When i was taught how to use a depth finder, i was told that it detects  air. like a fishes air bladder. have i been wrong all of this time. i was just told something different on another thread.

The answer is yes - and no.  The sound waves from the transducer work only in liquid, so theoretically, a transducer pulse hitting an air bladder would not be reflected back to the transducer, thus leaving an area of the screen where no data would appear.  However, the air bladder always seems to have a fish surrounding it that will provide plenty of reflection, so the bladder is never "not seen" by the transducer.

It bounces off the surface area of the fish. Although it will also show air bubbles, so if you start your big motor and engage the prop then turn around and drive back through you will pick up all the air bubbles (or lack of data as mentioned above) that you prop caused.