Nutrients and aquaculture
What are the potential sources of nutrients from aquaculture activities?
Commercial shellfish and fish farming may significantly increase the amount of nutrients entering a system through the deposition of faeces, fish food, live shellfish, and shells from a farm. The impacts of any added nutrients from an aquaculture operation into a system is largely dependent on the size of the waterbody and farming operation, local currents, tidal impacts, and the ability of a system to flush out and replenish itself.
Customary harvesting of shellfish can also be threatened by increasing nutrients from the surrounding catchment that can cause phytoplanton blooms and eutrophication (either partially or completely) of a waterbody. Shellfish species are filter feeders, making them much more susceptible to contamination than non-bivalve species. Elevated levels of some phytoplankton are a common cause of shellfish toxicity (contamination).
Potential impacts of high nutrients on water quality and mahinga kai
- Eutrophication - excess nutrients into a waterbody can lead to an increase in primary productivity which stimulates excessive algae growth, thereby degrading water quality.
- Loss of species - an increase in algae growth, sometimes called an algal bloom, reduces dissolved oxygen (DO) in the water when dead material decomposes and can cause fish and invertebrates to die.
- Loss of habitat - eutrophication of the water can kill off plants that fish depend on for their habitat.
- Increase in turbidity and a decrease in visibility - when the phytoplankton community increases in response to nutrients this reduces water clarity, visibility and recreational suitability. It also reduces the ability of some fish to see their prey or predators.
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