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Aquaculture Effluent: Pollution of Coastal Waters
Finfish culture is usually an intensive industry that involves an addition of solids and nutrients to the marine environment, and is recognised as potentially causing environmental degradation through these inputs

Many species of farmed fish depend on a diet of artificial feed in pellet form. This feed is spread on the surface of the water, and is consumed by the fish as it sinks. Because not all is eaten, much of the food can reach the bottom where it is eaten by organisms living on the bottom or decomposed by microorganisms. This can alter the natural food web and significantly impact the local environment.

An increased food supply on the bottom may favour some species over others. Also animals which are fixed at the bottom may die in water because of decreased oxygen levels caused by decomposition by microorganisms. Animals which move more may migrate to other areas.

Antibiotics contained in the feed can affect bottom dwelling organisms as they are released as the uneaten pellets decompose. Also chemicals used to prevent the growth of organisms which foul netting and other structures, reducing water flow through the cages.

The nutrients from the breakdown of excess food and fish waste products can increase nutrient levels well above normal, creating an ideal environment for algal blooms to form, that is eutrophication occurs. When the algal blooms die, they settle to the bottom where their decomposition depletes the oxygen. Before they die, the algae may also produce toxic (poisonous) chemicals that can kill other organisms. Shellfish contaminated with toxic algae have caused many health problems for humans.

Pollution Control
In many countries around the world, aquaculture development has often occurred with little government control. This has resulted in the loss of natural habitats especially wetlands and pollution. The impact of coastal aquaculture depends on a many physical, chemical and biological factors, especially the water flow (e.g. currents) in the area. In areas of high currents, wastes are spread out. Excess nutrients are still present, but the lower level of waste is more easily handled by the local food web.

Water movement also helps to replace water with low oxygen levels with oxygen-rich water from surrounding areas. In coastal areas some interesting new techniques are being developed. For example, in China, scallops, sea cucumbers and kelp are being farmed together. This reduces eutrophication and the use of poisonous antifouling compounds. Nutrients from scallop wastes are taken in by the kelp, antifouling compounds and herbicides can be reduced because sea cucumbers feed on organisms which foul nets and other structures. (Also see IMTA)

Pellets now contain the correct levels of nutrients needed by the fish they are used to feed and they are being designed to float for longer, rather than rapidly sink to the bottom where they become unavailable to the fish.

In contrast, shellfish farming usually results in a net removal of nutrients from the water column, and is generally considered to cause less environmental damage. Nevertheless, shellfish production can cause a build up of organic material on the seabed below as a result of particulate fallout from the shellfish or from the altered water movement around the farm.

Additionally, a net removal of nutrients from the water column may have either positive or negative repercussions for the natural system. Positive impacts are apparent in nutrient enriched areas, while negative impacts can occur if the shellfish compete with other organisms for survival (eg. seagrass).

Different types of feed pellets