason Picciandra goes out after green crabs. The other reason is money: the green crab is so unpopular up here, it carries a price on its head. "The town puts a bounty on these crabs," he explains, "and I as a bounty hunter, I go out and catch the crabs." He'll only get 40 cents a pound for his green crabs, and most of them will end up on a compost pile, but he reasons that by the end of the day there'll be fewer 40 cent crabs eating up $8 scallops.

Only four years ago, the scallop fleet that motored out of Edgartown harbor on opening day numbered more than 100 boats. This year the winds are so high and the hopes are so low that the "fleet" totals less than 20 boats. For several weeks, Picciandra and other fishermen had been motoring out to Cape Pogue Bay and Edgartown Harbor and Sengekontacket Pond to check on this year's crop, and by opening day the word has gone round the island: another off year for scallops.

An Unwanted Bounty

Green crabs are probably not the primary culprit in the scallop decline - but they stand accused as accessories to the crime. Most scallop fishermen think of the green crab as a local pest, but few of them realize this small green predator is an immigrant into these coastal waters, a transplant from European seas like the North Sea and the Baltic.

For marine scientists, green crabs are a prime example of yet another successful invasion of American waters by an exotic species. That makes them an instructive case study for the developing science of "invasion ecology."

Green crabs are probably not the primary culprit in the scallop decline -- but they stand accused as accessories to the crime.

Green crabs may be exotics, but you catch them much the same way you catch blue crabs, their larger, more popular cousins. Picciandra uses a technique familiar to any Chesapeake Bay waterman: he builds pyramid-shaped wire mesh traps, baits them with fish and sets them in the water attached to crab buoys. Every two or three days, he checks his pots and makes his haul. "I weigh them up, tag them, and then once a week I submit a voucher to the town treasurer and they pay me."

That bounty on the green crab is paid by the town of Edgartown, the scenic village on Martha's Vineyard where Hollywood filmed the blockbuster movie Jaws 20 years ago. In that epic, a great white shark slid around eating hapless swimmers and hard-drinking fishermen. In Edgartown today, green crabs are leaving people alone, but they are attacking scallops and other native shellfish.

Food for crabs or food for people?

Paul Bagnall, a shellfish biologist, checks seed scallops at a nursery run by the Martha's Vineyard Shellfish Group. Last year the group planted 600 thousand seed scallops in the harbors, bays and ponds around the island.

"Green crabs are a problem in our ponds because they eat soft shell clams, quahogs and oysters - as well as scallops," explains Paul Bagnall, the shellfish biologist for Edgartown and the man responsible for putting a bounty on green crabs. He wants to protect the scallop fishing in Sengekontacket Pond, a two-mile saltwater embayment shared by Edgartown and the neighboring town of Oak Bluffs.

Over the last year, he placed more than a quarter million seed scallops in the pond from the town's small shellfish hatchery. He hopes most of those scallops are caught by fishermen rather than green crabs.

Bounty programs are somewhat controversial as a predator control technique. Proponents like Paul Bagnall make this simple argument: every time you catch a crab, you save a number of scallops, clams and oysters. Opponents to bounties argue that crab populations in a tidewater pond reach a steady state equilibrium. Every time you catch a crab, you simply open up space for another crab to move in. The bounty is wasted money and effort.

The arguments for and against are largely theoretical, the evidence mostly anecdotal. As a result, a number of New England towns have sporadically tried bounties in hopes of protecting their shellfish beds. And most have abandoned them without knowing whether they worked or not.

Good evidence might come from the work of Greg Ruiz, an ecologist who has been setting a lot of traps for green crabs along the Atlantic coastal marshes of Connecticut and along the Pacific coastal bays north of San Francisco. As a graduate student, Ruiz worked with Jim Carlton, one of the country's leading researchers in invasion ecology, and his early research centered on San Francisco Bay, this country's hottest site for invasive exotics. Now with the Smithsonian Environmental Research Center on Maryland's Chesapeake Bay, Ruiz has been studying the history and pattern of green crab immigrations. His question: can we predict the ecological impacts of future invasions?

Claws II

According to Ruiz, there have been two major green crab invasions of American waters: one on the east coast, one on the west coast. The green crabs of Martha's Vineyard are descendants of the first influx that began more than 150 years ago. Their ancestors reached the Atlantic coast under sail, riding along in the dry ballast of wooden ships or clinging to the mossy crevices of heavily fouled outer hulls.

Those green crabs first found suitable habitat in coastal embayments from New Jersey to Cape Cod. In the early 1900s, they began spreading northwards, ranging up through Maine and the maritime provinces of Canada, all the way to Cape Sable, Nova Scotia. Their arrival in Maine in the 1950s coincided with dramatic declines in the soft clam fishery, setting off an earlier search for predator control strategies.

The second great invasion of green crabs was discovered as recently as 1989 in San Francisco Bay. First a fisherman found a large male crab in his gill net. The next summer bait trappers began finding green crabs in lagoons along the west side of the bay. Since scientists were able to track this west coast episode nearly from the start, this second invasion has revealed much more about the green crab's migratory strategy.

These recent immigrants probably arrived as stowaways in the ballast water of big commercial ships hauling televisions, trucks and cars - and the oil to run them. According to Jim Carlton, now an ecologist with Williams College, there are also a number of other less obvious routes. Green crabs, like their ancestors, could also be hanging onto the fouling found in seawater pipes of big ships. Or they could have come west directly from New England, tucked in with the seagrass and kelp used for packing and shipping Maine lobsters and Atlantic bait worms.

On the Western Front

The green crab invasion, according to Ruiz and Carlton, works something like this. Young green crabs thrive best in fairly protected coastal ponds and lagoons and embayments. There they eat molluscs, crustaceans, polychaetes and green algae. Though they have trouble cracking a hard clam, they can dig out soft clams buried six inches deep. In these food-rich coves and ponds and marshes they can grow and reproduce in sufficient numbers to create a "beachhead population." On the west coast, green crabs took three years to establish a beachhead in San Francisco Bay.

Their first major foray beyond the Golden Gate came in 1993 when they reached Bodega Harbor 75 miles north. According to Ruiz, it was not grown crabs but crab larvae, offspring of the beachhead crabs, that made the trip, gliding northwards at five miles a day on the current. These excursions probably occurred during short windows of opportunity lasting five to fifteen days when the normal northerly winds die down. Since 1993, green crabs have been building up a new beachhead in Bodega Harbor from where a new crop of larvae can travel north towards Oregon, Washington and Canada.

On the Global Front

As the entire globe becomes a free trade zone and shipping traffic continues to increase, green crabs will invade other estuaries - and so will other species. "What we've seen over the last few decades is really an explosion in the amount of commercial traffic that is bringing in ballast water to different parts of the world," explains Ruiz. "At an any one time, there may be tens of thousands of vessels moving around the world carrying ballast water. The effect has really been to open up a conduit for the transfer of species from one part of the world to another part of the world."

Whether bounty programs work remains an open question. In Edgartown, Paul Bagnall claims his bounty program is helping green crabs, scallops and fishermen coexist. "We have removed over 15,000 pounds of green crabs over the last five months from this pond," he says. "We have reaped the benefit of this by having a scallop harvest up here this year. It isn't the best the pond has ever seen, but there are certainly plenty of nice healthy scallops to be harvested."

Mike Picciandra ponders yet another exotic threat. The local name for this immigrant is Japanese grass, but is actually an alga (Codium fragile) that roots itself to the bottom, much like seagrass. It can replace eelgrass beds that scallops like to set on.

Will Edgartown - beset by green crabs and a growing population - ever see great scallop harvests again? Picciandra, for one, says it could happen - but only if the town deals with problems like reseeding, redredging, rising nitrogen levels, spider crabs, green crabs and the influx of yet another non-native species: Codium fragile, a bottom-rooting alga that locals call Japanese grass. In many parts of the pond Japanese grass is replacing the eelgrass beds that new scallops like to set on.

Though the threat of more invasions looms large with the approach of every tanker and container ship across the horizon, marine exotics remain a small, nearly invisible blip on the environmental radar screens of most Americans. "Look at the problems it is causing us," laughs Picciandra, the bounty hunter. "I'm killing this green crab for two years, and seeing it all around - and I didn't know that it didn't belong here."

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