Chemical contamination and aquaculture
What are the potential sources of chemical contamination from aquaculture activities?
The use of various pharmaceutical products, anti-fouling paints, and fish food are all potential sources of chemical contaminants from commercial aquaculture operations such as fish farms. Some have only small environmental impacts, while others can be significant, especially for shellfish species that are filter feeders (this makes them much more susceptible to contamination than non-bivalve species). Heavy metal concentrations in sediment (e.g., copper, lead, and zinc) are often locally elevated around built up human settlements.
Products that are highly water soluble, break down readily, and do not bind to sediments are less likely to cause significant impacts. Recent overseas studies have highlighted the potential for bioaccumulation of persistent chemicals in farmed fish. (Dioxins, polychlorinated biphenyls (PCBs), and heavy metals like mercury, are compounds that accumulate in animal tissue, including humans, via the food chain.) These impacts should be eliminated from any aquaculture systems.
Alternatively, customary harvesting of shellfish can be threatened by chemical contaminants and increasing nutrients (which stimulate phytoplankton blooms) from surrounding land and industry. Because they are filter feeders, shellfish accumulate toxins quickly, and outbreaks of toxic shellfish poisoning are not uncommon.
Monitoring of commercial bivalve molluscan shellfish for biotoxins, heavy metals, and toxic substances originating from phytoplankton is carried out under the Animal Products (Regulated Control Scheme – Bivalve Molluscan Shellfish) Regulations 2006. New Zealand Food Safety Authority (NZFSA) is responsible for monitoring the safety of non-commercially harvested shellfish from harmful marine biotoxins.
Potential impacts of chemical contaminants on water quality and mahinga kai
- Loss of fish species - discharges and runoff from land into estuaries or waterways can be detrimental to aquatic life depending on the strength of the toxin and size of the waterbody.
- Local loss of invertebrate species - some contaminants can be particularly lethal to invertebrates.
- Decreased dissolved oxygen (DO) levels - waste compounds released into waterways initiate biochemical reactions that use up oxygen as bacteria break down the organic matter (Biogeochemical Oxygen Demand, BOD).
- Some contaminants, such as mercury, may bioaccumulate in animal tissues and be carried on to human consumers of fish and shellfish.