Hi Larkin,
I have been trying to research if the hybrids are really sterile or not, and the best analogy that makes any sense compares them to mules. When breeding a male donkey to a female horse, similar, but different species, with different number of chromosones, ALL of the male mules that resulted were sterile. The female mules on the other hand, have been document approximately 60 times, from the 1500s, to 2007 in having in fact been fertile and reproduced. In the case of the 2007 foal, DNA testing confirmed the mare was a mule, and her 'son' was in fact hers off spring.
Based on the reading I have done so far, I still dont have an answer BUT if koi/comet hybrids were HGHLY fertile, I dont think we would be asking this question. I mean look at what people are doing in dogs with the "designer breeds"... Intentionally mixing two known breeds... Why wouldnt fish people do it too? Then take it a step further, that dedicated koi breeders do misc crosses (within Koi) with the hopes of developing new colors/patterns, etc.
It is human nature to "mess" with things. I am seeing some really pretty hybrids. Why wouldnt someone try to do it? I am in now way PROMOTING that anyone should be TRYING to make a "new breed"... I want a variety of pet fish. IF nature takes its course and they interbreed, so be it, BUT if I go through the efforts to do an intentional spawing, I want to know when I isolate out specific KOI to breed, that they are infact KOI.
Over the next few years, we have planned at least two more ponds (one large pond, the other for plants only), but I have half a mind to throw any known hybrids into the 'plant pond' to see if anything happens.