Several hundred demonstrators have taken part in a beach protest against plans by one of Scotland’s busiest ports to transfer millions of tonnes of crude oil between ships.

Campaigners formed a line carrying placards with the words “don’t spoil our dolphins with oil” and “Nairn says no to oil transfers here” on the town’s beach.

Others dressed as dolphins as they took the fight against the proposal to the shores of the Moray and Cromarty firths, which would be affected by any spill.

Opposition group, Cromarty Rising, fear an oil spill would damage marine life in a stretch of water which attracts more bottlenose dolphin visits than any other in Europe and is close to other designated nature sites.

They say it demonstrates how desperate The Cromarty Port Authority is to make money with the proposal expected to undercut an established transfer operation in Shetland by almost 70 per cent.

They claim the price is low only because it would be intrinsically less safe than what has been happening at Sullom Voe for many years, but the Cromarty Firth Port Authority (CPFA) says it has to be competitive.

Ship-to-ship (STS) transfers off Shetland take place in relatively sheltered waters between tankers tied alongside a jetty with other vessels in attendance for safety.

The CPFA wants to see up to nine million tonnes of crude oil transferred a year between tankers riding at anchor just outside the Cromarty Firth in the open waters of the Moray Firth.

Cromarty Rising fears an oil spill would would herald environmental catastrophe. They have calculated that the charge for three tankers transferring a total of 120,000 tonnes of oil at Sullom Voe would be £49,792, while the Cromarty Firth would charge just £15,120.

Shetland Islands Council, which operates Sullom Voe as the harbour authority, and CFPA, confirmed these figures respectively as being accurate.

A council spokeswoman said the charges at Sullom Voe reflected the fact that the council could not subsidise the harbour’s operation. They also covered all the safety precautions including extra vessels.

Sullom Voe was a designated Site of Special Scientific Interest so there had to be environmental safeguards, she said.

The CPFA also stressed how seriously it takes it environmental responsibilities, and a spokeswoman said: “We continue to receive requests from customers who would like to carry out these operations in the Firth, which is our reason for pursuing an additional licence (for STS ).”

She said the pricing was competitive to attract the maximum number of projects to the Cromarty Firth.

But economist Toy Mackay, an authority on the oil industry, said: “I am surprised that CFPA wants to get involved in ship-to-ship transfers because it is a declining business. Most of the oil involved is from the West of Shetland fields and the Sullom Voe terminal is the obvious location for that work.

This reinforces campaigners’ fears that the oil would come from the Baltic States, using flag of convenience vessels, underlining how irresponsible the port authority’s strategy is.

Marine biologist Dr Greg Fullarton, a Cromarty Rising spokesman, said “The port authority’s chief executive recently admitted to community council representatives that the STS money was required to service the authority’s loans for the £25 million upgrade of facilities at Invergordon.

“Certainly it has a lot of debt to service, £6m, and they may have to borrow the same again for phase four of the work. But why on earth has it left itself so exposed financially, so dependent on an activity for which it had no consent, and still doesn’t?”

As well as the protesters, 22 community councils around the Cromarty and Moray firths oppose the plan.

The port authority would not comment on the provenance of the oil, but said: “The port’s bank loans increased from £216,145 in 2014 to £6,086,562 in 2015. This is primarily due to the investment the port made in its phase three of development.””

The increased quayside capacity had placed it ahead of many competing ports in the facilities it can offer for large oil, gas and renewable energy projects.

She said funding partners were satisfied with the authority’s ability to repay its debts and were content to extend the financing facility to make further investments such as a new cruise liner berth.