Crans bully (Gobiomorphus basalis) is a stocky little fish that is hard to distinguish from common and upland bullies. It is brown with dark and pale patterns on its side and a blunter head than its common bully relatives. On mature males, the top edge of the first dorsal fin is a bright pinkish-orange.
Crans bully is strictly a North Island inhabitant and is found in most areas. Its rarity in the arc northeast of Lake Taupo is thought to be an effect of the Taupo eruption over 1800 years ago.
Because the Crans bully has no marine phase (it does not swim out to the ocean), their ability to colonize new river systems is limited and once they are gone from an area it is unlikely they will re-colonize on their own.
With no requirement to go to the sea, Crans bully is most common at sites at mid range altitudes and some distance inland. It dwells in stony rivers and streams and does not establish lake populations. Breeding behaviour is similar to the other bullies, with the male establishing a territory, and remaining to guard the eggs after they are laid by the female. Each female lays up to a thousand eggs in the nest and when the young hatch they swim around at the edges of the stream.
Because the Crans bully does not travel between the river and the sea, its young hatch and live in the same stream. It lives under small stones in gently-flowing streams where it feeds on stream insects. The Crans bully is a very friendly fish and will rest on your hand if you hold it still on the bottom of the stream.
Did you know? When spring comes, the male Crans bully turns black with an orange fin to attract females.