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Water Column Effects

Water Quality

Fish wastes and uneaten food from salmon farms can increase nutrients and reduce dissolved oxygen levels in the water column.

Such potential water column effects can be mitigated by siting farms in well-flushed areas. They can be further reduced through farm management practices that control stocking densities and limit feed wastage.

Having lots of nutrients in the water column can cause excessive algal growth and the wider flow-on effects of this – reduced water clarity and physical smothering. Nutrient enrichment can also increase the likelihood of harmful algal blooms (algal species that produce biotoxins). 

Due to current farm management practices and the small scale of farming, salmon farming in New Zealand has not been found to result in these types of enrichment effects or an increase in harmful algal bloom events. The monitoring of algae (phytoplankton) in the water column in the Marlborough Sounds has not revealed an increase in their numbers around salmon farms.

International studies show that reduced oxygen levels can occur in the waters in and around farms due to the respiration of farmed fish and the microbial breakdown of seabed wastes.  This effect is of most concern to the farmers themselves, as low oxygen can damage farmed fish stock.

In New Zealand, salmon farms are sited in areas with sufficient water flushing, so dissolved oxygen concentrations are well maintained.

Nutrient Cycling

Nutrients are released into the water column from waste material on the sea floor as part of the nutrient cycling process.

Good water circulation minimises the impacts of nutrient enrichment by distributing and diluting waste products and soluble nutrients over a wide area, and encouraging oxygen and nutrient exchange between the seabed and the water body.

As part of the assessment of ecological effects required for a marine farm application in New Zealand, scientists take samples of sediment from the seabed at the proposed site and measure them for sediment size, organic matter content and REDOX depth (the depth at which sediment becomes anoxic). These parameters provide a very general indication of the natural level of nutrient enrichment at the site, which helps to predict the likely effects of the proposed farm on nutrient cycling.  Enrichment values at the site are also compared to regional values to assess the site in the context of the wider area, and can also take into account the nutrient loads entering the system from the ocean, rivers and from land runoff.