I meant just a deep hole (lined or unlined) right at your drain outlet.
It serves two purposes. You can draw down your water and concentrate all of your fish in the sump. That way it is easy for you to pick good broodstock to save, sample the sizes of your fish, select for males or females, etc.
The other purpose is to collect the junk that will get in your pond specifically to pi$$ you off and clog your drain. With a sump installed, you can start flowing some water and most of the leaves, pine needles, etc. will collect in the sump and you can more easily scoop them out to keep your drain running and clean the bottom of your pond.
