The divide emerged after campaigners pledged to fight proposals by the Scottish Salmon Company to establish the site just off Gometra, less than three miles from world-famous tourist destination Fingal's Cave on Staffa and nine miles from Iona.
Only five adults, and their children during the summer holidays, live on the island. They are all opposed to the plan.
However, there are others on Mull and Ulva who would welcome the jobs on offer from this scheme, along with another planned by the same company at nearby Loch Scridain. Seven jobs would be created between the two sites, in an area where employment opportunities are precious.
But Gometra islanders say the scale of the development is inappropriate, claiming the cages would stretch to the equivalent of five Wembley stadium football pitches, and the whole site equivalent to 42 Wembleys.
Crucially, the islanders fear the fish farm would make travelling to and from the island by boat more dangerous, forcing them out from the shelter of the island's north coast into more exposed water in Loch Tuath.
Gometra is connected by a bridge to the neighbouring island of Ulva but neither have proper roads. It takes about 50 minutes by quad bike and about one-and-a-quarter hours in a 4x4 vehicle to drive across rough hill tracks to get to the ferry to Mull at the east end of Ulva. By boat it is just 20 minutes, but the cages would be situated on that route.
Residents are also convinced the fish farm would damage the marine environment and kill the green tourism that brings people to see dolphins, porpoises, seal, basking sharks and minke whales. It is fear they would be repelled by the anti-seal sonar equipment the farm would use.
Gometra and Ulva are part of the Loch na Keal National Scenic Area (NSA), one of 40 such areas protected due to their outstanding scenery. Scottish Natural Heritage has told Argyll and Bute Council, which is due to consider the application next month, it opposes the application.
Mull Community Council said of the two sites: "We are frankly aghast that something of this scale is even being considered."
Iona Community Council also objects, not least because it would present "a direct conflict of interest with wildlife/eco tourism, which Mull and Iona are increasingly famous for and is now the basis of the local economy."
Roc Sandford bought the island 20 years ago when it was deserted and had one habitable house. He said: "This proposal would devastate the experience of living on the island or of visiting the Staffa Archipelago. If there is one place in Scotland not to site a salmon farm, this is it."
But there have been expressions of support from businesses on Mull and local residents.
Ulva resident Rebecca Munro, who lives close to where the fish farm would be located, reportedly said she was backing the development. She said: "There are a lot of people in favour but who don't speak up."
Stewart McLelland, chief executive of The Scottish Salmon Company said: "We have been overwhelmed by the local support for our plans to expand our operations in Mull."
A spokeswoman said it consulted with the Northern Lighthouse Board, Mull Aquaculture and Fishermen's Association, West Highland Anchorage and Mooring Association and the Royal Yachting Association, and none had raised concerns about navigating around the site.