Goldfish

Identification

Wild goldfish bear little resemblance to their colorful aquarium variants. Feral goldfish typically have an olive-green to bronze color — the orange coloring disappears after a few generations in the wild. The body is deep and laterally compressed, similar to the crucian carp. Reaches 15 to 35 cm in the wild.

Behavior & ecology

The feral goldfish is a tough survivor that tolerates extreme conditions — low oxygen, high temperatures, and freezing. Like the crucian carp, it can switch to anaerobic metabolism, producing ethanol instead of lactic acid. The diet is varied: algae, aquatic plants, insect larvae, worms, and small crustaceans. It can hybridize with crucian carp and Prussian carp.

In the Netherlands

The goldfish is not a native species but occurs throughout the Netherlands wherever aquarium fish are released — city ponds, park waters, and polder ditches. With 441 records, widespread but at low densities. Releasing is discouraged due to disease transmission risk and displacement of native species.