In the winter, fish metabolisms slow significantly and they enter a semi-hibernation state called torpor. Since fish are cold blooded, their internal temperature is regulated by the temperatures around them. This means that once things begin to cool down, your fish's internal systems quickly recognize it, and know it's time to prepare for winter. Even in the depths of winter, fish will sometimes move around during torpor to assess their surroundings or move to areas where the water has more available oxygen or is slightly warmer. In winter, the deepest parts of the pond are the warmest, so you'll actually rarely see them.
The torpor state means your fish will move very slowly, if at all, and their metabolism will slow significantly. This is why, in wintertime, it's critical to stop feeding once water temperatures drop below 50 degrees Farenheit. You'll also need to avoid using a pump that recirculates water, and definitely don't perform a water change - this activity will mix the thermal layers of your pond and remove the warm pockets where your fish have settled.
