Goldfish are sexually dimorphic, so you can see the differences between males and females.
Stephen M. Meyer
Q. I have enjoyed reading about pond fish in AFI and started with goldfish about six months ago. I would like to know how to tell if a goldfish is male or female, especially during non-breeding seasons.
A. Goldfish are weakly dimorphic. This means that while there do tend to be specific physical differences between males and females, they are not so distinctive as to allow great certainty in sexing the fish.
For example, females tend to be larger and rounder than males, but relative age (even a few months difference) and genetics easily mask such tendencies. Males usually have thicker main rays on their pectoral fins, but, once again, genetic differences easily mask this. Moreover, these weak differences are even harder to spot in young, immature, goldfish — which are what you are most likely looking at in the neighborhood aquarium store.
Determining the sex of mature goldfish is much easier and more reliable at breeding time — which in most parts of the U.S. is the middle of spring. Females develop bloated and rounded abdomens. It looks very similar to dropsy, and I know of many instances where hobbyists, thinking their goldfish were sick, treated them with medications. The unfortunate result was an immediate halt to spawning, and, in more than a few cases, the death of the fish.
Male fish at spawning time develop white bumps on their opercula (gill covers) and the leading edge of their pectoral and anal fins. Being lazy I prefer to wait for the fish to begin spawning to determine which are males and which are females.
Goldfish are school spawners, meaning that groups of males chase a single female. Thus, the chasers are males and the chasee is female — most of the time. In ponds where there are no females, groups of males may nevertheless choose some hapless companion and begin the spawning chase. The same can happen in ponds with only female fish.
In this latter case, eggs are scattered, but obviously no fertilization takes place. By the way, this holds true for koi as well.