PORTLAND – The cruise ferry Scotia Prince makes its last overnight voyage of the season Tuesday from Portland to Yarmouth, Nova Scotia, but its future after that is uncertain.

Despite $10 million spent on improvements to the ship, ridership dropped by 23 percent this year, according to Scotia Prince Cruises, which plans to keep the vessel in Portland over the winter while its owners decide whether to continue operations next year.

“We want to sail [next year], so we’re planning to do so,” said Mark Hudson, vice president of finance and communications for Scotia Prince Cruises. “We’ll wait and see as we come out of this season what the final results were.”

The 35-year-old ferry operation faces problems that go beyond the drop in business.

It could be getting competition from Bay Ferries Ltd., operator of The Cat, the high-speed ferry which now runs between Bar Harbor and Nova Scotia, and is hoping to expand to Portland.

The Federal Maritime Commission is reviewing the legality of a 30-year-old agreement that prevents Portland from becoming home to another ferry to Yarmouth.

A new concern for the Scotia Prince surfaced this summer when it was forced to vacate its city-owned terminal because of mold and structural problems with the building that has been home to the ferry since 1969.

Those problems hit as the company was already trying to deal with a gradual decline in overall tourist traffic between the United States and Canada. The number of drivers from the United States to Nova Scotia has plummeted 42 percent since 1972, a trend that accelerated in recent years amid fears of terrorism and concerns about border security.

The Scotia Prince has been getting a larger share of the market, but the market is shrinking too fast, Hudson said.

“We think we’re hitting the wall now,” he said. “Unless there’s a turnaround in the total volume of Americans going to Nova Scotia, it’s going to be very difficult for us or impossible for us to increase our loads.”