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Plankton Extraction

Pacific oysters feed by filtering plankton (small free-floating plants and animals) from the water column. This means oyster farmers don’t need to feed their crop, but it also means farmed oysters depend on the plankton naturally available in the water column.

While oysters filter both plant and animal plankton from the water, they also secrete nutrients into the water, which stimulate the growth of plant plankton.

Oyster farms could affect coastal and estuarine food webs if they significantly reduce either the amount of phytoplankton that is available to wild species, or the amount of fish eggs and larvae in an ecosystem.  However, it is important to note that research in the main marine farming areas of New Zealand shows that nutrient inputs from the ocean and rivers have a larger effect on plankton abundance in the ecosystem than oyster farms. 

The amount of plankton uptake is an essential part of the assessment of effects of a proposed oyster farm because it influences how much food is available to wild species and how many fish eggs and larvae may be removed from an ecosystem. 

Applications for new oyster farms must include research on the estimated level of plankton depletion that is likely to occur with oyster farm development.  If there is more than one existing marine farm in a bay, applications for new farms will often investigate the cumulative effects of further farms on the marine environment.

It is difficult to calculate the actual effects of oyster farming on plankton availability and the flow-on effects to the ecosystem; however, New Zealand researchers use the best available methods to estimate the effects of oyster farming on plankton, both on a site-specific and a cumulative scale. 

The assessment of ecological effects which accompany farm applications, often provide Geographic Information System (GIS) modelling of the ‘footprint’ of potential phytoplankton uptake using the percentage of water processed as a proxy for plankton depletion.  Scientists can also measure the actual concentrations of chlorophyll a in the water around oyster farms as a good indicator of phytoplankton levels.

Based on previous experience from research on marine farming regions around the country, New Zealand scientists from the National Institute of Water and Atmospheric Research (NIWA) generally consider plankton depletion of around 5% to 10% to be ecologically sustainable.  The limits of acceptable change, however, can vary greatly depending upon specific conditions of the site.