Mariculture has rapidly expanded over the last two decades due to new technology, improvements in formulated feeds, greater biological understanding of farmed species, increased water quality within closed farm systems, greater demand for
seafood products, site expansion and government interest. As a consequence, mariculture has been subject to some controversy regarding its social and
environmental impacts. Commonly identified environmental impacts from marine farms are: • Wastes from cage cultures • Farm escapees and
invasives •
Genetic pollution and disease and parasite transfer •
Habitat modification As with most farming practices, the degree of environmental impact depends on the size of the farm, the cultured species, stock density, type of feed,
hydrography of the site, and
husbandry methods. The adjacent diagram connects these causes and effects.
Wastes from cage cultures Mariculture of
finfish can require a significant amount of
fishmeal or other high protein food sources. In cage culture, several different methods are used for feeding farmed fish – from simple hand feeding to sophisticated computer-controlled systems with automated food dispensers coupled with
in situ uptake sensors that detect consumption rates. In coastal fish farms, overfeeding primarily leads to increased disposition of detritus on the seafloor (potentially smothering seafloor dwelling invertebrates and altering the physical environment), while in hatcheries and land-based farms, excess food goes to waste and can potentially impact the surrounding catchment and local coastal environment. Escapees can adversely impact local ecosystems through
hybridization and loss of genetic diversity in native stocks, increase negative interactions within an ecosystem (such as
predation and
competition), disease transmission and habitat changes (from
trophic cascades and ecosystem shifts to varying sediment regimes and thus
turbidity). The accidental introduction of invasive species is also of concern. Aquaculture is one of the main vectors for invasives following accidental releases of farmed stocks into the wild. One example is the Siberian sturgeon (
Acipenser baerii) which accidentally escaped from a fish farm into the
Gironde Estuary (Southwest France) following a severe storm in December 1999 (5,000 individual fish escaped into the estuary which had never hosted this species before).
Molluscan farming is another example whereby species can be introduced to new environments by ‘hitchhiking’ on farmed molluscs. Also, farmed molluscs themselves can become dominate predators and/or competitors, as well as potentially spread pathogens and parasites. Also, non-indigenous species which are farmed may have resistance to, or carry, particular diseases (which they picked up in their native habitats) which could be spread through wild populations if they escape into those wild populations. Such ‘new’ diseases would be devastating for those wild populations because they would have no immunity to them.
Habitat modification With the exception of
benthic habitats directly beneath marine farms, most mariculture causes minimal destruction to habitats. However, the destruction of
mangrove forests from the farming of shrimps is of concern. Furthermore, they act as buffering systems whereby they reduce coastal erosion, and improve water quality for in situ animals by processing material and ‘filtering’ sediments.
Others In addition,
nitrogen and
phosphorus compounds from food and waste may lead to blooms of
phytoplankton, whose subsequent degradation can drastically reduce
oxygen levels. If the
algae are toxic,
fish are killed and
shellfish contaminated. These algal blooms are sometimes referred to as harmful algal blooms, which are caused by a high influx of nutrients, such as nitrogen and phosphorus, into the water due to run-off from land based human operations. Over the course of rearing various species, the sediment on bottom of the specific body of water becomes highly metallic with influx of copper, zinc and lead that is being introduced to the area. This influx of these heavy metals is likely due to the buildup of fish waste, uneaten fish feed, and the paint that comes off the boats and floats that are used in the mariculture operations. ==Sustainability==