Here's my guess. Fish kill was likely caused by overpopulation vs poison goose poops. While catch and release is great fun without the removal of fish overtime they will overpopulate and die as a result. Fish kill happened right after a stressful event (hot summer temps)?

There are some ways to help with this....Fishing derby for the kiddos, scout troop (boys or girls or both). Team up with a local shelter to see if they would take fresh fish after so it doesn't go to waste.

Could go for an annual electro shock survey where all fish are removed...

Encourage the people that fish the lake to report what they catch and please take some home to fry them up....

I'm completely guessing though. I live in the KC area and we just had some really warm weather roll through. Guessing you guys got it too. One of the other more experienced guys may have a different opinion but from what I've read it sounds like nature decided to clean out the garden. The good news is that Nature abhors a vacuum and those remaining fish will refill the pond.

1.5acre LMB, YP, BG, RES, GSH, Seasonal Tilapia
I subscribe to Pond Boss Magazine