Overfishing may be the buzz word on the waterfront to explain the decline of many stocks of fish around Vineyard waters, but it isn’t heard often enough when it comes to explaining the loss of bait fish.

On Wednesday night at the Chilmark Public Library, a lone man stood before an audience of anglers and commercial fishermen to report the worst environmental tale needing to be told is the loss of one of the most valued forage fish in the ocean, which used to swim in abundance in these waters but is almost gone — menhaden.

The loss of menhaden, often called bunker in these waters, is having a far more devastating impact on the ecosystem than the community at large realizes. The decline of menhaden is not just affecting other species of fish, it is causing a decline in water quality.

H. Bruce Franklin, the author of The Most Important Fish in the Sea, spoke for an hour about a fish that used to fill the waters from Maine to the Gulf of Mexico but has been reduced to a fraction of its former presence.

“Menhaden is the nation’s largest fishery,” Mr. Franklin said. “Since the end of the Civil War more menhaden has been caught, not just by numbers but by weight, than the combined landings of all commercial catch of all other finfish put together.”

Mr. Franklin outlined how the fishery was huge, eclipsing all other fisheries. Ships went up and down the coast using football-field-size nets and harvesting menhaden to the edge of collapse.

Yet not one fish is eaten by a human. All the fish are harvested, ground up for industry, made into fertilizer, farm feed and other products.

Given that these products can be made from less costly sources, Mr. Franklin said this use of menhaden is too costly to the health of the nation.

Menhaden are essential not only as food for bigger fish but as filter feeders, responsible for clearing waters of harmful overabundance of algae. Mr. Franklin said a single menhaden is able to filter four gallons of water a minute.

Using scientific data, Mr. Franklin blames the ecological collapse of the Chesapeake Bay, including the decline of the oyster fishery there and the rising incidence of diseased striped bass in the bay, to a drop in water quality, which he ties to lack of abundance of the central character in his book. He said similar stories can be told of the decline of estuaries in Maine.

No one is alive today who can remember when acres of menhaden used to fill the waters of New England to the degree that European colonists found centuries ago. Mr. Franklin shared stories of what was once the most abundant fish and now is considered increasingly rare in these waters. At the height of the New England menhaden fishery, there were 13 factories processing menhaden in Narragansett Bay alone.

Mr. Franklin speculated that the decline now of alewives and shad in New England waters is tied to the decline of menhaden. It is widely understood that striped bass, bluefish and other predators prefer to feed on menhaden. Without enough of this fish in their diet, these predators are pursuing and putting pressure on other vulnerable species.

The reduction of menhaden by industry over the last century is so thorough, he said, that “supporting the profit for a few individuals has torn down the entire structure of marine life as we know it.”

A number of Island commercial and recreational fishermen attended the talk, including Janet Messineo of Vineyard Haven, president of the Martha’s Vineyard Surfcasters Association.

On Thursday, Ms. Messineo said she remembered seeing large schools of menhaden in Vineyard waters.

“I have not seen a school of bunker in Vineyard waters since 1986,” she said. “I really took to what he said about menhaden being the liver of the ocean. Menhaden clean the waters as filter feeders. Once you have taken them out of ecosystem, the estuaries become choked with toxins. I love this book.”

Jonathan Mayhew, a commercial fisherman from Chilmark, said Thursday: “I found the talk very impressive. It was an eye-opener. I think the author is correct in stating people don’t realize what a huge industry the fishery was in this region. I think he was right on; regarding the impact of taking these important filter feeders out of the equation, in the waters of the Atlantic seaboard and in the Gulf of Maine.”

In an interview Thursday, avid recreational fly-fisherman Kib Bramhall said, “I think the book is one of the most important books ever written about fish. I think the guy is the Rachel Carson of the sea, pointing out the dangers just in time to save it.”

“I was awed by the passion and intelligence that he brings to this situation,” Mr. Bramhall said. “This is like one man’s crusade, that could be a critical part in protecting habitat.”

Warren Doty, president of the Menemsha Fisheries Development Fund, who was responsible for inviting the author to speak, said afterward: “I think we should start a petition drive to get this menhaden reduction fishery stopped.”