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Mark,
Interesting question. I did some research on this awhile back when my good friend JR and I had discussion on the physiological differences in Japanese and domestic koi and common carp. We were actually bantering about the ability of Japanese koi to winter over in cold climates. JR made some comments that the physiology of the Japanese koi has altered over generations as they were bred for color and conformation. He noted that the gut, for instance, in a Japanese koi was shorter than that of a common carp or even some domestic koi. And he may well be right as the feeding habits and even the food available to pond-raised (and pampered) koi is significantly different than what our common carp have available to them. Even mud-pond raised koi have a different cuisine than river or lake carp. So, it stands to reason with better quality and more abundant food, the Japanese koi could ostensibly have shorter and more effecient guts.
Now, I said all of that so we can understand that fish raised by breeding for color and conformity are also raised to be LARGE. And for the most part, LARGE beyond the means of their other physiological functions to support them. And here is where your comments about a set a symptomatic conditions is spot on. The "sinking" disease mentioned (but not often) is problematic in koi bred for size. Conventional wisdom says that the swim bladder has not developed enough to allow the fish to handle the increased mass that "good" breeding has allowed it to have. And at some point in time, the physical size of the fish cannot be managed by the swim bladder volume and the fish "sinks." In actuallity, the fish does not sink so much as it lacks the energy to keep itself buoyant.
And we all know that the chagoi/magoi/x.goi family is only a step or two away from the common carp and so they have an inherent ability to grow large AND have their physiologies handle the load, so to speak.
Like I said, this is a more common problem than advertised. But a darn good topic to bring up.
REC
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