Aquaculture is the farming of aquatic organisms in both coastal and inland areas involving interventions in the rearing process to enhance production. It is one of the fastest growing food-producing sector and now accounts fo 50 % of the world’s fish that is used for food.
The intensity and type of environmental impacts of aquaculture are dependant upon the species farmed, the intensity of production and on the farm location.
Fish culture is usually an intensive industry that involves an addition of solids and nutrients to the marine environment, and is recognised as potentially causing environmental degradation through these inputs. 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.
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.
Additional concern is that escapes from fish farms—particularly where farmed species are non-native—may compete with fish from wild populations for food, potentially displacing wild fish.
In contrast, shellfish farming is generally considered to cause less environmental damage. Shellfish aquaculture operations actually improve water quality by filtering out pollutants, sediments, and nutrients from the water column. Filter-feeding bivalve shellfish – oysters, mussels, clams and scallops – are successfully farmed across the world as a sustainable food source while also enhancing the marine environment.
It is estimated that an adult shellfish can filter up to more than 150 l of water a day removing suspended solids from surrounding waters, thereby increasing water clarity allowing sunlight to penetrate for enabling sea grass growth and the foundations of the marine food chain to flourish. Beds of bivalve shellfish provide ecosystem services by naturally filtering silt and also removing bacteria, viruses and nutrients from the water.
Scientists calculate that bacteria in sediment around bivalve beds biologically remove at least 20 percent of the nitrogen in wastes through the same process used in modern wastewater treatment plants. While most of the nutrients filtered from the water by shellfish are recycled back into the water column, the flux of undigested plant matter into the sediments stimulates bacterial processes known as denitrification. This process of turning fertilizer ammonia into nitrate and then into harmless nitrogen gas allows its escape into the atmosphere instead of stimulating phytoplankton blooms.
Ultimately, sustainable shellfish aquaculture can provide greater global food security and when farmed locally, mitigate the carbon transportation footprint.