RET is being piloted on all Western Isles routes
The success of a government pilot project aimed at lowering ferry fares in the Outer Hebrides has sparked a fresh call for reductions elsewhere.
Allan Wishart, chairman of Shetland transport body, Zettrans, said the road equivalent tariff (RET) trial had shown the benefits of cheaper rates.
He said, in the Northern Isles, it could encourage tourism and trade.
But RET champion and councillor, Roy Pedersen, said, if applied to Shetland routes, it would increase ferry fares.
The RET pilot scheme on all ferry routes to the Western Isles has increased traffic significantly, according to the Scottish Government.
Its launch last October brought complaints from Orkney, Shetland and Argyll because they were not included.
Mr Wishart said the Northern Isles lay the furthest from the mainland and reducing fares would help the islands' economy.
He said: "I think the figures from the trial on the Western Isles show it is very successful when the fares are reduced."
Mr Wishart added: "We have the highest fares and people have to dig deeper to get to and from Shetland, I think this can breed inequality in the system."
Highland SNP councillor Mr Pedersen, a long-time advocate of RET, said: "First of all, this is a trial and you have to have comparison - an area which has RET and an area which has not.
"The fares within Shetland, the local ferries, are all below RET level. Fares on the Aberdeen-Lerwick route are already below RET and the subsidy for Orkney and Shetland are among the highest in the world."