Hopefully by now you have stopped feeding your fish. Although your pond tenants don't exactly hibernate, their metabolism slows way down. When the water temps are consistently below 55° your koi or goldfish are not able to fully digest food. Giving them a mid winter snack is not healthy and could weaken their immune system, making them more susceptible to disease and infection.
They will be perfectly fine until spring. Koi are cold-blooded, so their activity level coincides with the water temperature.