"Ride The Ducks" sightseeing rides - recognized by their distinctive amphibious vehicles and passengers blowing duck calls - ceased in Baltimore as of Tuesday.
Company officials said Wednesday that they had decided to focus investments in other cities.
Union representatives say the company closed to retaliate against tour guides who were trying to organize. Phil Ornot, a United Steelworkers organizer, said he filed charges Wednesday with the National Labor Relations Board, alleging that the company intimidated, coerced and threatened staff.
Bob Salmon, vice president of marketing and sales for the Georgia-based company, denied that the petition to form a union had prompted the move.
"We've realized that in order to get Baltimore Ducks to the level it needs to be would require additional investments on our part," he said. "At the end of the day, we didn't see the potential to be significant enough in terms of sales potential to warrant that kind of investment."
The company operated 35 to 40 trips on peak days in eight "ducks" - vehicles based on a World War II design that travel on roads or in the water with passengers blowing duck calls on "quackers." The tour company, founded in Branson, Mo., started its Baltimore business in 2002.
Ride the Ducks had six year-round employees and 23 seasonal employees in Baltimore. They were notified Tuesday about the decision to cease operations and will receive severance pay, Salmon said. He said the company began considering whether to shut down the Baltimore operation in July. "The process had begun before we had even heard the captains were organizing," he said.
Ornot said the operators contacted him in July. Their concerns primarily focus on safety, including scheduling breaks so operators weren't taking back-to-back tours, as well as the safety of the vehicles themselves, he said.
Salmon said the company has never had an incident with a vehicle in which passenger safety was an issue.
The union filed a petition to organize on behalf of the Ducks operators Aug. 6. Mail-in ballots for an election were scheduled to be sent out Friday, said Wayne Gold, regional director of the NLRB office in Baltimore.
As of Tuesday, that election had not been canceled, he said. The board will investigate the charges to see if they have merit and determine whether to file a complaint against the employer. If an investigation reveals that the business closed as a retaliatory action, one possible remedy would be to restore the operation, he said.
Tom Noonan, president and CEO of Visit Baltimore, the Baltimore-area tourism and convention organization, said he was disappointed to see Ride The Ducks go.
"It was one of those activities that our tourists in town really enjoyed," he said. "We hope somehow they'll be back."