The Wellington region’s rivers, lakes and wetlands are home to at least 20 species of native fish, which makes ours one of the most diverse regions for freshwater fish in New Zealand. Our native fish are among the hidden treasures of New Zealand’s animal life because they are seldom seen and we know very little about most of them.
Freshwater ecosystems include streams, rivers, lakes, springs and wetlands, and there are some distinctive types within these categories. For example, a wide shallow river with a sandy bottom is habitat for a different set of species than a swift and stony mountain stream. There are also ‘in between’ habitats like estuaries which are influenced by the salt water of the sea.
16 of the region’s 20 native species need to migrate between freshwater and the sea during their lives. The survival of these species depends on it.
Migration between freshwater and the sea (known as diadromy) is common in New Zealand making our fish community unusual compared with other countries. This is similar to the way New Zealand’s native birds are regarded as unusual because so many of them are flightless.
Several of our fish lay their eggs on streamside plants in the lower reaches of rivers and streams, and their offspring then swim upstream to live further up the catchments. An example is inanga, which is one of five native freshwater fish species that are known as whitebait.
Longfin eels also need to migrate downstream to spawn, and they then go far out into the sea between Tonga and New Caledonia. This only happens once for each eel, at the end of their lives. They can live to be over 100 years old.
Smelt and lamprey are different again. They spend most of their adult lives at sea and migrate up rivers where they lay their eggs.
Migrating native fish don’t move upstream by jumping over low barriers the way trout and salmon do. Some of them have an effective alternative – they can climb! Kōaro and elvers (juvenile eels) have climbing perfected. They can get up waterfalls and are found in good numbers above some of the highest weirs in the region’s rivers, such as the Kaitoke weir on the Hutt River and the water supply weir on the Orongorongo River.
The kōaro uses its pelvic fins like broad, flat arms to help it cling to steep surfaces. They usually leave the main areas of water flow and use surface water tension on wet rocks to stay attached while they wriggle their way upwards with surprising ease. Other native fish with reasonable climbing ability are shortjaw kōkopu and banded kōkopu.
Not all native fish are good climbers. Inanga, whose juveniles make up to 95% of the whitebait catch, have trouble getting over barriers with a vertical drop of more than 30 centimetres. Other poor climbers include giant kōkopu, torrentfish, most species of bully, smelt and lamprey.
There are a range of barriers in the region’s streams and rivers that can prevent fish migration. Some are natural, like waterfalls, and some barriers are only in place occasionally, such as when water flows are very high or very low. But there are many fords, culverts, weirs and other structures that are in place year round and that block the passage of fish getting up or downstream.
Poorly installed culverts, weirs or fords can restrict passage and reduce the amount of habitat available to fish. We can help them by removing structures from streams and rivers or installing ramps or fish passes to help them get where they need to go.
Some native fish have an amazing ability to live in fast-flowing rivers and streams, and there are plenty of these rivers in the region. Bluegill bullies and torrentfish live in the swift white rapids of stony rivers and streams, where people would find it hard to stand up. These fish stick to the bottom by clinging onto the gravel with their fins.
Another native fish found in the region is the brown mudfish. It can survive out of water during droughts. Mudfish live in swamps, drains and forest pools that may dry up in summer. These fish were first discovered curled up in the mud when people dug up wet areas for drainage and cultivation. When the water disappears, they can be found underneath logs and other debris, or they burrow into holes where tree roots have rotted. They can breathe through their skins and survive in damp places (without any surface water) for weeks, even months. You can find brown mudfish in wetlands in the Wairarapa valley. They once were abundant on the Kapiti Coast but it has been some time since they have been recorded there.
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