The little torrentfish (Cheimarrichthys fosteri) spends most of its life in flowing torrents. Its superbly adapted fins act as depressors, keeping it on the stream bed under swiftly flowing water. It may be a close relative of the marine blue cod.
Like many New Zealand freshwater fish, torrentfish spend a part of their lives at sea, though it is uncertain how this happens. Nothing is known of their reproduction. What is known is that in spring, juveniles (around 2 centimetres long) can be found swimming into estuaries from the sea. Scientists suspect that they migrate to sea as newly hatched larvae.
Lamprey are parasitic fishes, and New Zealand’s one species (Geotria australis) spends most of its life at sea. It attaches itself to larger marine fish with its sucking mouth, and rasps away its host’s flesh.
Lamprey migrate into rivers in late winter and spring. When they first arrive they are bright silver, with a blue back that has two vivid, paler blue stripes. But they soon lose their brilliant colours, becoming dull, drab and brownish. They penetrate long distances up rivers and probably spawn in the headwaters of small bush streams. The larvae hatch, and spend a few years living in sandy or silty sediments of stream margins, before moving to sea for several years. Nothing is known of their breeding habits.
The black flounder (Rhombosolea retiaria) spends much of its life in either rivers or lowland lakes. It is notable for the bright brick-red spots on its back. It migrates to sea to spawn, probably during winter, though little is known about this. Small juveniles, around the size of a thumbnail, can be found making their way into the rivers over spring.