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Were you jigging or was your jig just hanging in one spot? Trying to figure out the (red) lines on the graph. If you were jigging and pounding the bottom, it could be debris stirred up off the bottom. If the jig was just hanging there and that was a fish that swam up and took off it could just be air bubbles caused by the fish. Also you might want to see what your settings are at, if your sensitivity is maxed out that could be causing you to pick up everything in the water column.