Identifying the problem

Identifying the factors causing fish numbers to drop will allow you to determine which restoration tools you need to employ.

There are two parts to this. Firstly, you need to identify which species are lacking in your stream. Secondly, you need to identify the factors limiting these species.

Step One

What species should be present in your stream?

First, identify which fish species should be present in your stream.  Then, check to see if they are actually present. The links below will help identify what species should be present:

  • Fish Finder – shows the distribution of all fish species in New Zealand streams and rivers
  • Predictive maps – provides distribution models based on probabilities of where fish species might be located. Contact NIWA's Hamilton office to obtain predictions of specific stream reaches.

What species are present in your stream?

A search of records in the New Zealand Freshwater Fish Database will reveal what sampling has been carried out in the vicinity of your stream, provide data on the fish species found to date, and highlight what sampling methods were used. If no data are available, or the data are limited, a field survey (see below) may be necessary, possibly involving electric fishing equipment to target the micro-habitats of the species you would expect to find in your stream.

Information on the physical habitat of New Zealand's main fish species will help identify the species that should be present in your stream.

Fish survey methods (see pg 89-93)

What species are missing or scarce?

The next step is to identify species that are missing or scarce. If one or more of the species that should be present is either absent or scarce, it is likely there is a problem. A further option for assessing what species should be present is to produce a model from an Index of Biotic Integrity (IBI). Contact Dr Mike Joy for more information on IBI models. 

Some fish such as the giant and shortjaw kokopu are naturally rare, while others such as torrentfish and bluegill bullies are only present in specific micro-habitats which may not be present.

Step Two

What is reducing fish in your stream?

Declining numbers of freshwater fish are commonly the result of one or more of the following:

  1. Poor recruitment
  2. Habitat loss
  3. Biotic limitations

One or more of these three factors may affect your stream and reduce some species but not others. This diagram shows a generalised species-specific decision support system (DSS) to help identify the main causes of low native fish abundance in NZ streams.

Electric fishing survey to identify fish present. Credit: NIWA