I had around 20 larger koi (average 1.5ft long) and several smaller koi from a couple inches up to 8 inches just disappear from my pond over the winter. I'm in Michigan, and my pond is natural spring flowing water with a small over flow that has a screen. It is about 50ft by 20ft, and 2ft to 5 ft deep (varies) and is peanut shaped. I have had problems with herons, but never noticed any tracks in the snow all winter. I know they were all there before the first big freeze. We had several thaws over the winter, but my koi generally stay in the deepest part when its cold and so I never stressed over not seeing them, thinking they were in hiding under the sediment. I only have a handful of fish left, 3 largest being about 12 inches. No scales, no carcasses, no sign of anything happening. I don't have muskrats. We do have eagles in the area. Is it possible they took all my fish , and if so, how do I deter it from happening again?