The fish will survive the winter by burrowing down into the mud at the bottom. A big pond like that may or may not freeze right over in winter, I guess it depends upon how cold it gets for how long. If you have a solar operated pump aerator (which I would if it were mine) that will help keep the water moving and slow down the freezing.
Again, if you get the water balanced, with aeration, lots of water plants, and fish to eat the mosquitoes, it will be a nice feature. I would think the banks are much too steep for your cows to use it as a drinking source. They'll fall in and you'll need a crane to get them out. Or they'll swim to the island and be there the rest of their lives . . .
