Human impact has affected the ecology and biodiversity of the Waikato River. Scientists recognise the three key areas of impact that have affected the distribution of native species in the lower part of the river and catchment area are:
- exotic species
- habitat destruction and modification.
The number of glass eels is substantially smaller now compared with records in the 1970s. A smaller proportion of juvenile longfin eels are returning from the sea to the river. There has also been a decline in commercial fish caught (contributing factors to this are habitat loss and reduced access to streams, lakes and wetlands).
Harvesting pressure has also contributed to declining numbers of whitebait. The average annual whitebait catch from 1931–1950 was 46 tonnes compared with 14 tonnes from 1968–1985, 10.7 tonnes in 1998 and 3 tonnes in 2000.
Exotic or introduced animals and plants can affect native species through competition for space and food and through modification to habitats. The biomass and high density of koi carp in the lower river regions mean they have a more significant ecological effect than other exotic fish species. Koi are known to uproot aquatic plants, increase turbidity of the water and cause bank erosion by their feeding activities. Catfish compete with other species including trout, and gambusia (mosquito fish) will attack native fish species. The introduction of trout and smelt was associated with the decline of the native kōaro. Several introduced invertebrates, mostly snails, now appear in the river. Daphnia dentifera (parasitic water fleas) make up almost half of the planktonic cladoceran crustaceans.
Habitat destruction and modification
Sediment loss and riparian planting
Within the Waikato catchment, 52% of the river flows through farmland, 15% through exotic forest, 13% through native forest and 1% through urban areas. A move to intensive farming has resulted in habitat modification. Forest clearance and intensive farming have increased sediment losses and erosion.
In recent years, riparian planting to create a vegetation filtering system for overland flow and suitable vegetation to provide shade and bank stability has partially mitigated many of the effects associated with land use intensification and land activities that generate sediment.
The quality of water can have an effect on species. Nutrient enrichment (from fertiliser use on farmland) and associated algal levels (and eutrophication) have increased in catchment areas, progressively moving down the river. The water becomes murky and greener. Moderate numbers of toxic algae are present in some lakes. This can reflect an increase in farming activity and stock numbers in the Waikato catchment over recent decades. Levels of faecal bacteria arising from farm animals are not problematic in the main lower river but are moderate to high in catchment areas that feed into the river.
Turbidity or clarity of water refers to the concentration of fine sediment in suspension. Turbidity is more prevalent in the lower parts of the river and surrounding catchment. This affects fish migration, habitat and feeding ability.
Increased water temperatures mainly affect the invertebrate communities, with mayflies becoming less dominant when temperatures exceed around 22°C. Temperature increases in the lower river catchments occur in the summer in exposed pastoral streams. Increased temperatures in the river have previously resulted from thermal power station discharges, but controls have been put in place to regulate discharge temperatures.
The Waikato River has a highly modified flow regime as a result of hydroelectricity generation and the construction of floodgates and stopbanks. Dams to create water storage have transformed areas of the river to slow-flowing or still-water habitats. Flow management in sections below the Karāpiro dam down to Waipā creates changeable water levels. This is likely to affect species that can occur in periodically wetted habitat, such as invertebrates and algal communities originally adapted to fast-water environments.
The exchange of water between the river, flood plain, wetland, lake complex and tidal areas helps to provide food for inhabitants and keeps the areas ecologically productive. The construction of 259 km of stopbanks along the main river and 255 floodgates now influences this interaction during high flows and tidal changes. As well as reducing food supplies, changes in water flow throughout these areas restrict access for some fish species.
Fish passage and spawning habitat
Floodgates potentially restrict fish access to 212 km of tributary habitat (including whitebait spawning habitat). Below the Tuakau Bridge, culverts at road crossings restrict some whitebait species and eels from gaining access to suitable habitat upstream – limiting the number of adults that mature and produce eggs for the next generation. In 2008, 25% of the 717 culverts were considered by Environment Waikato to pose barriers to fish migration. In addition, much of the preferred adult spawning habitat for inanga has been lost due to land development.
Mitigation of human impacts
Key management activities to help relieve the effects of human impact on the lower river are:
- improved land management and riparian protection
- alleviation of fish passage restrictions in areas with suitable upstream habitat for native galaxiid and eel species
- enhanced water flow linkages in the flood plain, wetland and lake areas
- extended and enhanced whitebait spawning areas.
Nature of science
Scientific research sometimes reveals that environmental problems can be linked to human activity. This balance between environmental needs and our needs is often the subject of debate involving scientists, iwi, environmentalists, authorities and local people. Such discussions lead to further scientific exploration and possible solutions and mitigation.