The Atlantic menhaden is one of those big things that come in small packages. It’s a pipsqueak of a fish, but it feeds some of the most important fish in the ocean. If it vanished, marine biologists say, the ecosystems of the Atlantic and the Chesapeake Bay could come crashing down.

As the population of this once abundant fish dwindles in dramatic fashion, that theory might be put to a test. Humans don’t eat the oily and bony menhaden, but it’s caught by the metric ton each year, ground into meal and fed to farm fish and livestock.

Environmentalists fear that the commercial catch takes food from striped bass, bluefish, swordfish, king mackerel, tuna, loons and eagles that rely on menhaden.

The reduction of menhaden, widely dubbed “the most important fish in the ocean,” is such a concern that the Atlantic States Marine Fisheries Commission is scheduled to meet Tuesday to consider whether its harvest for commercial products and sport-fishing bait should be significantly lowered for the first time in years.

“Menhaden is ecologically critical to the marine ecosystem along the east coast,” said Bill Goldsborough, fisheries director for the Chesapeake Bay Foundation. “It’s not much of an exaggeration to call it the most important fish in the sea. It’s an essential link in the food chain.”

In addition to feeding salt­water fish that Americans love to fish and eat, tiny menhaden feed on phytoplankton that contribute to algae blooms and oxygen-depleted “dead zones.” A dead zone currently in the Chesapeake is on track to be the largest ever, Maryland and Virginia state biologists have said.

At the meeting in Alexandria, commissioners will be guided by an assessment that says menhaden has not been overfished, spokeswoman Tina Berger said. But the commissioners are concerned about other data that show the number of young fish entering the population is falling, and that the number of eggs that sustain menhaden has started to dip below a standard they set.

The commissioners are expected to consider a proposal to increase the number of young menhaden, as well as egg production, possibly by reducing the menhaden catch, experts say. A final decision could be made in November after a three-month public comment period is held on whatever proposal the commission adopts, Berger said.

The meeting will be closely watched by Omega Protein, which last year fished about 160,000 metric tons of menhaden in Atlantic coastal waters — 80 percent of the total catch. The other 20 percent is collected by small companies that fish it for bait.

The fisheries commission assessments of the menhaden stock show a dramatic decline: Fifty years ago, the abundance of menhaden a year old or less was nearly 90 billion. Twenty-five years ago, it was 70 billion.

Now, after continued fishing that environmentalists say is loosely regulated, only 18 billion menhaden of that age remain.

Omega Protein spokesman Ben Landry said factors such as poor water quality have reduced the fish.

Nevertheless, 13 coastal states from Maine to Florida under the commission’s jurisdiction have banned Omega Protein from harvesting menhaden in state waters with its huge ships and large purse seine nets.

Only Virginia allows the company full access to its waters in the Atlantic and the Chesapeake Bay. North Carolina gives very limited access, according to Landry and the Coastal Conservation Association that monitors the menhaden harvest.

Virginia also stands alone in managing its menhaden fishery from an unusual place, the state General Assembly.

Others rely on specialists at environmental agencies such as Maryland’s Department of Natural Resources and New Jersey’s Division of Fish, Game and Wildlife.

The Virginia Marine Resources Commission declined to comment on why state politicians gave it control over all other fish species but held on to the menhaden to manage themselves.

“It’s political,” said John Bello, who sits on the board of directors for the Coastal Conservation Association. The association has unsuccessfully pushed legislation to transfer management of the menhaden fishery to professionals at the VMRC for 10 years.

Omega Protein has a processing plant in Reedville, Va.; a reduction in the harvest might threaten 250 jobs there.

“I think there’s this notion that environmentalists have put out for a long time that this is a depleted stock and the reason behind the depletion is Omega Protein,” said Landry, the spokesman. “Overfishing occurred in one year, 2008, in the last 10.”

Landry said the abundance of menhaden is “far over the commission’s threshold,” and the commission, he said, should keep that in mind. He said there are enough spawners and eggs in the population to replenish itself.

“Trillions of eggs are released in the water,” he said. If they do not survive, poor water quality, water temperatures and predators share more of the blame than Omega Protein’s giant nets.

But environmentalists say those factors cannot account for an 88 percent drop in menhaden since 1984.

Its schools were once so abundant that rivers and streams turned silver when menhaden swam. Native Americans and European explorers sometimes tried to catch menhaden by hand.

Without menhaden, the ocean’s ecology and food chain would be completely thrown out of whack, marine biologists and environmentalists say.

Search the bellies of dolphins, whales, sailfish and seafood-eating humans and at least traces of menhaden will show up.

“There’s plenty of science showing that striped bass in the Chesapeake Bay have an infection, a wasting disease that’s fatal, and the most likely culprit is poor nutrition,” Goldsborough said.

Berger, the commission spokeswoman, played down that science as old. She said studies have since shown that diseases suffered by striped bass were attributed to other factors, such as water quality.

The commission’s focus should rest on the menhaden’s importance to the Atlantic’s food web, said Jay Odell, director of the Nature Conservancy’s Mid-Atlantic marine program.

“It’s impossible to imagine that reducing the menhaden stock so much has not had some negative impact,” Odell said. “Fisheries scholars differ on exactly what the cause and effect of the different changes are. But menhaden sit at the very base of the food chain, and scientists around the world are saying they need to be managed more conservatively.”