Aquaculture, the farming of aquatic organisms such as fish, shellfish, and aquatic plants, has grown significantly to meet global food demands. This practice involves cultivating species in controlled environments, ranging from freshwater ponds to marine net pens. As wild fish stocks face increasing pressure, aquaculture has emerged as a method to provide a consistent supply of seafood to consumers worldwide. Its expansion is a response to the rising demand for protein and the limitations of traditional capture fisheries.
Environmental Contamination
Aquaculture operations can release various pollutants into surrounding aquatic environments. Nutrient pollution, which results from fish waste and uneaten feed accumulating in the water, is a significant issue. This excess of nitrogen and phosphorus can lead to eutrophication, a process where nutrient enrichment promotes rapid growth of algae. These algal blooms can deplete oxygen levels when they decompose, creating “dead zones” that harm or kill aquatic life. For example, a salmon farm with 3.5 million fish can release waste equivalent to the sewage from a city of 169,000 people.
Chemical pollution also poses a threat, as substances are used in aquaculture to manage diseases and maintain infrastructure. Antibiotics, parasiticides, pesticides, and antifoulants are commonly applied. In open net-pen operations, these chemicals can disperse into the marine environment, potentially impacting non-target organisms and altering microbial communities. The widespread use of antibiotics raises concerns about the development of antibiotic-resistant bacteria spreading beyond the farm environment. Some chemicals, like mercury, can bioaccumulate in farmed fish and transfer to humans through consumption.
Ecological Disruptions to Wild Species
Aquaculture practices can directly impact wild aquatic populations through disease and parasite transmission. Farmed fish, kept in high densities, can become reservoirs for pathogens and parasites like sea lice. These parasites spread to vulnerable wild fish populations, particularly juvenile salmon, as they migrate past aquaculture sites. Heavy infestations of sea lice can cause significant physiological damage to wild fish, including skin erosion and reduced survival rates.
Escaped farmed fish also pose a genetic threat to wild populations. When farmed fish, selectively bred for traits like fast growth, escape into natural ecosystems, they can interbreed with native species. This interbreeding can dilute the genetic diversity of wild stocks, potentially reducing their fitness and ability to adapt. Studies have shown that escaped farmed salmon can lead to a predicted population collapse in wild pink salmon due to increased sea lice mortality.
Furthermore, escaped farmed species can compete with native species for resources and habitat. This competition can place additional stress on wild populations. The introduction of non-native farmed species through escapes can also disrupt established food webs and ecological balances in the receiving environment.
Habitat Destruction and Coastal Alteration
Establishing aquaculture facilities often involves the physical alteration and destruction of natural habitats, particularly in sensitive coastal areas. Construction of aquaculture ponds and infrastructure frequently requires clearing vital ecosystems like mangroves, salt marshes, and wetlands. These coastal habitats serve as nurseries for many marine species, protect coastlines from erosion, and act as natural water filters.
The conversion of these areas to aquaculture sites leads to a significant loss of biodiversity and ecosystem services. For example, approximately 28% of mangrove habitat in Asia has been converted to aquaculture ponds, primarily for shrimp and fish farming. Mangrove destruction can increase coastal erosion, reduce natural protection against storms and sea-level rise, and release stored carbon dioxide, reversing their role as carbon sinks.
Beyond coastal clearing, the physical presence of net pens and waste accumulation beneath them can alter the seabed. This leads to changes in sediment composition and benthic communities, further impacting the local marine environment. Placing farms in sheltered bays and estuaries, while beneficial for farmed fish, often coincides with areas important for wild fish and other marine life.
Resource Strain and Feed Sourcing
Many farmed aquatic species, especially carnivorous ones like salmon, rely on feed derived from wild-caught fish, leading to resource strain. This reliance is quantified using the “Fish In, Fish Out” (FIFO) ratio, indicating how much wild fish is needed to produce a kilogram of farmed fish. For some carnivorous species, this ratio can be greater than 1:1, meaning more wild fish protein is consumed than produced. For instance, producing 1 kg of farmed salmon can require at least 4 kg of wild fish, primarily as fishmeal and fish oil.
The demand for fishmeal and fish oil for aquaculture feed contributes to the overfishing of small pelagic fish species, often called forage fish, such as anchovies, sardines, and mackerel. Almost all global fishmeal (87%) and fish oil (74%) production is currently used in aquaculture. The depletion of these forage fish stocks can disrupt marine food webs, as they form a foundational link for larger wild fish, marine mammals, and seabirds.
While there are efforts to reduce reliance on wild fish by incorporating alternative protein sources like by-products from fish processing, the demand for marine ingredients remains substantial. The continued pressure on forage fish populations can have cascading effects throughout the marine environment, impacting the health and stability of diverse marine life.