wondering about the ventral fin on koi, some dont have the single fin below, others do. Not indication of sex?

Answer 1

Ditto Copperhead's answer - clamped fins are almost ALWAYS a sign of stress in koi. If you can't see the anal fin on one, first check all your water parameters to rule out ammonia poisoning or nitrogen build-up, and then treat for parasites (flukes are all-too common in koi), if indicated.

As for sexing koi, the sand-papery feel of the gill plates in males during the spring breeding season is the most sure-fire indicator of gender. However, if it's not spring, or you don't want to put your fish through the stress of handling, you can generally tell the gender of the fish through observation, assuming that the fish is at least 8-10" or longer (smaller than that and it is REALLY tricky to tell gender with any sort of reliability). Males tend to be longer and thinner, more torpedo-shaped, than females. Females, on the other hand, are fatter, generally larger overall, and shaped more like a football. In addition, many females of breeding age (generally 12" or larger) will have a "hump" on their neck/shoulder area that is very rare to see in males (in fact, so rare, I've never seen one with a hump....)

Hope this helps!

Answer 2

Although some Koi differ greatly (with fins) from others icluding a dorsal fin, when it comes to the ventral fin the Males are usually distinguished from females by the larger ventral fin This is difficult unless you know the fish are roughly the same age. As far as a definate ID on the sex of the fish, no. A good guess, yes.

Answer 3

The fin your asking about is the anal fin, and it should be present on all the fish. It may not always be extended (open) though. If it's held tightly against their body at all times, this can be a sign of a disease or parasite affecting the fish.

At any rate, it isn't a way to determine gender. The best way of doing this is to examine the gill coverings of the fish when they're close to spawning season (generally spring). Males will have a rough surface that you can feel. This si similar to the breeding tubercles in male goldfish (to which they're related) but is less pronounced in koi.

Answer 4

I have never heard of a way to tell the gender on a koi.