Fishponds were places where the early Hawaiians used to raise fish for consumption. Lahuipua’a Fishpond at Mauna Lani is a very large pond which holds many Milkfish. Awa is their Hawaiian name.
These Milkfish were congregated at the entrance gate, no doubt hoping for someone to open it and let them out. The gates can be used for either purpose, but the idea of the ponds is that small fish can enter, but as they get bigger, they can’t get out. The fish in the top photo are far too big to escape through the grill.
There is another way out, as the sign in the second photo notes. Jacks and barracudas sometimes manage to get into the pools and will feed on the juvenile fishes. There are supposedly a couple of very large barracudas in this pool that have so far evaded capture and they’re probably living well off the inhabitants there.