I run my ponds and pumps all year round since 1991, in Canada, where temps can fall below -40 Celsius and water temps get close to freezing, bottom pond can get lots of thick ice. Running water year round keeps ice thinner in upper connecting ponds. Two of my oldest Koi have been outside and alive since 1991. Everybody doing just great after one of the longest hardest winters I can remember in years... I am pretty sure your koi will be just fine as your winters are very short. America has some wild koi now in some lakes, as KOI are "cold water fish" and should be fine if your koi are healthy. Many people now wintering koi all over.