Loch Etive, north of Oban, has the famous Falls of Lora at its mouth and Ben Cruachan to the east.
Fish farm firm Dawnfresh has five trout farms on the loch but wants a sixth which will be the largest. The £2.5 million development would be a 14-pen rainbow trout farm.
Campaigners say it would cover an area the size of three-and-a-half football pitches on the surface but its mooring grid would be equivalent to 14 pitches.
Lawyer Guy Linley-Adams, who is representing a local estate fighting the plan, said: "It will physically bar others using the loch from a huge area of the loch surface including removing safe and easy access in an inlet for the anchoring and mooring boats."
Gideon Pringle, fish farm manager for Dawnfresh, said: "Trout farming has been established on Loch Etive for 30 odd years without causing the damage being predicted."