Common Bully Gudgeon

Gobiomorphus cotidianus is the most abundant of New Zealand's native bully species, found in rivers, streams, lakes, and estuaries throughout both main islands. It is a compact, well-proportioned sleeper goby with a mottled brown-and-cream body and distinctive orange-red colouration on the pelvic fins of males during breeding season. Like other New Zealand bullies, it is a benthic predator feeding on invertebrates among gravel and cobble. It is an excellent choice for cool-water or temperate aquariums maintained without heating, tolerating temperatures close to 10 °C without difficulty. Some populations are migratory (diadromous) while others are fully landlocked, making it an ecologically interesting species. In captivity it is hardy, personable, and becomes tame quickly. It is one of very few New Zealand native fish regularly available in the hobby.

Water: 10–20 °C, pH 7.0–8.0, GH 5–20, KH 3–15; cool, well-oxygenated water essential; unheated tanks often ideal. Tank: 100 L minimum; river cobble and gravel; flat stone caves; moderate to strong flow. Feeding: Carnivore; frozen bloodworm, aquatic invertebrates, small earthworms, and high-quality sinking pellets. Breeding: Cave spawner; male guards egg mass; diadromous populations exist but landlocked forms may be breed-able. Compatibility: Semi-aggressive toward conspecifics; fine with similarly sized cool-water species.