Mike Bell captured the remarkable image while he was taking a group of tourists for a trip on Loch Ness on June 27.
The sonar picture shows the bottom of the loch, a fish – and a long, thin object about 115ft (35m) below the surface.
When the 24-year-old circled and took readings at the same spot the object had disappeared, tending to rule out a log or other inanimate object.
The image shows a the sonar device with 101 metres at the top left hand corner indicating the total distance to the bottom at that point.
On the right hand side the device is going down in increments of 20 metres and on it about 15 metres there is a big blip which suggest a big fish.
However, at 35 metres there is a long zig-zag line suggesting a very large object, which Mike believes could be the Loch Ness Monster.