Risks Associated with Aquaculture

The practice of aquaculture is not without negative impacts. The harmful consequences and risks associated with aquaculture can be broken down into the three subcategories below. They are: risks to the farmed fish themselves; risks to the surrounding environment and organisms; and risks to the human population.


Risks to the Farmed Fish

Disease and Sea Lice When any type of organism is confined in high density the spread of disease and parasites can become more rapid. While parasitic copepods are common on cultured and wild marine finfish, it has only been with the development of semi-intensive and intensive aquaculture that their importance as disease-causing agents has become evident.(4) Sea lice, copepods in the family Caligidae, are a known and reported problem to the success of aquaculture programs. Sea lice have the potential to affect the growth, fecundity, and survival of their hosts. Fish farms are ideal breeding grounds for sea lice due to the high density of stocked fish as well as the increased susceptibility of fish to sea lice resulting from the stress of confined quarters. There are many types of sea lice that infect different types of fish including salmon, cod, haddock, trout, halibut, flounder and wrasse. However, the effect of sea lice on salmon is the only area that has been studied at large, likely resulting from the huge detrimental effect that the lice have had on numerous salmon aquaculture efforts. Two sea lice species, Lepeophtheirus salmonis and Caligus elongatus have reeked the most havoc on farmed salmon stocks.


Infectious Salmon Anemia (ISA) ISA (aka hemorrhagic kidney syndrome) is a lethal disease of Atlantic salmon caused by an orthomyxovirus. While this virus appears to only cause disease in Atlantic salmon, both wild and farmed, it can also infect the sea run brown trout, rainbow trout and other wild fish such as herring. ISA was first discovered in Norway in 1984, and has since been documented in Sweden, Nova Scotia, Canada and more recently, the United States. Transmission occurs by direct contact, through parts from infected fish (mucus, blood, viscera, trimming, feces), contact with equipment contaminated with parts from infected fish, people who handled infected fish, and sea lice. Symptoms of ISA include lethargy, swelling and hemorrhaging in the kidney and other organs, protruding eyes, pale gills, darkening of the posterior gut, and swelling of the spleen. Mortality is highly variable and can be from 2 - 50% over one production cycle. It is suspected that the first case of ISA in the US, in Cobscook Bay, Maine, in 2001 was transferred from pens 3 miles away in New Brunswick, Canada, found to be infested in 1999. In response to the outbreak in Cobscook Bay, 1.5 million fish were slaughtered in hopes of containing the virus as ordered by the US Department of Agriculture's Animal and Health Plant Inspection Service.(5)

Other Fish Diseases Occurring in Aquaculture Spring viremia of carp (SVC) is a systemic, acute and highly contagious viral disease caused by Rhabdovirus. SVC primarily affects common carp and most susceptible fish belong to the Family Cyprinidae. Outbreaks of SVC have occurred and required fish kills for control in the US in multiple states including Missouri, Illinois, Washington, Wisconsin and North Carolina. (5)

Risks to the Surrounding Environment and Organisms

Pollution from Aquaculture Sedimentation of feces and uneaten fish feed can add large quantities of nutrient-rich organic matter into the sea (marine or coastal pens), rivers (raceways) or soil (inland ponds). In nutrient-limited oceanic environs, a moderate input of organic matter can enhance the abundance and diversity of the benthic fauna by increasing the supply of food. However, as the organic load increases, the faunal diversity declines and the sea bottom becomes increasingly dominated by a small number of hardy, opportunistic species, often polychaetes. If the organic input is severe enough even these animals decline and the bottom becomes covered by a whitish blanket of bacteria (Beggiatoa) which obtain energy by oxidizing sulfide diffusing out of anoxic sediments. Temperature, sediment type, local hydrography and distance from the point source (i.e. fish cage) influence the severity of the Beggiatoal mat.

A beggiatoal mat.
Increased Fishing Pressure on Wild Stocks Many farmed fish are carnivorous species that require more feed products than can be supported from natural food sources either growing within or passing through fish ponds or pens. This is largely due to the high densities of fish (as demonstrated in the picture to the right) in the farming environment. Many intensive and semi-intensive aquaculture systems use 2-5 times more fish protein, in the form of fish meal and fish oil, to feed the farmed species than is supplied by the farmed product. This is the reason that aquaculture may not only increase pressure on wild stocks but provide less fish product to the market than wild fish.
High density fish farming in Ohio

Escaped Farmed Fish = Invasive Species The National Research Council has ranked invasive species among the most serious threats to native marine biodiversity (9). In the United States and abroad, aquaculture has become a leading vector of aquatic invasive species worldwide introducing unwanted seaweeds, fish, invertebrates, parasites, and pathogens. Most major aquatic species cultured in the United States are not native to their farm sites (8) and when fish inevitably escape from their marine (or riparian) pens they become competitors with wild species. For instance, as much as 40% of Atlantic salmon caught in the North Atlantic are of farmed origin. The 1973 introduction of a seaweed species to Hawaii with little regulatory control and the subsequent escape lead to its rapid spread across the state's coral reef, shading essential sunlight and stealing nutrients. Likewise, bighead and silver carps, imported from Asia for confined food culture and biological control in the 1970s, have become established in rivers throughout the Mississippi Basin and compete with native fish. Invasives have been documented numerous times as outcompeting native species. Observations by Einum and Fleming (18) of habitat use and diet suggested that the wild and farmed populations compete for territory and food, and both farmed fish and hybrids expressed higher growth rates than native fish. As the number of farmed fish grows to exceed the number of wild fish, even small fractional rates of escape may result in the local presence of large numbers, and high frequencies, of escaped fish. Some studies disagree, however, saying that while wild fish may be smaller and less aggressive than hatchery fish, they have more local experience and a prior residence advantage. Indeed, some studies on salmon have shown superior competitive and reproductive abilities of wild types over farmed escapees. (16, 17)

On the other hand, transgenic species (hybrids of wild and farmed types) have the potential to be more of a menace, altering the gene pool and showing evidence of increased vigor. (18) Genetically modified (GM) fish, such as the growth-enhanced Atlantic Salmon manufactured by Aqua Bounty Farms Inc. of Waltham, Massachusetts, would almost certainly outcompete native species if accidentally released. Additional policy measures, improved containment methods, the maintenance of robust wild populations and GM fish sterility are all suggests to control the negative impacts of escaped farmed fish.

New parasites are also introduced such as Terebrasabella heterouncinata, a parasitic worm brought to California on South African abalone in the 1980s. It now deforms wild and cultured shells of abalone resulting in reduced market prices and even causing several farm closures. Other major pests transferred through molluscan aquaculture include the Japanese oyster drill, turbellarian flatworm and Asian eelgrass. (8)

Spread of Sea Lice to Wild Fish A growing number of scientific studies are reporting on the movement of sea lice from farmed salmon stocks to wild salmon stocks. Some evidence links salmon farms in Atlantic coastal waters to frequent lice infestations among neighboring wild salmon, and scientists have warned that expanding the aquaculture now present in Pacific waters could create new reservoirs of disease. Researchers at the University of Alberta in Edmonton examined juvenile wild pink and chum salmon at points along migration corridors that pass close to a British Columbia fish farm. Twenty kilometers up a fjord from the farm, few wild salmon had lice, whereas immature lice were common on wild fish passing the farm, and older lice generally predominated on salmon farther along on their journey to the sea. The authors suggest that lice are 73 times as prevalent on salmon near the farm as in distant waters and that the farm elevates infestation rates in salmon as far as 75 kilometers beyond its pens. (6) The parasitic sea lice are able to survive up to 22 days off the host making transfer from farmed to wild salmon possible. (7)

Sea lice on young wild salmon found near a fish farm in British Columbia
Risks to the Human Population

Health effects from farmed fish consumption: an understudied topic

Bioaccumulation of toxins in Salmon: Not only are farmed salmon artificially colored, higher in fat and lower on Omega-3 fatty acids than their wild counterparts, but a new report showed elevated levels of PCBs and other toxins in farmed stocks. Salmon are relatively fatty carnivorous fish that feed high in the food web, and thus bioaccumulate contaminants. Scientists analyzed over 700 farmed and wild salmon (over 2 metric tons) from around the world for organochlorine contaminants and compared concentrations among farmed fish stocks and between farmed and wild stocks. Analyses revealed that thirteen of the fourteen contaminant groups tested were significantly more concentrated in the farmed salmon as a group than in the wild salmon, and concentrations in farmed salmon from North America were significantly higher than those in wild salmon for all 14 contaminants. The large differences between the farmed and wild salmon contaminant concentrations are most likely a function of diet. The researchers also found increased levels of toxins in fish meal they purchased that is made largely from small pelagic fishes. (11)

The human health effects of exposure to PCBs, toxaphene, and dieldrin in salmon tissues are a function of contaminant toxicity, concentration in fish tissues, and fish consumption rates. Individual contaminant concentrations in farmed and wild salmon do not exceed U.S. Food and Drug Administration (FDA) action or tolerance levels for PCBs and dieldrin. However, FDA action and tolerance levels do not address the health risks of concurrent exposure to more than one contaminant, and do not provide guidance for acceptable levels of toxaphene and dioxins in fish tissue. The U.S. Environmental Protection Agency takes a more stringent approach, designed to manage health risks by providing risk-based consumption advice regarding contaminated fish (for example, one should limit consumption of a particular species to a specified number of meals per month or week). The amounts of some contaminants in the farmed fish in the above study fall within acceptable levels by FDA standards, but surpass the guidelines set by the EPA.

This study also demonstrates the importance of labeling seafood as either farmed or fished and identifying the country of origin, particularly as pregnant women and children are at higher risk for contaminant-related health problems from farmed fish. This labeling is not currently required in the US except voluntarily. However, a recent study undertaken by a New York Times reporter showed that six out of eight salmon advertised as wild in New York fish markets were actually farmed. (15)

Concentrations (in ng/g wet weight, except dioxins) of 14 contaminants found in farm-raised (red bars) and wild (green bars) salmon. The vertical lines represent the 10th, 50th, and 90th percentiles, and the boxes represent the 25th to 75th percentiles. Dioxins are in pg of World Health Organization toxic equivalents (TEQ) per g of wet weight and include polychlorinated dibenzo-p-dioxins and dibenzofurans and dioxin-like PCBs. Typically 75% of the total TEQ was due to the dioxin-like PCBs. Other abbreviations are as follows: Tot DDT, the p,p' and o,p' isomers of DDT, DDD, and DDE; Nona, nonachlor; Chlor, chlordane; Hep Epox, heptachlor epoxide.

Jobs and the Economy If a state chooses to allow aquaculture, it may be able to increase job availability, but it may also be displacing traditional commercial fishermen that do not desire to leave their occupation. Some states have opted to ban certain types of farming, such as Alaska's ban on salmon farming. While Alaska reduces its risk of the harmful environmental impacts of aquaculture, this decision has negative economic impacts. Alaska's salmon market share has declined from more than 50 percent in 1980 to about 15 percent today. This decline has occurred despite the fact that Alaskan production has remained stable; in fact, the total amount of salmon produced by fishing and farming worldwide is now more than double what it was a decade ago. The sharp increase in the production of fresh, farm salmon has rearranged the economic and political landscape of the fishing industry. While commercial landings have remained constant, salmon prices have declined dramatically. Low prices, while good for consumers, hurt the financial prospects for both farmers and fishermen. However, low prices have thus far had a greater impact on the fishing industry. (10)