In Ireland the ponds rarely freeze but the optimum temperature above & below 50 degrees faranheit is the diffrence between the fish moving about or going to the bottom & staying there.
The optimum over 50 degre period is only 4-5months of the year.
In order to keep the temperature up I have 2 sheets of bubblewrap which go onto the pond either side of the summer. This keeps the fish moving and up above the optimum,
Later in the year I have a coil of 100metres of polypipe fixed in circles around six uprights and connected to the house boiler. this gets turned on for a month or so, when I let the winter do its damdest.
This gives me a season of 8-9 months in a 4-5month climate & my fish grow rapidly (10" per year)
Would a simular system work for you or do you have bigger summers/