Improving water quality

The known water quality preferences and tolerances (temperature, pH, oxygen, ammonia and turbidity) for common native fish are shown in the table at the bottom of the page.

What to do:

You will need to determine whether any of these water quality variables are monitored by the regional council. If not, they may need to be monitored primarily in the summer months to see whether any are limiting fish.

Improvement in water quality can be achieved by the following actions:

• where summer water temperatures are excessively high, use riparian tree planting to shade the stream’s upstream reaches
• where oxygen levels are low, re-aerate the water by placing rocks in the streambed to create small instream rapids (do not plant oxygen weeds because they withdraw oxygen at night and contribute to dangerously low levels of oxygen in the water. Increasing the surface area to depth ratio of pools will also help.
• where low oxygen levels are caused by organic pollution, high ammonia levels and a lower pH are also likely to occur. Reduce the source of pollution to restore water quality. Information on this is available on pages 58-75 of the Stream Restoration Toolkit. Please also see our page Dissolved oxygen criteria for fish
• if your stream is affected by excessive turbidity, consult the regional council to identify the source of the problem. Refer to NIWA’s system for identifying turbidity limits for freshwater fish
  
• where fish numbers are low due to a one-off chemical spill, recolonisation will take place naturally provided there are no further spills. However, this takes time and you may need to use stocking to accelerate recovery.

Table: Known water quality preferences and tolerances for common native fish