The proposals, submitted by Mowi (formerly Marine Harvest), is the third attempt by the company to get the project underway.
In a letter to Highland Council the conservation charity lists key concerns about the farm’s location in biologically “the richest part of the Small Isles Marine Protected Area.”
These include the risk level to the marine environment; a projected biomass of 2,500 tonnes of salmon in open pens producing organic waste (mainly faeces) – on a par with a town of 33,000 people (for contrast, Oban’s winter population is 8,500 and 25,000 in the summer,) and the inadequacy of Mowi’s assessment to accurately determine the effects and spread of this waste.
It also says Mowi has underestimated the number of boats and cruise ships calling into Canna and on which the tourism economy depends.
The trust also raises fears pens and infrastructure would impede sea access and prevent larger ships from anchoring and manoeuvring.
The islands of Canna and Sanday were given to the trust at the bequest of their previous owner, the Gaelic folklorist and scholar John Lorne Campbell, in 1981.
Canna, 4.3 miles long and one mile wide, has a population of 18.
In 2017 the trust handed responsibility for Canna’s regeneration to a development trust run by residents.
Stuart Brooks, the trust’s head of conservation and policy, said supporting the proposals would betray the wishes of John Lorne Campbell.
“The protection of the cultural and natural heritage of Canna on behalf of the nation is our responsibility.
“Based on the information provided by Mowi so far about their fish farm proposals, we have no confidence their plans would not undermine, if not destroy, the very things that make Canna so important.
“In all conscience, we cannot support Mowi’s proposals. We would be failing in our core purpose as a conservation charity if we did, as well as betraying the wishes of John Lorne Campbell, who donated Canna and Sanday to our care.”
Clea Warner, the trust’s general manager for the North West and the Islands said: “This is clearly a sensitive matter for the residents of Canna.
“The Isle of Canna Community Development Trust will form its own views and communicate when they are ready. We have shared our response with them and remain committed to working with the Canna community.
A spokesman for Mowi said: “We are disappointed to see the NTS publicly campaign so early in this scoping process. Our science experts will continue to work with the Canna Development Trust to explore the potential for a sustainable aquaculture development, and look forward to discussing scientific evidence that is the basis for Canna’s decision whether to proceed.”