Wow! Do you have seventy-one fish ponds?
If you counted that many then yeah seems so
That's crazy.
How many fish do you remove at once and how long does it take them all to respawn?
usually the 1st fish pond quest is easy (and easy/costless to reset) so most of the ponds stay at 5 population while any that have easy quests after that go up to 7 and sometimes 10.
I fish out 4 fish from each pond, and 5 from the ponds with 7 and 10 fish (as I use the quest marker on the 5 population ones as an indicator to harvest). That's currently around 300 fish per harvest.
As for how often they reproduce, that's up to the fish, anchovy are exceptionally fast, only taking a single day, but some like stingray take up to 4 days. Currently the only fish I'm done is albacore, though anchovy is pretty close.
And are those coops on Grandpa's memorial island? Can you get inside them all?
Yep, they're all accessible and no building clipping is required either, it's a pretty tight fit though!