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– The queen of the sea, a monster mollusk that inspired its own republic in Florida but now is as likely to be found in a frying pan or a gift shop as it is the ocean floor, is in trouble.

A marine preserve in the Bahamas famed for its abundance of queen conchs and intended to help keep the country’s population thriving is missing something: young conchs. Researchers studying the no-take park off Exuma, one of hundreds throughout the Caribbean, found that over the past two decades, the number of young has sharply declined as adult conchs steadily matured and died off.

The population hasn’t crashed yet like it has in the Florida Keys, but in the past five years, the number of adult conchs in one of the Bahamas’ healthiest populations dropped by 71 percent.

For the hundreds of slow-moving slugs that gather to mate, scientists fear a new, unexpected threat may doom the park’s population: old age.

The discovery also raises questions about the effectiveness of marine preserves, long viewed as a solution to reviving overfished stocks. If one of the Caribbean’s oldest and best marine preserves isn’t working to replenish one of its biggest exports — now regulated as tightly as lobster — what does that mean for other preserves?

“We can see [the preserve] works for grouper and sharks,” said Andrew Kough, lead author of a study published this month and a larval expert at Chicago’s Shedd Aquarium. “But for a lot of the animals you don’t consider as much, for example conch that are tied to a complex life cycle of larval dispersal, it’s not working.”

To find out why, Kough and a team of researchers set sail this month from Miami aboard a Shedd research boat. For 12 days, they’ll dive the deep channels surrounding the park in search of young conchs to count and measure. They’ll also take DNA samples to determine where the conchs are coming from. If they can trace the path of the young conchs, the hope is they can find a better way to protect them and manage the fishery.

In the Florida Keys, the conch looms large in oversized highway replicas, on T-shirts and as horns. But in the Caribbean, conch is a vital part of the economy, and the reason governments are so concerned.

Conchs used to be prevalent in Florida, too. But decades of overfishing nearly wiped them out. In the mid-1980s, the U.S. banned their harvest to save what was left. Yet more than three decades later, they still have not recovered in Florida waters, an inauspicious sign for the Caribbean.

Across the Caribbean, conchs are as good as currency. Almost anyone who can swim can grab one from the ocean floor and sell it or serve it. Cracked conch or conch salad appear on almost every menu. Their shells line porches and walkways. They are used for jewelry and bait. Whole industries, from fishermen to exporters, depend on a healthy population.

But regulating them has been uneven. While some islands impose seasons and limits on takes — in the Turks and Caicos, conch season starts in October and there are set limits on numbers and size — others have not. Populations have plummeted in Haiti, the Dominican Republic and Honduras, prompting the U.S. to ban their import.

The Bahamas has taken an aggressive approach. In 2013, the government launched a campaign to save what it considers a national treasure.

In recent years, Kough said the herds have thinned considerably, driving populations down. In the Berry Islands, he said, previous surveys found the sea bottom littered with conchs, which can live up to 40 years and are known to graze on algae that can kill sea grass. The last time his team visited, he said, they found hardly any big adults.

Fishermen are going out further to get them, he said. “It’s the adults that are in decline, and that just screams fishing.”

Scientists believe a healthy population needs 50 to 100 adult conchs for every 2.5 acres to sustain itself.

Working with the Bahamian government, Kough hopes to better understand how the baby conchs are circulating. About five days after female conchs release their eggs in long sandy strands, larvae emerge and get caught up in currents. Because the larval stage can last up to a month, the babies can float more than 100 miles. Kough suspects the young conchs from the preserve are winding up in unprotected areas hammered by harvesting.

Although the Bahamas restricts fishing, Kough said tighter measures may be needed. Existing regulations allow the take of any conch with a flared lip, the smooth curve on its rosy shell. Scientists now believe the thickness of the shell is a better measure of maturity, triggering a local move to change rules to require shells be at least as thick as a Bahamian penny.

“You don’t want to pull up juveniles,” Kough said. “You want animals to reproduce.”