Among Aotearoa-New Zealand's mix of unique creatures, torrentfish get special mention because they are one of the few freshwater fishes not to have origins in Australia.
It's thought to have emerged from the seas and adapted to freshwater in NZ and nowhere else. Its closest relative may be blue cod , a saltwater species that's found along rocky coasts up to depths of 150 metres.
Torrentfish (panoko, Cheimarrichthys fosteri ), however, prefer shallow, fast-flowing rapids or riffles in freshwater rivers over much of New Zealand.
Their only connection with saltwater is a brief period in the larval stage. After spawning near river mouths, larvae are swept out to sea before returning to rivers as juveniles.
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They are "particularly enigmatic" and there are large gaps in the scientific knowledge. It wasn't until 2014, for example, that panoko larvae emigrating to sea were captured and later identified, and this was the byproduct of an effort to capture larval bluegill bully and common bully.
The researchers captured over 22,000 fish larvae and 99 per cent were the bullys. But some were a mystery and eventually 56 were classified as torrentfish based on physical characteristics and other factors. They looked something like a tadpole and 2-3 millimetres long, wrote Matt Jarvis, Manna Warburton and others from the University of Otago.
Other research by Warburton and his PhD supervisor Professor Gerry Closs showed that adult torrentfish in the South Island migrate upstream considerable distances. Females travel the farthest inland – almost 90 kilometres on the Rakaia River in Canterbury – and other researchers have found females almost 300kms inland.
Adult males seem not to travel so far inland, but are still found some distance from river mouths. "Extremely high" numbers of juvenile fish enter river mouths at certain times of the year and food competition probably drives panoko inland.
The males and females travel downstream to spawn near the sea. This probably happens because the tiny larvae are "poorly provisioned" and a shorter migration distance to rearing habit in the ocean is advantageous.
It's not known what happens after spawning. The adults may die or they may migrate upstream again, before returning for a second or third spawn.
"Based on the detection of ripe females across a broad range of total lengths (70-135 mm) it seems likely that torrentfish are iteroparous and multiple spawning migrations are taking place," wrote Warburton Closs and another researcher in a paper published earlier this month in the New Zealand Journal of Marine and Freshwater Research .
In 2017, DOC classified panoko as "at risk – declining " and in 2014, the International Union for Conservation of Nature listed them as "vulnerable" .
Panoko have streamlined bodies that allow running water to pass over them while the fish tucks into loose gravels and rocks. They have an undercut jaw and a fleshy upper lip that allows them to eat invertebrates off the surface of stones.
Dams, irrigation, water pollution and climate change probably affect the cool, fast-flowing water they prefer. River sedimentation is also a problem because they need these rocky bottoms to thrive.
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