I have just witnessed this myself , I have just netted my koi at bottom of pond with a frog on top of it-the frog had wedged its front feet into the fish gills killing the fish. It is mating season at the moment - so this is possible. I was a bit quick removing the frog from the koi to see if the fish was alive ... had I thought I could have taken a pic as I was pretty shocked as not witnessed anything like this before !