Salmon fish farms can have a major impact on the marine environment. Most sea cages are moored in sea lochs and inlets that previously received minimal waste from the sparse population in the surrounding area. With the arrival of a fish farm there is a considerable increase in the amount of waste entering the water. Figure 19 summarizes the main sources of these wastes.
Fish are fed artificially from the surface with pelleted feed which slowly sinks through the cage. The feeding rate is designed to minimize wastage because fish feed is expensive. Nevertheless, excess, uneaten feed falls through the water and is deposited on the sea bed. In addition, there is a constant 'rain' of faeces from the fish which contains organic matter as well as traces of minerals and nutrients. The fish also excrete urine so there is an input of soluble waste into the water.
The main concerns are about the accumulation of organic matter on the sea bed, and the addition of extra amounts of nitrogen and phosphorus to the water. There are also unproven fears that the residues of antibiotics and other treatment chemicals may harm other marine life in the surrounding sea area.
The organic matter settles as a thin layer on the sea bed where it smothers the sand, stones, rocks or mud. If the accumulation is severe, then the decomposition of the organic matter by naturally occurring bacteria results in a depletion of the dissolved oxygen in the water within the sediment. This is the same effect as described in Chapter 2 when a poor quality sewage effluent enters a river. Sometimes, the growth of bacteria is so vigorous that thick white mats of them develop over the waste. The bacteria consume the organic matter and use up oxygen dissolved in the water at a greater rate than it can be replaced by clean water. This can give rise to anoxic conditions (no oxygen) in the sediment and result in the formation of harmful and objectionable by-products. In the worst cases, hydrogen sulphide, ammonia and methane gases bubble up from the sea bed to the surface. Hydrogen sulphide is a poisonous gas and can kill the fish which are the originators of the waste: a good example of 'fouling one's own nest'. These bad conditions are rare because there is usually sufficient movement of water over
Figure 19. Environmental impacts of cage fish farming on the marine environment
Figure 19. Environmental impacts of cage fish farming on the marine environment the sea bed to ensure enough oxygen is present for the decomposition of the waste. A more common effect of the excess organic matter is to encourage a build-up of a population of marine organisms which thrive on the waste. The uneaten food and faeces provide a valuable food source for marine worms such as the polychaetes Capitella capitata and large numbers accumulate below fish cages.
Once the source of the excess organic matter is removed, either by the fish cage being moved or it being left empty for a while (to fallow), conditions slowly return to normal. This is because the organic matter is consumed or broken down by the worms and bacteria until there is none left and the organisms starve. This recovery can take between two and ten years depending on many factors such as the amount of accumulated waste, water temperature, water movement, etc.
The phosphorus in the waste is usually present in its insoluble form and is not considered to be a serious problem in sea water. It is a problem though in fresh water (see below) because it is a limiting nutrient (see page 30). In sea water, the limiting nutrient is nitrogen, so extra inputs of this can encourage the growth of algae (phytoplankton). There is a possibility that the soluble nitrogen waste produced by fish in a fish farm could result in the formation of algal blooms. However, there is no clear evidence that this has occurred despite a number of investigations.
The doubling time for marine phytoplankton in favourable conditions is about two days but for this to happen, there has to be sufficient sunlight, nutrients and calm water. The combination of these factors does not happen often because most fish farms in the UK are located on the West coast of Scotland which is not renowned for calm sunny weather. Algal blooms therefore are rare events. They have occurred though and have had disastrous effects because some marine algae produce toxins in the water as a result of their photosynthesis. Blooms of toxic algae have been responsible for mortalities of farmed and wild fish in Scotland, Norway and Ireland.3
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