Agriculture and Environment: Salmon

Environmental Impacts of Production: Waste & Nutrient Loading

Salmon produced from aquaculture are efficient at converting feed to flesh.

For example, 1 kilogram of salmon can be produced with as little as 0.9 to 1.1 kilograms of feed and only 0.27 to 1.1 kilograms of waste.

Even so, because salmon are produced in a water column that can be up to 20 metres deep, wastes can accumulate and degrade water quality. This in turn can smother plant and animal communities living beneath the net cages (Weber 1997).

Waste from faeces and uneaten food results in increased nitrogen and phosphorous released into marine environments. In 1998 Scotland produced 115,000 metric tons of Atlantic salmon. Nutrient inputs to the marine environment for that year were 6,900 metric tons of nitrogen and 1,140 metric tons of phosphorous (i.e., 1 metric ton of salmon released 60 kilograms of nitrogen and 10 kilograms of phosphorous).

Significant amount of Nutrient pollution....
Ellis and associates (1996) found that each metric ton of salmon production resulted in waste equivalent to that from 9-20 people. So for nitrogen, the total nutrient input for 1998 was equivalent to the sewage from 3.2 million people, and for phosphorous, 9.4 million people. In 1997 Scotland's human population was 5.1 million people (MacGarvin 2000).

....leading to eutrophication
Nutrient pollution leads to eutrophication, which often results in increased plant growth. Even small changes in nutrients can have major impacts on phytoplankton communities. Increased phytoplankton populations reduce light availability below the surface, and as a result, threaten seaweed and eelgrass communities. Elevated nutrient concentrations, along with climatic conditions, can contribute to blooms of plankton and toxic algae (MacGarvin 2000).

The perils of toxic algal blooms
Blooms can have devastating effects on farmed fish. Some plankton species have sharp spicules (needle like pointed structures) that can damage gill tissue, making fish more susceptible to disease. Depending on cage depth, salmon raised in net cages may not be able to evade the surface plankton (Ellis and Associates 1996).

The frequency of mortalities due to algal blooms around salmon farms is increasing. When these mortalities occur, salmon farmers suffer huge financial losses but can also make compensation claims. In certain areas, the evidence suggests that salmon farm pollution is the main, or at least a contributing, cause of toxic algal blooms (Staniford 2002).

Algal toxins can also be transmitted via plankton-feeding fish up the food web to other marine species including birds and marine animals (MacGarvin 2000). In Scotland, faecal waste from fish farms has been linked to toxic algal blooms and outbreaks of the algal toxins that cause diseases in humans, most notably amnesiac shellfish disease (ASD).

Both diarrhetic shellfish poisoning (DSP) and paralytic shellfish poisoning (PSP) are also of concern in the region. Such blooms have severely depressed the shellfish farming industry in Scotland. Fish farm sediments are deposited on the ocean floor, disrupting and altering the community of macrofauna that live there. Benthic communities play important roles in sediment nutrient cycling. The structure of the community can change as species with low tolerance to pollution, or species that are no longer suited to the organically enriched environment, die or move to other areas.

The rapid deposition of waste can overwhelm organisms that promote aerobic decomposition on the ocean floor. Anaerobic decomposition by a different community is then favoured, causing a drastic shift in the ability of the original benthic community to survive in the area (Ellis and Associates 1996).

A possible solution
Incorporating seaweed and/or shellfish into the salmon farming system can help to solve the waste problem, since these organisms filter and utilise waste products. An integrated system of, for example, salmon and seaweed or salmon and shellfish could reduce nutrients significantly.

There is some question, however, as to whether such systems could reduce significantly the overall impact of having so much organic matter concentrated in one place. In addition, such a system does not help solve the problem of toxic chemicals entering the marine environment (Staniford 2002).


Extracts from "World Agriculture & Environment" by Jason Clay - buy the book online from Island Press

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