I would also like to know if you've actually checked this.
Nothing personal really, but if you could only catch as much fish as you've planted in that pond, what would be the purpose of stocking a pond at all? Fish never spoil in one's inventory, so why couldn't you just have them there instead? However, I believe I read somewhere that if you catch enough fish in one place it will become harder and harder to catch more unless you allow the fish to recover. Is it that effect you're referring to?
I've always thought that the reason you would need 10 was that there should be enough fish for their reproduction to kick in.