Great read AP. Guy brings up some interesting points. Out here we have nothing like that as far as Winter temps yet almost all of th edie offs and crashes are in the Winter. And he hits the nail on the head with lack of O2. A lot of the novice koi owners think you only need a Spring cleaning. More important is a preWinter cleaning. All that vegetative matter just sucks up or removes the O2. How many posts do we see here on this forum about Northerners having die off in the Winter with any species?
Plus with Koi, he says you have to due several things, limit the desirable population, cull all diseased fish. Provide non stop O2/aeration even before the pond ices over. This is just not feasible in a large trout/koi pond.
As far as koi diseases, and why they kill them off in the Winter. The immune systems just start shutting down below 70. Digestion stops at 50. They are dormant, or clinging on to life, waiting it out. If they are weak, diseased, parasites, over overpopulated they just don't make it through the Winter. I don't have the space to list all of the diseases and parasites these fish can get or have. I think fungus is the number one killer though. And again, you just can't treat that in cold water, the medicines won't work. There are even procedures on how to raise water temps to treat koi with medicines.
There is no happy medium for these two species to coexist. There is no water temp for both of them to thrive. Koi can withstand very cold water temps, but only for thier Winter cycle. As soon as you heat them up to get them fat and happy your trout are dead.
