Heron.We had to recently cover our koi pond with netting like yours for first time in 30 years. Two came up missing. Probably racoon. They are sly little devils.
I tried the motion-activated sprinklers. They worked for a while. The heron learned that if they approached really, really, slow, they could still get in. The sprinklers work by detecting heat. And herons are covered with feathers. Pretty well insulated! And, I can’t use the sprinklers in freezing weather… so, I resorted to the net.