Have your pond fish being chasing each other and splashing around lately? They’re probably spawning.

Most of the fish species available to pond owners in the UK can be bred, and most breed in the same way. Goldfish (including shubunkins and Sarasa comets,) Koi, Tench, Orfe and Rudd are all egg scatterers.

How to sex goldfish

Summer temperatures warm pond water and the female’s bellies start to swell up with eggs. You may notice that some of your fish are bigger and fatter than others, and they’re the females. The slimmer males then start to chase the females around the pond, nudging them and trying to drive them into roots or shallows where they can wedge them.

The males have thicker pectoral fin rays along their leading edge and in goldfish, you can see them get white, serrated pectorals and white spots on their gill covers. The white spots are called Tubercles, not to be confused with Whitepot disease, and they use those, now rough, pectoral fins to grip the females and guide them as they chase them. 

On a sunny morning, if the water is warm, gangs of males drive females into the shallows and wiggle vigorously next to them. The female’s skin thins and their bellies become soft and the squeezing of the males (ideally one on each side,) pushes out hundreds or thousands of tiny eggs which stick to plants, algae and roots. The fish may spawn several times in a day and across several days or weeks.

Neither the males nor females take care of the eggs and will actually start feasting on them not long after they spawn. Fish eggs and fry will be eaten by aquatic invertebrates too, so by laying so many eggs, it helps to ensure some survive. The eggs are visible as tiny apricot coloured balls stuck to plants and the ones that weren’t fertilised turn white and get covered in fungus within a few days. 

What to do with pond fish eggs

If you spot eggs or fry in the pond the best thing to do is just leave them. The majority may get eaten but even if ten or fewer survive each year that’s another ten adult fish you’ll have in the future, and they’ll spawn when adult too. You won’t see it, but the pond will also be full of microscopic life which is perfect for fry and will feed them until they are large enough to take pellet foods with the other fish. 

You can move the eggs on plants but it’s often not the best thing to do as you will then need to feed the tiny fry when they hatch with special fry foods, and you’ll need a mature, air-powered filter and plenty of water changes. 

If you hatch thousands of fry its often difficult to feed them enough and provide them with enough space to grow, so they often stunt when raised in a tank compared to the room they would have had in the pond. 

Use spawning brushes to breed fish in a pond

If you do want to intentionally remove eggs and raise the baby fish, add spawning mops or spawning brushes to a shallow, south-facing area of the pond and secure them so the spawning fish don’t push them around the pond. 

Fish won’t always breed true and often your young goldfish will be a mix of tail length and body colours and goldfish fry a mix of mostly undesirable colours. 

To solve this, handpick two males and one female (eg shubunkins or Kohaku koi,) and place them in a large vat filled with spawning brushes. On a sunny morning, they will spawn and can then be returned to the pond, but expert breeders might wait until the event and then catch and “hand strip” male and female fish, by massaging their bellies. This practice isn’t recommended for beginners.

Why won’t my pond fish breed?

You may have young fish, only males or only females, or the water may be too cold, and the pond too dark. You need sexually mature males and females, a water temperature of 20C and something for the fish to spawn into like spawning mops. In a bare pond, they may not spawn or any eggs could be eaten or sucked into the filter.

Fish often spawn really early in the morning in the summertime so they may be spawning and eating the eggs and you may be missing it, and although it’s common for goldfish varieties to spawn, koi rarely do and it’s even rarer to raise nice koi from eggs laid in your pond. Make sure you feed your fish with a high protein diet to condition them for spawning.  

Take precautions when pond fish breed

If your fish do create a spawning frenzy in shallow water, females can get beached or stuck in cover netting, and all that shallow water commotion also makes them very vulnerable to predatory cats and herons. 

Female goldfish often develop fungal infections from having their flanks scraped, so keep an eye on them after spawning and using an anti-fungal medication if you see any cotton wool-like growths on their sides in the coming weeks. Antifungal treatments are also good for preventing fungus from infertile eggs spreading to fertile ones.  

Koi carp (koi fish,) don't get the tubercles or the pectoral fin ridges like goldfish do, but those and other carp can be sexed by slender male fish and larger, plump females when viewed from above. Its the same for most other egg scattering pond fish like Orfe, Tench, Rudd and of course other goldfish varieties.