Counting Fish

I need a figure for the fish population in a lake.

I catch 40 fish and mark a scale on each fish so that they can be identified if caught again.

The fish are then released and one week later I once more catch 40 fish and look to see which of these I caught before.

How does this help me come up with a figure for the fish population in a lake?



A poster of this problem can be found here.