Shellfish gathering and farming take place in marine and freshwater environments.
Shellfish farming (mussels and oysters)
Mussels, like the native green-lipped mussels or ‘GreenshellTM Mussels’ (Perna canaliculus) are grown in the marine environment on long-lines or ‘dropper’ ropes that hang down through the water column suspended by floats. Pacific oysters (Crassostrea gigas) are grown on wooden racks in sheltered and shallow bays. Both oyster and mussel farming rely on catching young shellfish (spat) from the wild, with animals then taking approximately 12 to 20 months to grow big enough to sell. No food is added, as shellfish are able to feed by filtering natural plankton (free-floating animals and plants) out of the water column. Farmers commonly use barges to access their sites, although on intertidal oyster farms, where the water isn’t too deep, farmers often walk among racks while conducting maintenance or harvesting.
Ecological impacts of shellfish farms
Shellfish farming can cause localised disturbances to the physical, chemical and biological nature of the seabed through the deposition of organic and inorganic nutrients, including faeces, live shellfish, and shell from the farm. Accumulation of this organic-rich material results in increased microbial activity, which can lead to oxygen depletion in the sediment. Other mobile organisms (e.g., starfish, crabs, and fish) may be attracted to the accumulated material, as it provides a potential food source, while the hard substrate provided by deposited shell may allow reef-type communities to colonise under farms.
Fine waste materials may be suspended in the water and increase turbidity. When these settle they may smother benthic species. As mentioned above, shellfish feed by filtering plankton and other suspended particles from the water. Filtering may enhance water quality in the location. However, shellfish farming could potentially affect coastal and estuarine food webs if they significantly reduce the amount of plankton available for wild species, or the amounts of fish eggs and larvae in an ecosystem.
Some oyster racks are constructed from treated timber, from which chemical contaminants such as copper, chromium, and arsenic may leach out into waterways. Farming structures shade the area under the farm, which potentially affects light-dependent species such as algae and sea grasses, and the species that live around these communities. Structures also create obstructions in the water that can alter local current speed and flow patterns. This can potentially affect the ecology of the surrounding area by altering the rate at which food is replenished or waste dispersed, or by increasing the resettlement of sediment and nutrients.
Good management has proven to be an effective way to minimise the spread of pests and disease of aquaculture activities.
Most farmed salmon are born in land-based hatcheries, then transferred to sea cages where they are grown to harvestable size, with some fish also grown at inland freshwater farms. Sea based salmon farms tend to be positioned in areas that are flushed out by currents, which reduces the detrimental effects of salmon waste and uneaten food (nutrients) in the environment. These effects can also be controlled through farm management practices, particularly ensuring appropriate stocking densities and feeding. Salmon are fed an artificial diet of pellets that are high in protein and fat, and have vitamins and minerals added to provide nutrition (standard feed in Aotearoa does not contain antibiotics, vaccines, steroids, or other growth enhancers). Salmon generally reach a harvestable size after about 10 to 18 months in seawater. The potential environmental impacts of fish farming other fish species using similar techniques to salmon farming will likely produce similar environmental impacts.
Fish feeding and cages
The most common effect of fish farming using sea cages are localised changes to the seabed and water through the release of uneaten food and fish waste. This can change the physical, chemical, and biological nature of habitats below the farm where the wastes are deposited. This waste is a source of nutrients that can result in increased growth of phytoplankton, which can lead to eutrophication and potentially harmful algal blooms, and increased microbial activity. Oxygen concentrations in the sediment can drop drastically with increased microbial activity and may result in deaths of fish or other organisms. Seabed effects are largely reversible, although recovery may take many months or years, depending on local water flushing characteristics.
The feed supplied to farmed fish contains small amounts of zinc, which is an essential micronutrient for the prevention of cataract formation and other fish health problems. Zinc can accumulate in sediments beneath fish farms and can be toxic in high concentrations. Feed companies are presently working to minimise zinc discharges to the seabed, mainly by reducing the content in the feed.
The design and positioning of cages may affect sediment deposition patterns through altering water currents around a farm site. Antifouling paint that is used on fish farming structures to reduce build-up of fouling organisms may contain chemical contaminants like copper which can accumulate in sediments below farms and are toxic in high concentrations. Salmon farming companies in Aotearoa are encouraged to actively minimise the use of antifouling paints that contain copper and use manual defouling methods.
The magnitude and extent of potential environmental impacts from aquaculture depends on the site conditions, including water depth, wave exposure, benthic sediment type, currents, and farm management practices.
Customary fishing and harvesting
Fishing and harvesting of marine and freshwater mahinga kai is common for both customary interests and commercial aquaculture purposes. A Tangata Kaitiaki/Tiaki appointed under the Kaimoana Customary Fishing regulations may authorise any individuals to take any fish, aquatic life, or seaweed for customary food gathering purposes from within the whole or any part of an area/rohe moana, for which the Tangata Kaitiaki/Tiaki has been appointed. Management of areas that have special significance (i.e., areas near a marae, a certain type of fishery, or a breeding ground) and any quotas (restrictions on harvesting) are managed by tangata whenua.
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