Granada, L., Sousa, N., Lopes, S. and Lemos, M.F. (2016). Is integrated multitrophic aquaculture the solution to the sectors’ major challenges? – a review. Rev Aquacult, 8: 283-300. doi:10.1111/raq.12093
Goldburg, R.J., Elliott, M.S. , and Naylor, R.L. (2001). Marine Aquaculture in the United States: Environmental Impacts and Policy Options. Pew Oceans Commission, Arlington, Virginia.
What is Aquaculture?
Aquaculture is the growing or farming of aquatic species, namely finfish and shellfish, to help meet the increasing demands of seafood, especially in developing countries. It is a rapidly growing industry designed to keep up with food demands while taking pressure of off wild fish stocks, many of which are over-exploited.
Traditional aquaculture has been highly regarded as a benefit to the seafood industry. In 2010, aquaculture produced approximately 47% of total fish (around 160 million tons) for human consumption (Granada et al.). Fish grown in aquaculture settings are largely exported to developing countries, providing a positive nutritional benefit to these growing communities. As the aquaculture industry continues to thrive, it brings job opportunities and economic benefits to communities. In many cases, aquaculture operations are located in rural areas where an economic boost is needed.
The traditional aquaculture industry has been under scrutiny for several negative environmental side effects that need to be addressed including an excess of nitrogen waste, disease occurrence, chemical pollution, and use of natural resources.
The discharge of fish feces and uneaten food contributes to a substantial amount of pollution from aquaculture facilities, which increases nitrogen and phosphorous levels in surrounding water bodies and sediments. This can be damaging to wild species found in the affected areas.
When housing many fish together in a small space, this creates an environment susceptible to infection by pathogens or infestation of parasites. While these occur in the wild as well, in an aquaculture facility they are intensified due to the close proximity of the organisms, increased stress levels, and inadequate water exchange. Diseases and parasites often result in serious economic or ecological consequences, the most critical of which is the death of cultured species and the financial loss that follows.
Further, the use of pesticides and antibiotics in aquaculture is common although regulated differently by each nation. These are considered chemical contaminants, and though it is necessary to treat diseases and parasite infestations in an aquaculture system, antibiotics are often used as a preventative measure, which can result in antibiotic resistance. At the same time, the antibiotics can enter the natural environment which leads to accumulation of antibiotic residues in ponds, sediments, and wild fish.
Water, land, and forage fish harvest are some of the natural resources required to maintain a functioning aquaculture facility. In order to flush out nitrogen build up, frequent water exchanges must be used. Inland aquaculture facilities require lots of land and cage-based farms utilize the seabed. In addition, most of the feed for aquacultured fish comes from the harvest of forage fish such as menhaden, sardines, and anchovies. This puts pressure on these fish stocks and can ultimately lead to a decrease of these fish populations, which is counter-productive to the original goal of aquaculture.
IMTA as a solution
In recent years, aquaculturists have been trying to address the side effects of their practice. One innovative way to ameliorate some of these issues is the use of integrated multitrophic aquaculture (IMTA). IMTA is an approach to aquaculture where the wastes from a target fish species are recycled to become food and energy for another species. This requires cultivating a typical aquaculture species (fish) with species that extract organic waste from the ecosystem (invertebrates), and species that extract inorganic material from the ecosystem (plants or macroalgae). For example, in the Bay of Fundy, Canada, there is an IMTA system growing Atlantic salmon along with blue mussels and kelp. As the salmon create waste in the form of feces or leftover food, the blue mussels filter the water and remove most of the organic wastes. The blue mussels produce cleaner water as they eat. At the same time, the kelp extracts inorganic dissolved nutrients, ultimately reusing the bulk of the waste produced by the salmon. In this case, the salmon, mussels, and kelp can all be efficiently grown and harvested.
There has been a growing need to create a more sustainable aquaculture performance and IMTA is one solution. As a relatively new idea, there are still studies being done to understand the overall benefit that IMTA has over traditional aquaculture. However, due to its ability to recycle organic and inorganic waste as well as grow multiple species efficiently and simultaneously, IMTA will likely be an increasingly common practice in the coming years.