Egg Binding in Koi

Oh, goody! Another “disease” for our Fake Fish Diagnoses file. I recently received an email explaining how “egg binding” kills fish, and I figured it would be an excellent opportunity to set the record straight.

Many hobbyists are unaware that koi, and many of their fish cousins, are constantly carrying eggs. In the warmer spring months, some of these eggs will mature and be released into the water column. The males will release their sperm at the same time to externally fertilize the eggs. If conditions aren’t right or there are no males to spawn with, females will resorb their mature eggs and wait until next year. This is a NORMAL process. If you open up a dead koi and see eggs, do not assume they died from “egg binding.”


Graphic pictures below!

The bilaterally symmetrical brown/red masses are immature ovaries in a large female koi.
Adult koi with mature ovaries

Sometimes, fish do not release their eggs. Why would they do this?


Most likely the #1 cause of non-spawning is that your fishes’ environment isn’t suitable to spawning. This can be an individual fish issue or an entire group. If the water quality is poor, the male:female ratio is off, the pond is overcrowded or the fish aren’t receiving enough nutrients, the fishes’ brain will signal ‘don’t spawn.’ This is NORMAL. If your fish aren’t spawning, they know better than you.

Large koi females carrying eggs require extra nutrients and oxygen to keep their large egg mass alive. Adding in the aggressive spawning practice of male koi, it is rough on these females to reproduce. If you don’t have issues with spawning in your pond, this is not a bad thing! Too many ponds are overstocked due to “accidental” spawnings.


Neoplasia (Cancer)

Koi with benign cysts or gonadal sarcomas will not spawn due to changes to their internal body structure. These fish will usually start to look lopsided in their bellies. Ultrasound will confirm the presence of a cyst, tumor or asymmetrical ovary. Cysts and gonadal sarcomas require veterinary care in order to resolve. DO NOT ATTEMPT SURGERY ON YOUR FISH.

The chances of your fishes’ eggs getting stuck inside them and causing the fish to die is very, very, very low. Just because your fish dies with eggs inside them does NOT mean the eggs caused them to die.

References (in case you think I like making all this up)

  • Stoskpof. Fish Medicine. 1993. W.B. Saunders Company. p. 470-72.
  • Smith, Stephen A. Fish Diseases and Medicine. 2019. CRC Press. p. 244-45.

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