New Zealand’s freshwater fish have strong connections with fish of other southern lands. The shortfin eel, īnanga and kōaro are found in eastern Australia, and īnanga in Patagonian South America and the Falkland Islands. In the past some researchers suggested that this spread was because New Zealand, like Australia, was once part of the supercontinent Gondwana. Recent DNA research indicates that it is far more likely that these fish are more recent arrivals, carried around the southern hemisphere on oceanic currents. Some endemic groups such as the pencil galaxias may have an ancient Gondwana heritage.
Evolving from marine species
Some species that evolved as marine fish have established themselves in fresh water. Just how this happens is unknown, but at some stage an event must have caused a shift into fresh water. Perhaps a lack of fish diversity in river rapids provided an opportunity for a marine species to invade this environment.
The torrentfish still retains its marine connections by living at sea during larval and early juvenile life. The black flounder must still return to sea for spawning and early juvenile life. Several flounder (mainly marine) can also live in river estuaries and lowland lakes. But the black flounder has taken the process a little further – it may be found many kilometres up some rivers.
Links to the sea
Nearly half the native freshwater species are found in the sea at some life stage. This may be as larvae and juveniles (as with whitebait species and several bullies), after which they return to fresh water. Some adults (such as eels) may migrate to sea to spawn. In another example, the smelts living in rivers spend most of their lives at sea before returning to fresh water as adults, to spawn.
Fish migrate between rivers and the sea at most times of the year, but especially in spring and autumn. These species are known as diadromous (from Greek words meaning ‘running across’).
The distribution of the migratory species depends on how far upstream they can move. Rapids and waterfalls are not necessarily barriers. Some species have extraordinary climbing abilities, and can be found upstream of waterfalls tens of metres high. Eels are able to climb like this, and some of the whitebait species, especially kōaro, banded kōkopu and shortjaw kōkopu.
These fish climb mostly when small, moving up the wet margins of falls, and using their fins to hold onto rocks by surface adhesion. Some are well known for climbing out of buckets, and if in captivity, often climb out of aquariums (they can climb glass as long as it is damp).
Stuck in the Nevis
For the past half million years Otago’s Nevis River has flowed north into the Kawarau River, which then flows into the Clutha River. But it is thought that the Nevis once flowed south, into Southland’s Mataura River. Supporting evidence is that the Nevis has a Galaxiid species (Galaxias gollumoides) that is otherwise found only in the Mataura and other Southland waterways. The fish is found only in one other isolated locality in the Clutha catchment.
Many species can vary their behaviour. Although the ancestral pattern is for them to go to sea, they can establish landlocked populations in the open water of lakes rather than the sea – mostly at the juvenile stage.
A high number of native species are nocturnal, moving from under cover to be active at night. Why they are so nocturnal is not understood. The most likely explanation is that it might minimise predation by aquatic birds, especially shags, and perhaps herons. But if it is an avoidance strategy then a paradox emerges. Some fish are habitual prey for large eels, which are also more active at night, emerging from cover to feed.