Chemical contamination and agriculture
What are the potential sources of chemical contamination from agricultural activities?
Chemical contaminants in the agricultural industry include a range of compounds. They can be found in fertilisers applied to crops, pesticides and herbicides used for weed and pest control, and antibiotics and steroid hormones for animal health and growth.
Chemical contaminants may eventually make their way into an adjacent waterway through direct application, by spray drift, or via wastewater discharges from effluent ponds and surface runoff from the land. The use of chemicals can have long term effects on mahinga kai species and water quality.
Potential impacts of chemical contaminants on water quality and mahinga kai
- Local loss of fish species - fish may be harmed by contaminated water. Discharges and runoff into waterways can be lethal to aquatic life depending on the strength of the toxin and size of the waterway and can cause fish kills.
- Local loss of invertebrate species - contaminants such as synthetic pyrethroids (in sheep dip) can be particularly lethal to invertebrates. Invertebrates are also food for fish and persistent discharges that kill invertebrates could cause fish to travel farther in search of food and expose them to greater risks and stress.
- Decreased dissolved oxygen (DO) levels - waste compounds released into waterways initiate biochemical reactions that use up oxygen as the stream bacteria break down the organic matter (Biogeochemical Oxygen Demand, BOD). Excess nutrients can also lead to algal blooms; oxygen is used up when the algae die and decompose. Because fish ‘breathe’ oxygen through their gills, a decrease in available oxygen (anoxia) in the water column threatens their ability to respire, which may lead to death. Fish that tolerate low levels of dissolved oxygen (such as the introduced species gambusia) may replace native populations that are less tolerant.
- Increased turbidity and decreased water clarity - water may become cloudy or discoloured with chemical contamination, which reduces the ability of fish to see, prey, and detect predators.
- Damage to species - repeated exposure to sub-lethal doses of some contaminants can cause physiological and behavioural changes in fish that have long term effects on the population, such as reduced reproductive success, abandonment of nests and broods, a decreased immunity to disease, tumors and lesions, impairment of the central nervous system, and increased failure to avoid predators.
- Some contaminants, such as mercury, may bioaccumulate in animal tissues and be carried to human consumers of the fish.