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 . Description 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. Distribution and habitat G. huttoni is endemic to New Zealand, found throughout both main islands as well as Stewart Island and the Chatham Islands. Redfins are quite rare along the east coast of the South Island north of Oamaru, except for Banks Peninsula. They are very good climbers, able to traverse waterfalls when going upstream, and prefer to live near the coast; they do not establish landlocked populations. Redfins mainly live in the runs and pools of small, boulder-filled streams, and prefer a habitat with a moderate flow of water with pools and riffles, in quite large gravelly streams with cobble substrates. They do not need a dense overhead canopy, but prefer a high proportion of native trees. Life cycle Redfin bullies are amphidromous – they migrate between fresh water and the sea as part of their life cycle. Over winter and spring, the male establishes and defends a ‘nest’ – usually a hollow beneath a rock. 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