Goldfish are egg-layers, bred by the Chinese from common carp. These long-lived, cold-water fish are typically in their prime during their third year. Goldfish in outdoor ponds will prepare to spawn as the weather becomes warmer and can be sexed at this time, by the physical changes that take place in their bodies. Behavioral changes will also take place. By understanding and looking out for these things, the hobbyist will easily be able to gauge when their goldfish are preparing to lay eggs.
Observe your goldfish closely during winter to form a clear picture of what each adult looks like prior to spawning. This makes it easier to notice the physical changes that appear as the weather warms up.
Observe the goldfish during May and June for the first signs of spawning behavior and physical changes. Male goldfish will develop raised, white colored tubercles on their opercula, or hard gill plates, and along the first ray of their pectoral fins. Male goldfish also appear to be streamlined, but this is dependent on the their exact strain or type.
Watch the goldfish closely on a daily basis. Female goldfish will fill out and take on a plump appearance as the roe develop within their bodies.
View each goldfish from above. The midriff of pregnant females will become noticeably rounded. These females will also be rounder on one side, when seen from above.
Take note of any spawning behavior. Females that are preparing to release eggs will show interest in the spawning mop. These fish will remain in the vicinity of the mop and will be seen swimming through and around it.
Look for those fish that show interest in the plump females. These are males, which will swim repeatedly into the side of a female, to encourage her to release the roe. Males will also drive the female towards and against the side of the pool.
Observe the final stages prior to the release of roe. The goldfish being rammed is the female. Male fish may already begin to release milt or sperm in the water and the female fish they are bumping into will release its eggs almost immediately.