The Casco Bay Lines ferry Aucocisco III crosses Casco Bay on its way back to Portland in April. Gregory Rec/Staff Photographer

The state Public Utilities Commission on Tuesday ruled out a formal investigation of fare changes for ferry service to Casco Bay islands from Portland, effectively approving a mix of higher round-trip ticket prices and reduced fares for longer-term passes.

The PUC in May opened a summary investigation into fare increases approved by the Casco Bay Island Transit District that upped the cost of a round-trip ticket by 82%, created a flat rate for trips to any of the islands and lowered prices for 30- and 90-day passes, benefiting residents and workers.

The district said its first price hike in 15 years is intended to offset a growing deficit as operating costs climb and revenue is flat.

Commissioner Patrick Scully said at a PUC meeting that state law “expressly enables exactly the kind of shifting that this rate design will effect and for that reason I would not disturb the district’s decision.”

“I would note that the principal concern raised here is really more an issue of rate design rather than the actual revenue requirement and it’s inherent in rate design changes that some customers will fare better and others will fare worse,” he said.

Peaks Island summer resident Andrew Doukas had urged the PUC to reject what he called an “unfair rate schedule” and submitted a petition signed by nearly five dozen others asking regulators to intervene. The PUC said the “vast majority” of comments it received support the rate changes.

Doukas, who owns a cottage on Peak’s Island and uses the ferry three or four times a week, said he was not surprised by the PUC’s decision. Regulators didn’t consider whether the rates that are applied differently to residents and visitors are fair, he said. “They just looked to see if they’re legal,” Doukas said.

A peak season flat rate will be $14 and an offseason fare will be $7.20, according to the PUC. A round-trip passenger rate will increase between $3.45 and $6.30 in peak season and for offseason the fare will range from a 25-cent decrease to a $3.20 increase depending on the passenger’s destination.

The transit district also will eliminate commuter books of five discounted tickets and will reduce the prices of 30- and 90-day passes.

Regulators said the transit district has “undertaken a major update of its passenger rate structure” that it had not done for 15 years.

“In so doing, the district has prioritized rates for residents, commuters and other frequent ferry users over rates for tourists and other less frequent users,” the PUC said.

Those who oppose the rate increase said it discriminates against visitors and less frequent ferry users, and violates the transit district’s authority that they say requires it to account for the needs of island inhabitants and other passengers, the PUC said.

However, regulators said opponents “gloss over” Maine law that directs the transit district to preserve the affordability of passenger, freight and vehicle transportation for year-round island residents and the viability of the transit district’s finances and of the island communities. State law also allows the transit district to “reasonably cross-subsidize” rates of year-round residents to benefit the finances of the transit district and of the island communities, regulators said.

Regulators said the transit district has “reasonably exercised the broad discretion” from the Legislature and that its new rate structure balances its financial needs and the interests of all the passengers “within the bounds of that discretion.”