Aquatic insects form the bulk of its diet. Stoneflies mayflies and caddisfly larvae are preferred prey. It feeds on drifting items in current or picked from rock surfaces in cold fast water. The Tarndale bully is a high-country specialist. It lives in streams above 1000 metres elevation, where water temperatures remain cold year-round. These waters flow through tussock country with minimal vegetation, exposing the fish to aerial predators but also keeping conditions suitable for survival. The altitude provides some protection from introduced trout, which struggle in the coldest highest reaches. But as climate warms, that refuge shrinks. Named for the Tarndale area in the Southern Alps, this species reaches about 100 millimetres in length. Its body is streamlined, coloured mottled brown and grey to blend with river stones. Large pectoral fins allow it to cling to rocks in fast currents. During breeding season, males develop darker colouration and guard nests under flat stones. Spawning occurs in late spring or early summer, later than lowland species due to colder water temperatures. Eggs are laid amongst rocks in shallow riffles. The male guards the eggs, fanning them to keep oxygenated. The larvae hatch and remain in freshwater, never migrating to sea. This non-diadromous life history means the species is entirely dependent on maintaining suitable high-altitude habitat. Classified as Nationally Vulnerable, the Tarndale bully faces threats from multiple directions. Climate change raises water temperatures, pushing the fish higher into mountains where less habitat exists. Hydroelectric development alters flow regimes. Introduced trout expand their range upward as waters warm. Each threat compounds the others. The Tarndale bully persists in shrinking pockets of suitable habitat, surviving where conditions remain cold and predators absent. It is a cold-water specialist in a warming world. A fish running out of mountain.