Intensive salmon farms have revolutionised the supply of salmon making it accessible to far more people than before, but they do have serious problems. Many of these are caused by the prodigious amount of food that the salmon are eating. This contains a large amount of nutrients, both in the form of energy, and in the form of nitrogen and phosphorous which act as fertilisers. Some of these end up making up the salmon, but most are either missed by the salmon, or pass straight through, and can cause algal blooms and seriously alter the balance of the ecosystem.
Kjell Reitan from the Norwegian University of Science and Technology is working on the old adage that "where there is muck there is brass", essentially considering these inputs as a resource rather than a problem.
Only about 30% of the fish food is used by the salmon, the rest is released into the water. So the trick is to find something which will eat this food and turn it into something useful. Filter feeders are the natural answer so surrounding fish farms with mussel beds could transform the excess food into mussels which can of course be sold. He has found that waste fish food increases the rate of growth of the mussels and could produce 15,000-20,000 tonnes of extra mussels in Norway per year.
This still leaves the nitrogen and phosphorous fertilisers, which should be able to be used to fertilize plant growth. This can be a problem if the plants are small algae which choke off everything else, but if they are large seaweeds, such as Kelp, they can clean up the water and produce a resource which can be collected as a biofuel. In Norway alone there is the potential to grow 0.6 to 1.7 million tonnes of kelp from fish farm waste.
Together these could both clean up the environment and produce a resource worth 600 million pounds a year, which is a good example of looking at problems as opportunities.