Q: I have koi in my pond and one of them is constantly chasing the other fish by bumping and nuzzling the other fish on their bottom side in front of the tail and behind the ventral fin. Is there a reason for this behavior?
A: Yes there is a reason for this. The fish getting bumped into is a female carrying eggs, while the fish doing the bumping is the male. The male fish is trying to knock the eggs out of the female so it can fertilize them. This is very common, but it is best to keep an eye on the fish during these times to make sure no injuries happen. Also, if you would like the koi to have babies, I would suggest making sure you have a sufficient amount of bog and submerged plants. Koi are known to eat their own young and adding aquatic plants will create hiding spaces for the babies to allow them time to grow.