- 1). Turn the fish finder on by pressing the power button on the front of the unit.
- 2). Locate the depth reading on your fish finder. It is a numeric figure in one of the upper corners of the screen. Also locate the line that shows the bottom of the body of water. It is a line that runs across the bottom of the screen. Read it left to right.
- 3). Motor toward the middle of the lake, keeping your eye on the screen. You should come across a drop-off where the shallow water near shore drops into deeper water. At this drop-off, the number representing the depth will get bigger, and the line that indicates the bottom will slope downward from left to right.
- 4). Motor until the numeric depth reading gets smaller and the line that indicates the bottom slopes upward. Continue motoring to see if the depth increases and the bottom slopes downward again. If it does, you may be on a sunken island, but you need to first determine whether the entire piece of structure is surrounded by deeper water. Drop a marker buoy at each edge you have located so far.
- 5). Position your boat in the middle of the marker buoys. Drive the boat in a straight line away from the marker buoys, watching your fish finder. If the depth increases and the line that shows the bottom slopes downward, you have located a third edge. Drop another marker buoy on the edge.
- 6). Drive away from the third marker buoy, back toward the first two buoys. Steer the boat so it travels through the mid-point of the two buoys. Continue driving and checking your fish finder for deeper water and a down-sloping bottom. If you locate such a spot, drop a fourth marker buoy.
- 7). Travel around the perimeter of the four buoys, watching your fish finder to ensure you remain in water deeper than that between the four buoys. If the water on all sides of the structure is deeper, you have located a sunken island.
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