Human impact has affected the ecology1 and biodiversity2 of the Waikato River. Scientists recognise the three key areas of impact that have affected the distribution of native3 species4 in the lower part of the river and catchment5 area are:
- harvesting
- exotic species
- habitat6 destruction and modification.
Harvesting
The number of glass eels7 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 decline8 in commercial fish caught (contributing factors to this are habitat loss and reduced access to streams, lakes and wetlands9).
Harvesting pressure10 has also contributed to declining11 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 species
Exotic or introduced animals and plants can affect native species through competition for space and food and through modification to habitats12. The biomass13 and high density14 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 aquatic15 plants, increase turbidity16 of the water and cause bank erosion17 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 smelt18 was associated with the decline of the native kōaro. Several introduced invertebrates19, mostly snails, now appear in the river. Daphnia dentifera (parasitic water fleas) make up almost half of the planktonic cladoceran crustaceans20.
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 sediment21 losses and erosion.
In recent years, riparian planting22 to create a vegetation23 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.
Water quality
The quality of water can have an effect on species. Nutrient24 enrichment (from fertiliser25 use on farmland) and associated algal26 levels (and eutrophication27) have increased in catchment areas, progressively moving down the river. The water becomes murky and greener. Moderate numbers of toxic28 algae29 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 faecal30 bacteria31 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 invertebrate32 communities, with mayflies becoming less dominant33 when temperatures exceed around 22°C. Temperature34 increases in the lower river catchments occur in the summer in exposed pastoral streams. Increased temperatures in the river have previously resulted from thermal power35 station discharges36, but controls have been put in place to regulate37 discharge38 temperatures.
Water flow
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 adapted39 to fast-water environments.
The exchange of water between the river, flood plain, wetland40, 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 tributary41 habitat (including whitebait spawning42 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, iwi43, environmentalists, authorities and local people. Such discussions lead to further scientific exploration and possible solutions and mitigation44.
Related content
The interactive Water quality indicators features several of the physical and biological indicators that scientists use to assess water quality.
- ecology: The study of the interactions of living organisms with each other and their environment.
- biodiversity: The range of species found in a particular region. The more species that exist (the higher the biodiversity), the more likely it is that an ecosystem will survive episodes of change.
- native: A species that lives naturally in a country, as opposed to species that have been introduced by the activity of humans.
- species: (Abbreviation sp. or spp.) A division used in the Linnean system of classification or taxonomy. A group of living organisms that can interbreed to produce viable offspring.
- catchment: An area that collects all the water that drains to a particular lake, river or reservoir. Also known as a watershed or a drainage basin.
- habitat: The natural environment in which an organism lives.
- glass eels: Transparent juvenile eels. Eel larvae – leptocephali – change into glass eels.
- decline: The gradual and continuous loss of something such as bird numbers or sea ice.
- wetland: An area of land that is saturated with water, often referred to as a swamp or bog. Wetlands may be seasonally or permanently water-logged with vegetation adapted for life under those soil conditions. Wetlands are known as repo in te reo Māori.
- pressure: The force per unit area that acts on the surface of an object.
- decline: The gradual and continuous loss of something such as bird numbers or sea ice.
- habitat: The natural environment in which an organism lives.
- biomass: 1. Organic matter, such as trees, plants, reject fruit, straw, algae, dairy effluent or tallow (waste fat), which can be turned into biofuel. 2. The mass of living biological organisms in a given area or ecosystem at a given time.
- density: How tightly a certain amount of matter (atoms or molecules) of a substance is compacted in a given volume. Density is commonly measured in grams per millilitre (g/ml) or cubic centimetre (g/cm3).
- aquatic: Growing or living in or near water (either freshwater or marine).
- turbidity: A measure of water clarity. High turbidity may occur when sediments are disturbed or other particles are suspended in the water.
- erosion: Wearing away of the land by mechanical action, such as by wind, water and glaciers, and by material carried in them. It can also be the gradual wearing away of a surface due to friction, particle collisions or chemical attack. Part of the process of erosion transports material away.
- smelt: To get metal from rock by heating it to a very high temperature. To melt objects made from metal in order to reuse the metal.
- invertebrates: An animal without a backbone, for example butterflies, worms, snails, insects, spiders and aquatic species such as crabs and jellyfish.
- crustaceans: A large group of arthropods, which includes animals such as crabs, lobsters, crayfish, shrimp, krill and barnacles.
- sediments: Material that settles to the bottom of a liquid. In geology, it describes the solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water or ice.
- riparian planting: Planting in the strip alongside a stream or river.
- vegetation: Plant life.
- nutrient: A substance that provides nourishment for growth or metabolism.
- fertiliser: Compounds that are given to plants to promote growth.
- algae: A large, diverse group of photosynthetic eukaryotic organisms. Algae have no stems or leaves and grow in water or on damp surfaces.
- eutrophication: A process where waterways, lakes and shallow sea areas receive excess nutrients that stimulate excessive plant or algae growth. This excessive growth depletes the available oxygen in the water and causes other organisms to die off.
- toxic: Poisonous and harmful.
- algae: A large, diverse group of photosynthetic eukaryotic organisms. Algae have no stems or leaves and grow in water or on damp surfaces.
- faeces: The excreted waste product of digestion in animals – poo.
- bacteria: (Singular: bacterium) Single-celled microorganisms that have no nucleus.
- invertebrates: An animal without a backbone, for example butterflies, worms, snails, insects, spiders and aquatic species such as crabs and jellyfish.
- dominant: An allele that is expressed if it is present.
- temperature: A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.
- power: 1. The rate at which work is done (defined as work divided by time taken). 2. Mechanical or physical energy, force or momentum.
- discharge: 1. The conversion of chemical energy to electric energy. 2. A flowing out or pouring out.
- regulated: Controlled by rules and laws. In biology: To adjust a bodily function or process. In genetics: To control the expression of a gene or genes.
- discharge: 1. The conversion of chemical energy to electric energy. 2. A flowing out or pouring out.
- adaptation: A change in the structure or function of something. In biology, a change in a species, as a result of natural selection. Individuals with a particular feature (adaptation) are more likely to survive and reproduce than individuals without this feature.
- wetland: An area of land that is saturated with water, often referred to as a swamp or bog. Wetlands may be seasonally or permanently water-logged with vegetation adapted for life under those soil conditions. Wetlands are known as repo in te reo Māori.
- tributary: A stream that flows into a larger stream or another body of water.
- spawning: The laying of eggs by aquatic animals like fish, frogs, crustaceans or mollusks.
- iwi: Māori tribe or large community, often consisting of several hapū (clans) bound together by common ancestors.
- mitigation: Reducing the severity of something.
