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Explore the environmental impacts of aquaculture, including physical structures, waste management, and biodiversity concerns. Learn about oxygen depletion, fecal matter, and organic waste release. Understand the effects on benthic communities and the importance of regulating fish farm sizes.
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A Company in the NIVA-group CONSULTANCY AND RESEARCH IN AQUACULTURE AND THE AQUATIC ENVIRONMENT Environmental impacts of aquaculture
Physical • physical structures, such as cages, pens, moorings and jetties, wharves, and with the waves created by boats. • The nets of the cages, pens and associated moorings changes the environment by preventing causing friction to the water currents and changing the current patterns. • The cages can hamper navigation routes • Associated impacts include debris and discharges associated from fish farms, notably fish bags, fish mortalities, petrol and diesel from outboard motors and even human faeces • Nets allowed to drop below the cages.
Aquaculture impacts • Physical • Physical structures, cages, pens, jetties • Visual • Hazzard to navigation • Net friction to exchange • Use of wetlands and mangroves for pond construction • Saline infiltration
Freshwater pond Saltwater infiltration Marine pond
Aquaculture impacts Impacts of aquaculture can be put into 3 categories • Physical • Physical structures, cages, pens, jetties • Visual • Net friction to exchange • Use of wetlands and mangroves for pond construction • Chemical • Oxygen depletion • Eutrophication • Antifoulants boats and nets • Medications and treatments • Biological • Faeces • Excretion • Waste food • Genetics and Biodiversity
Biological • Aquaculture produces wastes which may negatively affect the environment. • In intensive aquaculture, a considerable amount of organic wastes are produced in the form of particulate (mainly the uneaten food, faeces) and soluble substances (excreta) which increase biochemical oxygen demand, nitrates and phosphates in receiving waters.
Oxygen Depletion Oxygen is utilized in the vicinity of fish cages by • the consumption of oxygen by the fish • the consumption of oxygen by the release of organic compounds that decompose in the water column by chemical processes that use oxygen (Biological Oxygen Demand, BOD). • The consumption of oxygen by the primary production (algae) and secondary production (zooplankton) that are utilizing the additional nutrients released by aquaculture.
Oxygen Depletion • The absolute oxygen concentration at which the fish can effectively extract oxygen from the water is what matters most. Fish cannot extract oxygen efficiently below 40% saturation. Generally oxygen depletion is a localized phenomenon that affects mainly the farm itself, and seldom extends very far away from the farm site. • However the algae that is utilizing the released nutrients can bloom providing oxygen during the day but utilsing oxygen during the night competing with the fish. If it is a strong bloom, it can extract all the oxygen leading to an algal bloom collapse and fish kill.
Faecal Matter Fish faeces constitute a major and unavoidable form of nutrient enrichment affecting the environment. Modern fish feeds are produced in such a way as to minimize the loss of nutrients (extruded pellets) and by providing them in forms that can be easily assimilated by the fish (largely protein and good digestibility). The nutrients that are not assimilated are excreted mainly as soluble wastes such as urea and ammonia, so the fecal matter consists mostly of carbon and inert material.
Particulate material • Fish farming in net cages affects the environment by releasing large amounts of organic waste from uneaten food and faeces. • The amount of organic waste from cage farms can be roughly estimated as 1,000 kg per tonne of fish produced. • Inputs at this level can have a marked impact on benthic communities resulting in successional changes in response to increasing inputs. • There may be some consumption of particulate matter by wild fishes attracted by aquaculture facilities.
Particulate organic wastes • Severe effects are generally confined to the local area (a few hundred meters at most) and the total area of seabed used for this purpose is insignificant in terms of the total coastal resource. • The extent and severity of effects depend on: • The size and the production capacity of the farm • The depth of the farming site • The type of sediment beneath the cages • On cessation of farming, recovery may take several years. • The main method of regulating and controlling the size of fish farms such that the local environment is not overwhelmed.
Effects on the Benthos • Aquaculture impacts the structure of the benthic communities. • The number of macrobenthic species below fish cages decreases and the community becomes dominated by a few opportunistic species. • In extreme situations the seabed becomes azoic. • However this reduction in benthic diversity normally does not exceed 25m from the cage perimeter, followed by an increase in species richness and diversity in the transitory zone. • In case of coarse sediments the effect on sediment communities is considerably lower.
Benthic Loading • Benthic impacts are easy to identify and can be clearly associated with fish farms, especially when unconsumed feed or fecal pellets are present. • The first stage is the initial deposition of fish farm wastes on the bottom. • The second stage, is much more complicated and uncertain as to the fate of carbon and other nutrients after the initial deposition on the bottom, and are influenced by physical transport and biological degradation and assimilation by other organisms and removal.
Benthic Loading • The initial effect of nutrient enrichment is almost always an increase in benthic productivity, since there is more food to feed the benthic community. • This situation can persist for a long time if the degree of enrichment is low, but the amount of deposition under fish farms is usually so high that benthic scavengers cannot process all of it, and some begins to decompose through bacterial processes. • This leads to reduced oxygen levels and increases in sulfide concentrations, which are too stressful for many bottom dwellers and therefore many species are driven out and the species diversity falls.
Benthic Loading • At extreme levels, only a few species can persist, notably polychætes of the genus Capitella known as indicator species. • If the carbon loading is excessive then the capitellids die out and eventually we find just bacterial mats; the seabed becomes azooic and soon after totally anoxic.
Effects on Sediments • The sediments under the cages is characterised by • low values of redox potential, • high content of organic material • accumulation of nitrogenous and phosphorous compounds. • Occasionally this sediment layer is covered by Beggiatoa-type mats, i.e. white-coloured aggregations of bacteria living at the oxic-anoxic interfaces that may release gas bubbles of H2S or CH4.
Effects on Sediments • Sediment profiles taken at various distances from the edge of the cages have shown that the thickness of the farm sediment varies considerably with season and shows more obvious thichness fluctuations than the surface concentrations of chemical variables. • The effects on sediment geochemistry are much more severe in silty or muddy bottoms than on coarse sediments.
Biological • Sediments and nutrients may not necessarily be a problem as natural breakdown processes or dilution in the receiving waters can assimilate this, provided that natural waters are not overloaded. • The increased fertility of oligotrophic waters may even bring positive effects on the local ecosystem, enriching food availability for wild species. • The risk of negative impacts of aquaculture wastes are greatest in enclosed waters with poor water exchange rates, where excessive production from aquaculture can lead to eutrophication and other ecosystem changes (e.g. algal blooms and low dissolved oxygen levels).
Soluble Effluents • Soluble compounds, such as ammonia and urea are a large part of the wastes released by fish farms. However, unlike fecal matter which accumulates in the immediate vicinity of the cages, they can be flushed by movement of the water and thus tend to approach an equilibrium level where the rate of release into the water column is balanced by flushing. • Dissolved nutrients are not inevitably bad for the environment, and, like moderate benthic carbon loading, can even be beneficial.
Soluble Effluents • Phytoplankton require nutrients to grow, and by increasing primary production the release of nutrients can stimulate the total productivity of the system, including desirable commercially important species such as oysters and mussels. • Use of extractive species to extract nutrients from the water column • Mussels • Oysters • Seaweed
Dissolved nutrients In general the total amounts of N and P loading are linked with aquaculture intensity and with feed conversion factors. In Norwegian and Scottish coastal waters, around 55 percent and 17 percent, respectively, of all coastal phosphorus discharge was attributable to mariculture. These discharges also contribute to the overall load from inland and coastal environments in some locations, together with discharges from agriculture, forestry, industry and domestic waste.
Flushing rates • Bad flushing • Good flushing
Waste Feed A significant amount of feed does not get consumed by farmed fish and goes into the environment, which can have significant impacts. Some of the feed • is in the form of dust that is too small to be ingested by the fish • gets lost through over feeding of the fish • feed pellets are the wrong size for the fish. • Excess pellets fall through the pen and can be found on the bottom. These may be consumed by wild fish, consumed by benthic organisms or breakdown into nutrients by benthic assimilation.
Waste Feed • Fine particles from the break-up of feed pellets settle very slowly and are transported by currents in much the same way as soluble nutrients, and can, among other effects, contribute to Biological Oxygen Demand (BOD). • Larger particles that fall to the bottom enrich the sediments with carbon and other nutrients that can lead to substantial changes in the benthic community.
Waste Feed • The amount of wasted feed is decreasing, due to strong economic pressure to reduce wastage as much as possible since feed is usually the greatest expense in fish farming. • There has been a shift from trash fish to moist feed and from moist feed, which tends to break up in the water with consequent high loss rates, to the more efficient dry feeds. • Attention should be paid to paid to optimal feeding schedules, both in terms of • the amount of feed provided and when it is given • careful choice of the type and size of feed pellets used for fish at different stages of growth.
Turbidity • One impact is to make water more turbid because of the release of particulate matter. This is probably not a serious concern in coastal environments, where natural ambient turbidity is usually much higher than any increment from fish farms, but in clear water environments like lakes, and in isolated coastal bays, the effect may be significant. • The effect of increased turbidity is lower light penetration, which can reduce primary production by phytoplankton and by benthic macrophytes, and possibly could reduce the feeding efficiency of visual predators.
Disease Transmission – farmed to wild • There are impacts from the transmission of disease to wild stocks. • The high density of fish in cages provides breeding grounds for disease, and higher susceptibility to disease • Wild populations can receive a high level of exposure to diseases since they can swim very close to farm sites and often feed close by, but their susceptibility is much lower. • Little is known about the transmission of disease from farmed stocks to wild stocks, but the transmission of sealice from farmed salmon to wild salmon has been documented.
Disease Transmission – farm to farm • There are impacts from the transmission of disease to from one farm to another. • If one farm has a disease, this can be spread to another farm by transfer of the disease organism • Bacteria by currents • Sealice by swimming • Insurance companies will not insure farms that are too close to each other as the risk of disease transfer is high
Pharmaceuticals • There are impacts from the release of medication and pharmaceuticals that are used to treat disease • The effects of antibiotics and other pharmaceuticals on the environment is difficult to quantify. • Little is known about how the various chemicals used in fish farming affect wild populations that it is almost impossible to estimate the secondary effects, i.e., how they actually affect the environment.
Pharmaceuticals • Antibiotics • Vaccines • Treatments for fish disease • Baths • Oral • Copper sulphate for control of algae and fungus • Methylene blue bath for treating fungus • Formalin bath for treating parasites • Potassium permanganate bath for treating bacteria • Disinfectants for eggs
Chemicals • Detergents for cleaning • Disinfectants (floors, tanks) • Chlorine and Sodium thiosuphite for water sterilisation • Colouring in feeds (prawns and salmon) • Lime for treatment of ponds • Fertilisers for ponds • Tea seed cake for killing wild fish in a pond
Operational waste • Feed bags • Daily mortality of fish • Plastic sacs from growing algae • Kitchen paper from cleaning hatchery tank skimmers
Post harvest • Blood water from cutting gills • Fish rinse water • Waste fish from processing (Guts, head, skin, scales. Etc)
Genetic Mixing • Most farmed species, especially those of finfish, are genetically different from the native species, and there is concern about genetic contamination of the wild species • The viability of wild populations may be threatened by interbreeding with domesticated strains. • Genetic mixing can occur from • escape of farmed species into the wild (storm damage, holes in nets). • Use of wild caught fry from a different area • Use of broodstocks collected from a different area • Release of sperm or eggs from mature fish in cages • Purchase of fry from other countries
Biodiversity Aquaculture can affect local biodiversity in many ways. Wild caught fry is still common for some particular marine species. Repeated fishing for the juveniles of certain species can drastically alter species composition by preventing some of them from reproducing. • the escape of alien species such as salmon and tilapia can have deleterious effects on biodiversity. Tilapia are highly invasive and exist under feral conditions in every region in which they have been cultured or introduced. • If alien species allowed to escape, they can establish spawning populations in the country of introduction and dislodge native species from established food niches or worse become a pest. • A precautionary approach needs to be adopted with regard to the use of alien species for aquaculture purposes, particularly regarding biodiversity conservation.
