By David Rowe
The presence and abundance of fish in our rivers depends on many factors, water quality being one. Water temperature, the amount of oxygen dissolved in the water, acidity (pH), ammonia concentration, and suspended sediment level can all influence the suitability of water for fish. When changes in land-use occur in river catchments, or pollutants enter waterways, one or more of these key variables can change such that the water quality deteriorates and fish are affected. Permissible limits for the water quality variables therefore need to be identified to avoid such effects and knowledge of the responses of fish species to water quality is essential for setting these limits.
Trout (brown and rainbow) are among the more sensitive freshwater fish in NZ waters. When water temperatures increase beyond 19°C or oxygen levels drop below 5 ppm, trout are able to emigrate to more suitable waters because they possess temperature and oxygen sensors that warn them of unacceptable levels. But, when the pH drops below 4 or ammonia levels increase beyond 0.5 ppm, mortality occurs. In contrast, high turbidity triggers changes in behaviour rather than emigration or mortality. In turbid rivers and lakes, trout must rely less on the eyes and more on their sense of smell and motion detection in order to catch prey. This means that the types of prey they are able to detect and catch also changes. The reactions of fish to individual water quality variables therefore vary among the species and not all species have the same preferences or tolerances.
Shortfin eels tolerate (and may even prefer) poorer quality waters than other species. We know this because they are generally abundant in the warm, turbid and highly productive waters of agricultural drains and shallow eutrophic lakes. In the cold, clear bush-clad streams where water quality is generally high, they are scarce. Common bullies also proliferate in warm, turbid and highly productive waters. The distributions of these two species indicate that they are more tolerant of poor water quality conditions than many other native fish species. In contrast, the banded kokopu is the most sensitive of the native species tested to date. It prefers cool waters (< 17°C), its juvenile migrant stage avoids high turbidity ( pH 7) waters, and it has a low tolerance for ammonia. Smelt are also less tolerant of poor water quality than many other native species.
Because of such variations in tolerances among the species, the typical response of a fish community to a decline in water quality is a change in species composition rather than a decline in fish abundance. The tolerant species may become more numerous whereas the less tolerant species decline. As many of the introduced warm-water fish species present in NZ (e.g. carps, cyprinids, catfish, gambusia) are more tolerant of poor water quality, they too tend to proliferate in poor quality waters, and may out-compete many of the native species. A non-lethal decline in water quality may therefore increase overall fish abundance but at the cost of fewer desirable species and more undesirable ones.
As trout are among the more sensitive and better understood fish species, they are often used as the benchmark to set water quality targets. This is a workable approach to setting water quality limits for fish in NZ rivers. But the assumption that all native fish will always be protected is not necessarily correct in all waters. There are some water quality variables (e.g. turbidity) where trout are not the most sensitive species. Rare and/or endangered species (e.g. giant kokopu and shortjaw kokopu) have not been tested to date and may be more sensitive to some water quality variables than trout. Furthermore, the limits for adult fish may be higher than for eggs or larval stages (which have not been tested) and synergistic effects have not been rigorously determined yet. Then there are the rivers in the top half of the North Island where trout are not present. What limits should be set in these rivers? Whereas knowledge of the effects of water quality on freshwater is now well advanced on what it was several decades ago, there are still a number of important gaps to fill.
Dr David Rowe is a freshwater fish ecologist at NIWA.