Wood,
Your trout should make it through the winter O.K, but I prefer to leave an open hole in the ice by way of my diffuser on one end of the pond, and feed then ever few days in winter only once in the evening. This is just for maintenance and like I said you trout will be fine.
As you elluded to with temps as low as they are their metabolism is quite slow and they do not need much to eat nor do they metabolize their feed very well.
As to why you do not have an open hole from your diffuser I don't know. Is there enough flow etc?
Your air is probably escaping through cracks in the ice or where your ice meets the bank.As you probably know cracks are common in ice especially when it gets very cold.
If you want to see trout feeding in winter go to my website at
http://www.ligtel.com/~jjbaird/bairdfish2.htm and click on the brook trout page. You can also click on the address in my profile. I updated a little since last time.
I had brook trout coming up into my ice hole in my shanty looking for feed before I applied any to the water. One stuck his head out of the water and clacked his jaws scaring the dog to death! The dog almost knocked the shanty down trying to get out! My brook trout were much more active in the colder water than the browns and rainbows. However brook trout are actually a char and prever colder water as you probably know.
