Generally it is aquaculture in marine environments. If one takes a closer look there are different definitions. Some limit mariculture to the raising of marine plants and animals in the ocean itself (EEA, 2008). Others also include species from brackish water and include culture methods that take place in salty and brackish water that is not situated in the ocean (CBD, 2004, Wecker, 2006) . Here this wider definition is referred to. Mariculture can be distinguished from capture fisheries by two criteria: ownership of the stock and deliberate intervention in the production cycle (husbandry). (Naylor et al., 2000)
Mariculture includes a wide range of species and culture methods. It is growing fast on a global scale (CBD, 2004). This is due to the fact that many fish stocks are overfished and catches are declining (Neori et al., 2004, Wecker, 2006). At the same time the world population is rising and with it the need for dietary protein (Wecker, 2006). The expansion of mariculture can reduce pressure on wild fish, shrimps and molluscs, because they reduce their market price and by this the investments in fishing fleets, or they can increase the pressure due to the use of fishmeal in feed for some mariculture-species (Naylor et al., 2000)
Some forms of mariculture provide good quality food and the production is more efficient than that of terrestrial animals (roughly half the level of feed input per unit output is necessary) (CBD, 2004).
Due to freshwater scarcity in many areas of the world mariculture is expected to be the future of aquaculture (Wecker, 2006). These are the main species that are used in mariculture (Tab. 1):
Products obtained from mariculture are not only used directly as food items, but also as inputs for e.g. cosmetics, neutraceuticals, medicines, food additives and many more.
But mariculture can also have some disadvantages: there are several environmental problems that are associated with it. Their extend depends on species, culture method, stocking density, feed type, husbandry practice, hydrodynamic site conditions and the sensitivity of the receiving ecosystem. (Troell et al., 1999, Wu, 1995)
But these problems can be mitigated to a huge extend. And it should not be forgotten, that in the long run mariculture itself is dependent on a good quality of its environment.
Mariculture can play an important role, especially in rural areas for food security, economic and social welfare. In heavily populated coastal areas mariculture is in competition with other human activities for space and other resources. These other activities can for example be: fisheries, tourism, harbour operations, nature conservation and industry. Integrated Coastal Zone Management (ICZM) tries to bring these activities in the coastal zone together in a sustainable way. (Wu, 1995, Read and Fernandes, 2003, Wecker, 2006) In the different countries around the world legislation on mariculture and its enforcement vary widely, this is why they are not considered here.
Types of mariculture
There are different possibilities to group the kinds of mariculture. The first one presented here is the subdivision by species. This is done because different species require different systems that have different characteristics and effects. Only the most common systems will be mentioned (CBD, 2004).
Broodstock/seed supply: Bivalve mollusc larvae are either collected from natural grounds using material to which they adhere or produced in hatcheries by artificial fertilization.
Growout: Larvae that have set to their substrate are grown in hanging cultures (suspended from floating rafts or long lines on strings, trays, stacks or mesh bags), vertical or rack culture (sticks or platforms), bottom culture (shells, stones, rocks or cement slabs added to the ground), or in land-based systems (CBD, 2004).
Crustacean Culture (Fig. 1)
Broodstock/seed supply: Until last decade the global industry relied on wild-caught larvae or berried (= egg-carrying) females. Nowadays there is a trend towards hatcheries.
Growout: takes place in earthen ponds, concrete raceways and tanks (CBD, 2004).
Marine Plant Culture (Fig. 2)
Broodstock/seed supply: Cultured aquatic plants have complicates life cycles with several intermediate stages. The major source of broodstock is wild collection. Most culture is now dependent on hatchery production of the early life stages (monospores, zoospores, gametophytes, sporophytes) which are attached to growing media and transferred to marine sites. Other propagation methods involve fragmentation.
Growout: Young plants are cultured by 3 different methods: suspended (longline and raft), bottom cultures at the sea (large rocks or artificial shapes of concrete are placed on the seabed) and inland tank cultures (CBD, 2004).
Finfish Culture (Fig. 3)
Broodstock/seed supply: The broodstock can be domesticated or a mix of domesticated and wild animals. Most species are grown from larvae or fry produced in hatcheries. There spawning is often induced with a hormone application.
Growout: Cage culture can be divided into inshore and offshore cages and can be fixed, floating or submerged. Inshore cages are located in protected, shallow areas with less water circulation. Offshore cages are located in deep water and open areas with less protection from storm but with better water exchange. Nets and fish pen are located in shallow water and their edges are anchored to the bottom. A typical fish pond system consists of following basic components: pond compartments enclosed by dikes, canals for supply and drainage of water and gates or water control structures (CBD, 2004).
Enhancement or Sea Ranching is mostly developed with marine finfish. Both terms refer to the deliberate release of organisms from hatcheries into the natural ecosystem. In enhancement, fry are released to restock wild populations. In sea ranching, fish are harvested from artificially enclosed areas (CBD, 2004).
It is also possible to co-culture different species: this will be further described in the section about mitigation.
Another possibility to group different types of mariculture is depending on the intensity of farming systems (Fig. 4):
Negative environmental impacts
Their occurance and extend depend on husbandry parameters (species, culture method, feed type) and the nature of the receiving environment (physics, chemistry, biology). An additional factor that plays into the state of the receiving ecosystem is the release of waste products from other anthropogenic sources (e.g. effluents from industry or human settlements or agricultural runoff).
Nutrient pollution / Eutrophication
Eutrophication defined as nutrient enrichment (mainly N and P) is considered by some the most important pollution threat to marine waters (Wu, 1999). This problem is often mentioned in the context of intensive culture of fish and shrimp, where a lot of artificial feed are used. There waste consisting of uneaten feed and faeces moves down into the benthos: below fish cages in areas with low currents waste sedimentation leads to a shift in benthic populations towards pollutant-resistant-species. But this effect is mostly confined to within a distance of 50-100m from the mariculture facilities. Another part of the waste products consisting of CO2, dissolved organic carbon and various soluble nutrients (e.g. ammonia and phosphate) move into the water column (CBD, 2004, Troell et al., 1999).
Up to now a huge amount of anthropogenic input of nutrients (not only by mariculture) has caused major changes in structure and functioning of phyto-and zooplankton, benthic and fish communities (Wu, 1999, Troell et al., 1999). Areas with limited water exchange are at especially high risk. The effluents from fish farming have high N/P-ratios, which are considered a likely cause for the development of (non-) toxic algal blooms (Fig. 5).
Algal blooms can shade seafloor vegetation and when they collapse their decay on the seafloor may lead to hypoxia or anoxia and hence mass mortality of benthos and fish (Troell et al., 2003). If the species of algae additionally produce toxic substances there is also a public health risk associated with them, mainly via human consumption of filter-feeding shellfish contaminated with the biotoxins (Wu, 1995). Pretty much the opposite of eutrophication may occur at intensive open ocean bivalve cultures: They take nutrients away from the marine foodweb. If this takes place excessively there is less left for other herbivores and phytoplankton and those that live off them. Apart from that bivalves take suspended particulate matter and change it into denser particles that fall to the bottom. This might have an effect on benthic communities as well (CBD, 2004).
Another group of waste products from mariculture that is often released into the environment are certain chemicals (Tab. 2):
Spreading of parasites and diseases
Due to crowded and stressful conditions in intensive mariculture there are frequently outbreaks of diseases. The pathogens can be moved to previously disease-free regions by transport of hatchery products like shrimp-postlarvae. And later animals with infections or parasites might escape and spread the pathogens to wild stocks (CBD, 2004).
Escapes / Aliens / Biodiversity / Genetics
Alien species resulting from escaped culture stocks have established far from their home range. Some consider this just more biodiversity, while others think that they predate on or compete with native species and could eventually eliminate these (CBD, 2004).
There is also concern that the escaped fish might lead to a decrease in intraspecific genetic variablility via mixing of escaped cultured animals with wild stocks. By this locally adaptive features of populations will interbreed with fish subject to artificial selection. Research fishing in the Faroese ocean area showed that 20-30% of salmon there are escapes from farms. (Read and Fernandes, 2003) In the future genetically modified fish might also become a problem (CBD, 2004).
Farming up and fishing down the food chain / Food security
High value marine carnivorous finfish need animal sources of protein. Most of this comes from marine fish in the form of fish meal. The fish meal is made from small pelagic wild fish e.g. anchoveta and atlantic herring. This practise has two main disadvantages. One is that there is less left for marine predators like seals and seabirds and also commercially valuable predatory fish like cod (CBD, 2004). The other is concerning human food security. Often 2-5 times more fish protein is put into the farmed species than is supplied by the farmed product. In contrast herbivorous filter feeders contribute to food security (Naylor et al., 2000).
Catching broodstock from the wild
This practice poses several threats to the environment: Of course the natural stocks of the target specimen are depleted. This may also lead to problems for species that normally feed on them (e.g. shrimp larvae are a food source for many organisms). But there are also other side effects: bycatch may be very high in some cases and sometimes destructive gear like dredge nets is used (CBD, 2004).
Habitat degradation / modification
In general mariculture can (depending on cultivation method) take a lot of space, which can affect migratory routes and feeding patterns and reproduction of non-target species. One example is the destruction of mangroves to build shrimp ponds there. If these ponds are in operation later their effluents might pose a threat to adjacent mangrove ecosystems. Saltwater intrusion due to active pumping of groundwater into the ponds may lead to additional problems (Páez-Osuna, 2001, CBD, 2004).
Like underwater exploders are sometimes used in mariculture to deter predators from the farmed animals. This can also stress non-target animals (CBD, 2004).
Possibilities for mitigation
Many of the aforementioned possible negative environmental impacts of mariculture can be mitigated. In the following section some measures to do so are presented:
Use of enclosed and recirculating systems for shrimp and finfish
This prevents escapes and aerated settling tanks or other (bio-)filters prevent most particulate nutrients and parts of the dissolved nutrients to enter the natural ecosystems. A disadvantage of them is that they require high initial investment (CBD, 2004).
Integrated (multi trophic) aquaculture
Polyculture is defined by the Convention on Biological Diversity (CBD) as growing two or more species belonging to different trophic levels in the same system (CBD, 2004). Other refer to this as integrated (multi trophic) aquaculture and use the term polyculture only if the aspect of different trophic levels is not included. This form of mariculture is a managed imitation of natural ecoystems: The effluents from intensive fed culture of finfish or shrimps are taken up by bivalves and plants. Marine plants use sunlight and assimilate dissolved inorganic nutrients from the water, while bivalves filter organic suspended particles which can be uneaten feed or phytoplankton from the effluents. The marine plants can be phytoplankton that is then eaten by the bivalves or seaweeds that can be sold (like the bivalves). This has several advantages:
- waste of one species can be converted to products that have an economic value, this means a higher income and the diversification of the mariculture production also reduces the financial risk
- the adverse environmental impacts of intensive culture of carnivores are reduced and sustainability can be reached, as seaweeds do not only counterbalance the nutrient inputs by the fish and shrimp, but also other metabolic aspects such as dissolved oxygen, acidity and CO2-levels
- hardly extra effort is necessary to fulfil the requirements of organic aquaculture, a lucrative market.
Best site selection
An example is to take into account water exchange rates and currents that dilute the waste (CBD, 2004).
Whether dilution is a long-term-solution, is questionable.
Better management to reduce nutrient input effect
This could be realised by setting a threshold for stocking density or a careful selection of farmed species. Additionally the carrying capacity of the ecosystem to process waste products should be taken into account (CBD, 2004). Although it is difficult to assess, especially in coastal zones with cumulative pressure from several anthropogenic activities. Another new idea for intensive fish farming in cages that may help to prevent degradation of benthic habitats is this: fixing cages at only one mooring on a long line so that they can float over a large area (moved by e.g. wind and tidal currents) may help to reduce local amounts of sedimentation (Goudey et al., 2001).
Better feeding management: to reduce waste ...
One example for this is the improvement of feed formulations (reducing N and P in the diets (N is assumed to be the limiting nutrient for phytoplankton growth in marine waters)). Another example is the use of efficient strains of the farmed species. It might also be helpful to raise the awareness of farm workers. In shrimp ponds natural feed items like zooplankton and benthic organisms could be used as a supplement to artificial diets (CBD, 2004).
... and improve food security:
An expansion of farming of fish of low trophic levels and the reduction of fish meal and oils inputs in feed could be helpful (Naylor et al., 2000).
Reducing fish meal in feed and improving feed efficiency are already priorities in the mariculture industry as feed is the largest cost point in many intensive culture systems and the prices of fish meal continue to rise (Naylor et al., 2000).
Production of larvae in mariculture facilities instead of taking them from the wild (CBD, 2004)
Preventing disease outbreaks and transmission and with it the use of pesticides, piscicides and parasiticides and antibiotics
This can be achieved by establishing lower stocking densities and leaving larger distances between individual farms. Probiotics can be used to improve the water quality. Vaccination is available against some important infectious diseases. Improved monitoring and quarantine stations might also show positive effects (CBD, 2004).
Reducing the use of hormones
Alternatives can be proper genetic selection programmes and the use of photoperiod management in the industrial production of salmon (CBD, 2004).
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