Sometimes mussels really do show their muscle.

When they do, watch out.

It’s known as “overset” in the shellfish industry, and it occurred last November to Penn Cove’s famously plump delectable delights, set to star in the this weekend’s Penn Cove MusselFest.

It happens when baby mussels overtake adult mussels as they are growing on lines hanging from dozens of harvesting rafts.

“They smothered out all our adult mussels. Yes, they muscled them all out, they did,” explained Tim Jones, farm manager with Penn Cove Shellfish, LLC. “It was a bit of a problem. But the year before, it really was a major overset. We harvested but we lost a lot.”

Not to fear — stay clam, if you will — Penn Cove, the shellfish company, expects to harvest tons of mussels fresh from Penn Cove, the bay, just before hungry crowds descend on Coupeville in search of the bountiful bivalves.

Thousands are expected to participate in the Coupeville Penn Cove Musselfest, started 31 years ago. The two-day event features a mussel chowder tasting contest, chef demonstrations, boat tours of the mussel rafts and a mussel-eating contest. Saturday also includes a beer and wine tent with live music.

And, of course, heaps of steaming mussels are sold at vendors set-up outside and in the town’s restaurants and bars that are usually packed like sardines with out-of-town customers.

“MusselFest really helps brings traffic not just to town but it benefits the island far and wide,” said Vickie Chambers, executive director of the Coupeville Historic Waterfront Association, a nonprofit that sponsors the event. “This is one of the biggest weekends of the year for businesses. It provides a huge spike in revenue and for some, allows them to stay open during the shoulder season.”

Tourists often wonder why the event is held during the cold, windy, rainy days of March.

“It’s solely because that’s when our businesses need it!” said event coordinator Cindy Olson.

“It’s our second biggest weekend of the year and it really marks the start of tourist season,” added Christian Chambers, owner of Aqua Gifts on Coupeville’s Front Street. “We really work on stocking up on shelves. If there’s a mermaid in town anywhere, it sells.”

Every year, a mermaid stars as the logo for MusselFest. This year’s red, white and blue poster design was created by artist Andra Freet.

The event is also advertised on drink coasters that are distributed to the dozens of regional restaurants and clam bars where Penn Cove mussels are sold.

Penn Cove Shellfish supplies about 40 pounds of mussels to each of the 13 Coupeville restaurants competing in the mussel chowder contest. Participants pay $10 for a tasting ticket that includes 2-ounce servings from four restaurants.

The tasting tickets usually sell out first thing in morning as people wait in line outside the Coupeville Recreation Hall for the 10 a.m. distribution start.

“Years ago, we started with one tasting ticket choice,” Chambers said. “This year, we’ve added a fifth ticket of choices. We’re really hoping we won’t have to turn people away.”

While 13 restaurants are participating, a few agreed to be on two of the five tasting tickets, meaning they could have double the customers.

Over two days, some 20,000 mussel chowder tastings could be slurped out of the tiny colorful ceramic cups that come with the $10 tasting fee.

Despite that impressive number, organizers say MusselFest is much more than the tasting tour.

Chefs from Seattle and Portland will demonstrate a different mussel dish every 30 minutes in the recreation hall.

“MusselFest really shows the diversity of mussels and chefs enjoy coming up for the demonstrations,” says Jones, of Penn Cove Shellfish.

The company is the oldest and largest mussel farm in the United States, started in 1975 by the Jefferds family.

The company grows and harvests over two million pounds of mussels per year from farms located in Penn Cove and Quilcene Bay.

“We don’t feed the shellfish and we’re not changing the biology of the cove,” Jones explained.

In Penn Cove, mussels cling on some 50,000 collector lines. Each 20-feet long, hanging under 42 rafts; each line can weigh up to 50 lbs. Mussels are harvested at about 14 months, then the spawning season comes around again in the spring.

“We’re stewards of the cove. This is a resource that will be here for generations and generations,” Jones says. “We are in the business of sustainability.”

Tim Jones, farm manager for Penn Cove Shellfish, pulls up a line of mussels from one of the company’s rafts. He estimates 4,000 lbs of the bountiful bivalves will be harvested and used for this weekend’s MusselFest in Coupeville. Photo by Patricia Guthrie/Whidbey News-Times