A courting attempt most likely.. males have more of a torpedo shaped body while females typically are shorter and have bigger bellies or will have a bulge building by their back fins when eggs are being built up and are about to be released.. hope that helps..

You can easily tell the male form the femal looking straight down at them. The females have a round bulge in front of the back tail. Maybe you thought they were just fat? When they spawn, the male will chase the females and butt against them to release the eggs.

Or you may just have an aggressive fish. I had one like that who kept all the smaller fish, and a few of the big ones, from eating. They were all afraid of him. He had to go keep peace in the pond.

Well, it's hard to tell. The white fish seem slimmer than the orange. We've lived here three summers now, and this is the first time I've seen this behavior, nor have I seen evidence before of fry. Of course, I could be just oblivious, not knowing what to look for!

I have been told though, that if spawning does take place, the pond will take on a very distinctive odor, which I haven't noticed.

Could it be they're not getting enough to eat?

It's sex. And the smell of the pond doesn't change. But the plants may get a little ragged.

Never heard of a smell coming from the pond after a spawn or spawning.. your ammonia will spike afterwards and you will see the very small pencil tip clear eggs everywhere or in a concentrated place in your pond.. after a spawn your koi should become a bit dormant or lethargic.. but not sure if you are having a spawn or not.. you can feed them more.. but remove any food after about 20 minutes if they havent eaten it.. you also could have a trouble some fish.. I had one before that caused a ruckus when feeding.. anyway goodluck and happy ponding..

It sounds like you may have a mature male. He can only spawn one female at a time and he can badger her to death, literally, if he dosn't settle down mess with the pond, move plants, clean it, add water, feed them, any thing to get his mind off this poor female. Yes the water can smell, and when they spawn the other koi, females especially, will clean up fresh laid eggs and you may never see any eggs or babies. These are smaller fish so their won't be as many eggs anyway, or as big of a mess.

Aha! Well, we certainly gave them all something to think about, as the pond sprang a leak. Fortunately we discovered it and were able to repair it before any damage was done, but it does stir up the fish a bit.

I'm guessing you're spot on. Is this spawning behavior limited to a certain time, or can it go on all summer...fall...?