Story: Life in fresh water
Page 4 – Fish
Food for people
Fish are at or near the top of the food chain in most freshwater systems. Many species are valued by Māori as mahinga kai (traditional food sources), and the whitebait fishery is based on catching juveniles from five species of native galaxiids.
Some introduced fish are common in freshwater systems, where they compete for food – for instance, they eat invertebrates that are a food source for galaxiids. One common, introduced fish, trout, are implicated in serious damage to some native freshwater species, however, they are valued by anglers and are an important symbol for the protection and restoration of freshwater ecosystems.
The latest information indicates that New Zealand has around 40 native species of freshwater fish. This count may increase as new genetic techniques bring a better understanding of the diversity within some fish groups. Thirty-three species are known only in New Zealand. Kōaro, longfin eels and spotted eels are also found in Australia, while lamprey and īnanga also occur in Australia, Chile and Argentina.
New Zealand’s native freshwater fish belong to eight distinct families: the jawless lamprey, eels, smelts, southern graylings, galaxiids, torrentfish, bullies and flounder.
Most are relatively small, although the eels are an obvious exception – some female longfin eels are up to 2 metres long and 25 kilograms in weight.
Longfin eels are the longest-living native fish, generally taking 20–30 years to reach maturity. Some large female eels stay in fresh water for over 80 years. In contrast, smaller species, such as īnanga, reach maturity in just one year and rarely live longer than three years.
Almost all native fish prey on invertebrates or other fish. However, historical accounts indicate that the now-extinct grayling (Prototroctes oxyrhynchus) was an exception and grazed on algae from river rocks.
Migration and life cycles
Nearly half the native fish species migrate to and from the sea during their life cycle. As not all species can climb rapids and waterfalls, freshwater fish are most diverse at low altitudes, closer to the coast.
- Lampreys reproduce in fresh water but develop mostly at sea.
- Eels reproduce at sea, but most of their growth occurs in fresh waters.
- The five species of galaxiids that form the whitebait catch lay their eggs in fresh water, but their larvae quickly migrate to sea and spend a few months there before heading back to fresh water.
Although once common in New Zealand, grayling (Prototroctes oxyrhynchus) began to decline soon after European settlement. They were last noted in the early 1920s. It is not clear what caused their disappearance, but it could be linked to land use or the introduction of trout.
Although not found elsewhere, many of New Zealand’s freshwater fish do have close relatives in Australia, South America and Southern Africa. Two species that do exist beyond New Zealand are lampreys and īnanga. This distribution, with other evidence, indicates that migration across the oceans is an important factor explaining the global spread of these fish.