Seabed Effects
Salmon farms affect the seabed directly beneath them through faeces and uneaten feed falling to the seafloor. These organic-rich particles can alter the physical, chemical and biological nature of the seabed. Organic enrichment increases microbial activity that, in turn, can lead to oxygen depletion in the sediment (and subsequent changes to biodiversity).
In New Zealand, it is usual to site salmon farms over muddy habitats that are adapted to naturally higher rates of deposition. This minimises habitat modification and protects biodiversity of more sensitive habitats.
Information to date shows seabed effects are largely reversible, once the farm is removed, although recovery is likely to take many months or years, depending on water flushing characteristics of the farm site.
Organic enrichment can be pronounced directly beneath a salmon farm, but seabed conditions rapidly improve with increasing distance from farm structures (over tens or hundreds of metres).
How great these effects are depends mainly on the flushing characteristics of the site, and salmon stocking density and biomass (total weight of salmon in the farm). So these effects can be reduced by siting farms in deeper, well-flushed areas and through farm management practices that control stocking densities and limit feed wastage.
On New Zealand farms, stocking densities and feed wastage are managed at a level that their effects on the environment are considered acceptable by stakeholders.
As part of the ecological assessment for a salmon farm application, scientists check whether there are any unusual habitats or species at the site. Proposed farms are more likely to be declined if the sites contain or are very close to habitats or species of special conservation value or ecological importance.
The depositional 'footprint' of a proposed salmon farm is also checked. This is usually done using a predictive model that estimates the distance and direction salmon farm wastes could travel before reaching the seabed. The model takes into account local water current speed, water depth, and the time it takes for particles to settle to the seabed. The model also estimates the amount of deposition that would be likely to occur at increasing distances from the farm.
Scientists also study the composition of the sediments beneath the proposed site. This usually includes taking samples inside and outside the proposed site, to investigate the proportion of fine mud, sand, and shell/gravel, the organic matter content of the sediment, and the REDOX depth (the depth at which sediment becomes anoxic). These values are compared to the average values for other sediments in the region.
In New Zealand, established salmon farms are regularly monitored for their environmental effects. These monitoring programmes are set as conditions of the farm’s resource consent. Seabed samples can be taken, to detect the level and extent of the actual depositional “footprint” of an operating salmon farm (for example, by testing for organic content, REDOX depth, and what animals are living in the sediments).
