Seafood is now increasingly being raised in captivity, a practice known as aquaculture. Over one-third of all fish consumed in the United States is farm-raised, including shrimp, salmon, tilapia and catfish (1). Most of this food is raised outside the United States, although aquaculture is growing in all corners of the world, and is likely to continue to grow as demand for seafood rises.
Many fish farms are highly resource-intensive and have significant ecological impacts on the oceans or inland waterways where they are located. Below we have outlined problems with many existing systems, problems that need to be addressed if aquaculture is to supply a significant percentage of the world’s protein on a sustainable basis.
Many farmed fish are carnivores and depend on being fed wild fish. This means that instead of taking pressure off the oceans, fish farming may increase the demand for wild fish. For example, farmed salmon commonly requires three pounds of wild fish as feed for every pound of salmon raised. Current global farmed salmon production exceeds one million tons requiring 2 to 3.5 million tons of wild fish be processed into feed annually (2). This type of feed-intensive farming results in a net loss of protein. In other words, some farm operations use more fish than they generate.
On the other hand, some fish raised in farms are omnivores and can be fed plant-based diets. Catfish and tilapia are examples of species that can be raised without wild fish inputs. Thus, instead of resulting in a net loss of protein like salmon farms, these catfish and tilapia farms do not put additional stress on the oceans.
Another environmental concern of aquaculture is the pollution caused by net-pen farming. Many farmed fish, including salmon, are raised in net-pens in the ocean where thousands of them are concentrated in one area like cattle in a feed lot. These fish produce tons of feces which can significantly pollute the water. An average farm of 200,000 salmon produces the daily equivalent amount of feces as a town of 62,000 people. This waste accumulates on the sea floor below the farm pens and generates killer bacteria that consume the oxygen vital to wild bottom fish (3).
Chemicals and Antibiotics
Algae and shellfish growth on the net-pens is a problem for fish farmers because it leads to equipment damage. Therefore, fish farmers apply toxic chemicals (like pesticides and copper sulfates) on the nets. Since the purpose of these chemicals is to kill off algae and shellfish, contamination of the surrounding water poses a serious threat to native wild marine life.
Also, diseases are easily spread among farmed fish due to the high concentration of fish in each net-pen. Antibiotics are commonly used in aquaculture in order to prevent (or treat) a disease outbreak but overuse (especially if used for non-therapeutic purposes) may lead to drug-resistance in these farmed fish and wild fish if the antibiotics leak outside of the net-pen (3).
Farmed fish that escape from net-pens may have a significant impact on the surrounding marine ecosystem. They can compete for food with the wild fish and take over limited marine habitat area. Farmed fish can also spread diseases that did not previously exist in wild fish populations. In addition, farmed fish (that are often genetically modified) may interbreed with the wild fish and alter the natural genetic makeup of that species. A good example of this is farmed salmon. There have been known cases of millions of genetically modified Atlantic salmon that escaped and traveled thousands of miles from their farms, interbreeding with the wild salmon and disturbing nesting habitats (4).
Human Health Concerns
Fish farming is not only detrimental to the marine ecosystem but may also pose human health risks. Studies have shown that farmed salmon have 10 times more residues of PCBs and dioxins (chemicals linked to cancer) than wild salmon. This is because feed pellets produced for farmed salmon consists of concentrated fish meal made from wild fish that are most commonly exposed to environmental pollutants such as pesticides, dioxins and PCBs (5).
Although many fish farms can be resource-intensive and damage the environment, shellfish farms tend to be more eco-friendly. Farm-raised oysters, clams and mussels are examples of environmentally-friendly seafood choices. Because they mainly eat plankton, which they filter from the surrounding water, they don’t require extra feed. In addition, farmed shellfish are not raised concentrated net-pens like other farmed fish. Oysters and mussels are raised in bags suspended off the seafloor and clams are often raised in beds on the shore, so little ecological damage is done when the shellfish are harvested.
1. Monterey Bay Aquarium: Seafood Watch Program. www.seafoodwatch.org. Accessed March 16, 2007.
2. Naylor R, et al. Effects of aquaculture on world fish supplies. 2000, Nature 405: 1017-1024.
3. Mazurek R and Elliot M. Seafood Watch Seafood Report: Farmed Salmon. Monterey Bay Aquarium, April 2004.
4. Naylor R, et al. Fugitive Salmon: Assessing the risks of Escaped Fish from Net-Pen Aquaculture. 2005, BioScience 55(5):427-437.
5. Hites RA, et al. Global Assessment of Organic Contaminants in Farmed Salmon. 2004, Science 303:226-229.