ence and depth at individual sites. The settling velocity of marine salmonid feed pellets is greater than faecal particles because of their higher density. Consequently there is a tendency for faecal waste to spread over a wider area than feed pellets, although most waste solids tend to accumulate in the immediate vicinity of the cage site. The amount of solid accumulating below cages is very variable, from up to 40cm of soft, light brown flocculent material in low velocity sites to no visible accumulation on high velocity sites.

The solids falling to the seabed below fish cages are enriched in carbon, nitrogen and phosphorus relative to the natural sediments, hence fish farming may alter the physico-chemical nature of sediments below and adjacent to the operation but is usually limited to the vicinity (50 m) of the cages. This increase in carbon sedimentation results in an increase in oxygen consumption by bottom-living animals. The sediments will become anoxic (i.e.contain no oxygen) if this additional oxygen demand exceeds oxygen supply, at which point there may be severe consequences for both benthic organisms and the fish farming operation itself.

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