Koi Spawning Behavior: Aggression or Courtship?

You added a new, slightly smaller koi to your 2300-gallon pond. Immediately, your three established koi (just over 12 inches) began chasing, bumping, and nudging the newcomer—who has a distinct lump on her left side. Is this spawning? Territorial aggression? And what should you do?

Reading the Signs

This is almost certainly spawning behavior. The "lump" is likely a gravid (egg-filled) female. Male koi ram and chase females to stimulate egg release. They don't "know when to stop"—it looks brutal. Your established trio are likely males; the new fish is a ripe female. Water temperature (you mention 30 degrees—assuming Celsius / 86°F) is prime spawning range.

Territorial aggression happens, but usually involves defending a specific area, not relentless pursuit of one fish. The sudden onset upon introduction + the physical sign (lump) = spawning.

Risks to the Female

• Scale loss, torn fins, bruising from ramming.
• Exhaustion and stress-induced immune suppression.
• Egg binding if she can't release eggs due to stress/injury.
• Secondary infections (fungus, bacteria) on wounds.

Your Options

Option A: Let Nature Run (If You Want Fry)

• Provide spawning brushes/mats (floating frayed rope or commercial mops) for egg deposition.
• Monitor closely. If the female hides constantly, stops eating, or shows severe injury, intervene.
• Eggs hatch in 3–5 days. Fry need infusoria/green water initially. Most will be eaten by adults.

Option B: Separate & Protect (Recommended)

• Net the female into a large floating net pen or a separate quarantine tank with pond water, aeration, and a spawning mop.
• Introduce one male at a time for a few hours if you want controlled spawning.
• This prevents injury, lets you collect eggs, and reduces pond population explosion.

Option C: Remove the Female Temporarily

• If you don't want fry, keep her in a calm quarantine tank for a week until the males' hormones settle.
• Return her when chasing stops.

Identifying Males vs. Females

During spawning season, males develop breeding tubercles—tiny white bumps on gill covers and pectoral fin rays (feel like sandpaper). Females are softer, rounder in the vent area. Venting (gently turning the fish upside down) shows a convex vent in females, concave in males—but this stresses fish; tubercles are easier.

Bottom Line

Your pond is large enough (2300 gal) for this group, but spawning is violent. Protect the female. If you're not prepared for hundreds of fry, separate her. A floating net pen ($30) is the lowest-stress solution for everyone.