Redfin bully

Species of fish

The redfin bully (Gobiomorphus huttoni) is a species of freshwater fish in the family Eleotridae endemic to New Zealand. Being amphidromous, it spends part of its life cycle at sea. Males have distinctive bright red patterns and stripes on their fins. Adults grow to an average of total length, with a maximum of .

Male redfin bullies are the most colourful freshwater fish in New Zealand, with bright red markings on the dorsal, anal, and tail fins, as well as the body and cheeks. Additionally, males have a bluish green stripe on the outer edge of the first dorsal fin. Only the males have the red colouring; the females have the same patterns, but with brown in place of red. Redfin bullies of both sexes have distinctive diagonal stripes on their cheeks. These stripes are very useful for positive identification, as they are visible in small (about 30 mm long) and very pale fish. G. huttoni reaches a length of 120 mm. Males are larger than females. Redfins are quite rare along the east coast of the South Island north of Oamaru, except for Banks Peninsula. The male turns very dark, from brown to completely black, while defending the nest. When a female is ready to lay eggs, she enters the nest and turns upside down to lay 1,000–20,000 oval eggs in a close packed, single layer attached to the nest's ‘ceiling’. The male then fertilises the eggs. The female leaves the eggs in the care of the male, which guards them until they hatch two to four weeks later. Females may lay more than once over the spawning season, and one male may defend the eggs of more than one female. Upon hatching, the 3 mm fry are carried downstream to the sea; several months later, they return as 15–20 mm juveniles, and live the rest of their lives in fresh water. Redfin bullies seem obliged to spend their first few months at sea, because no landlocked populations have been found. Juveniles have the best climbing ability of the Gobiomorphus species, but are mostly found in lowland waterways. They reach sexual maturity in their second year and have an average lifespan around 3–4 years.

G. huttoni is an opportunistic feeder, eating the larvae of chironomid midges, mayflies, and caddisflies, small crustaceans, and aquatic snails.

The main threats to G. huttoni are competition and predation from introduced salmonid fishes, mainly brown trout (Salmo trutta), and habitat loss. Over the period of 2004 2014, redfin bully numbers declined by 20%, and they are now classified as a near threatened species.