Some fish may still be scarce or absent even though recruitment is occurring, habitat is present and there is no recent history of pollution or floods.

Is biological limitation present?

Identifying biological factors that may be limiting fish numbers requires sampling to ensure that key invertebrate prey species are not scarce and that pest fish, especially predators, are not abundant.

Food supply

If invertebrate food supply is limited (for instance, because of heavy siltation or excessive algal growth), fish abundance will decline. High densities of large insect larvae such as mayflies and caddisflies are an indication of good food supply for fish, whereas snails are a poorer food source.

If invertebrate food is scarce, then the factors limiting food need to be identified and remedied before fish can be fully restored. Flushing flows over dams can be used to simulate a flood and restore substrate for invertebrates. Better management of siltation via riparian planting upstream may also be required. 

Pest fish

Pest fish can reduce the abundance of native fish through competition, predation and aggressive behaviour. If juvenile trout are present, galaxiids are likely to be scarce; if gambusia are present, and the stream is shallow and still (i.e. a pool habit predominates), inanga and smelt will be scarce. A range of pest control methods is available to help reduce the impact of pest species on these fish.


Large eels may be scarce if they have been overfished. Detection of overfishing requires a comparison of the abundance and size of eels with a reference site (i.e. a similar reach where fishing pressure is light). Fishery management may be required to restrict harvest or to control pest species.


Disease and parasite infections can result in a sudden and dramatic decline in the abundance of vulnerable fish species. However, these have not been reported for native fish in New Zealand rivers to date, so they are unlikely to account for a reduction in fish abundance. 

To find out more, see Biosecurity New Zealand's Aquatic Animal Diseases Diagnostic Testing webpage.

The pestfish gambusia nips the tails off inanga and other small fish. Credit: NIWA