For all the positive effects aquaculture can have on food security and conservation of wild fisheries, it can also pose serious environmental risks and create sustainability challenges.
One of the environmental impacts that receives the most attention is the issue of nutrient and effluent buildup on the sea floor below the cages. Because the fish are contained in one place at high densities, their waste – which includes both solids and dissolved nutrients like nitrogen – has the potential to build up below the cages and in the surrounding area. This creates the potential for algal blooms, which deplete the water of oxygen and can create damaging dead zones near aquaculture sites. Regulatory agencies have recognized these problems and implemented measures to prevent them, including siting the cages in places with strong currents to wash away the effluent and moving the cages from year to year to reduce impact on any one area.
Another environmental concern is the effect of the farmed fish on local wild fisheries. Disease and parasite outbreaks in fish farms, though infrequent, can spread rapidly among farmed fish because of the high densities at which they are kept, and disease may spread to wild fish populations. Fish farmers used to combat these outbreaks with antibiotics and other chemicals in fish feed, but this created concern about the effect of the drugs on the ecosystems around the cages, as well as residual antibiotics winding up on consumers’ plates. More recently, safe and effective vaccinations for farmed fish have been developed and are widely used, and the use of antibiotics in aquaculture has almost ceased in the United States.
Another major concern is that escapees from fish farms—particularly where farmed species are non-native—may compete with fish from wild populations for food, potentially displacing wild fish. Cages are closely monitored by underwater cameras and regularly inspected by divers for damage, so the frequency of escape is minimal. There are also concerns about farmed fish interbreeding with wild fish and affecting the gene pool of the wild stock, but these concerns have been lessened by the exclusive use of genetically modified, sterile females in finfish aquaculture.
Some aquaculture operations can even have positive effects on environmental and human health. Farmed fish are generally free of environmental contaminants such as mercury and heavy metals, since they eat exclusively human-processed feed for which toxin levels are regulated. In addition, the farming of filter feeders such as shellfish can improve water quality, and in fact shellfish are often integrated into finfish production in integrated multi-trophic aquaculture (IMTA) systems, also known as polyculture. Because shellfish are filter feeders, they use uneaten feed and elements of fish waste as food, and can be cultivated near nets and cages containing finfish to improve water quality and even protect against disease outbreaks. Polyculture systems, which can also involve seaweed cultivation, are prominent in Maine, and there is ongoing polyculture research at schools like the University of Maine, the University of New Hampshire, and many others.
In addition to environmental risks, concerns about the long-term sustainability of aquaculture in meeting our protein needs remain. There are two main issues when it comes to the sustainability of various aquaculture practices: fish food and freshwater and energy use.
In certain forms of aquaculture, such as salmon farming, the fish being produced require large amounts of feed, which often contains fishmeal made from other fish species. This means using many smaller fish to create fewer big fish, wasting energy and protein in the process. Wild caught forage fish like herring and anchovies are the predominant source of fishmeal and oil in feed, and these fish are often from overexploited wild fisheries. Species like salmon that are higher on the food chain require more fishmeal and fish oil in their feed than herbivorous or omnivorous fish like carp and tilapia.
To make feeds more sustainable, the fishmeal and fish oil used in feeds may also come from trimmings produced in processing seafood for humans, and this type of recycling in the feed industry has been on the rise. It is also possible to substitute plant proteins for fishmeal in feed, but this can change the feed’s nutritional content and lower beneficial omega-3 fatty acids, which come from forage fish, in the final product, reducing the health benefits of eating fish.
Then there are the issues of energy and freshwater – both limited resources that are in high demand in the aquaculture industry. Land-based aquaculture systems like hatcheries often require huge amounts of water to be pumped into their systems. Pumping the water alone requires electricity, and depending on the design of the aquaculture system, cleaning and filtering the water may also require high energy input. Some fish farmers have found ways to minimize their freshwater and energy consumption by using recirculating water systems, where water from the fish tanks is pumped through a series of filtering tanks that use bacteria and algae to remove solids, nitrogen, and other waste materials. The same water is then allowed to flow back into the fish tanks, conserving vast amounts of fresh water and energy.
Clearly, the sustainability and environmental impacts of farmed seafood products depends on many factors and varies with the methods used by the fish farmers.