Mariculture

Definition

Mariculture is often defined as aquaculture in marine environments. Some limit mariculture to culture 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).

Introduction

Mariculture includes a wide range of species and culture methods. It is globally a fast-growing activity (CBD, 2004). This is due to the fact that many wild 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. However, they can also 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 it is expected that mariculture will become the dominant form of aquaculture (Wecker, 2006). The main species that are used in mariculture are indicated in Tab. 1.

Table 1: Top 10 mariculture species in 2000 (CBD, 2004); m = marine, b = brackish.

Products obtained from mariculture are not only used for food, but also as raw material for e.g. cosmetics, neutraceuticals, medicines, food additives and many more.

Several environmental problems are associated with mariculture. These problems depend 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). Many of these problems can be mitigated with appropriate measures. Farmers are in general aware that mariculture itself in the long run depends on good quality of the environment.

Mariculture can play an important role, especially in rural areas, for food security, economic and social welfare. In densely 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). Legislation on mariculture and its enforcement vary widely in different countries around the world. No further consideration is given here to this topic.

Types of mariculture

Figure 1: Shrimp ponds in Ecuador (www.fishfarming.com 2008).

There are different possibilities to group different 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 are mentioned (CBD, 2004).

Mollusc Culture

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).

Figure 2: Traditional Nori Growing (www.seaweed.ie 2008).

Crustacean Culture (Fig. 1)

Broodstock/seed supply: In the last century the global industry relied mainly 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).

Figure 3: Salmon farm (www.dfo-mpo.gc.ca 2008).

Marine Plant Culture (Fig. 2)

This includes macro- and microalgae as well as seagrasses.

Broodstock/seed supply: Cultured aquatic plants have complicated 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. Spawning is often accelerated 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 the following basic components: pond compartments enclosed by dikes, canals for supply and drainage of water and gates or water control structures (CBD, 2004).

Figure 4: Main differences between extensive, semi-intensive and intensive mariculture systems in terms of resource use and potential environmental risk (Tacon and Foster, 2003).

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

Environmental impacts depend on husbandry parameters (species, culture method, feed type) and the nature of the receiving environment (physical, chemical, biological characteristics). The state of the receiving ecosystem also depends on the release of waste products from other anthropogenic sources (e.g. effluents from industry or human settlements or agricultural runoff).

Nutrient pollution / Eutrophication

Figure 5: Algal bloom .

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. Waste consists of uneaten feed and faeces moving 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. This effect is mostly limited to a distance of 50-100 m from the mariculture facilities. Another part of the waste products consists of CO₂ , dissolved organic carbon and various soluble nutrients (e.g. ammonia and phosphate) that is dispersed into the water column (CBD, 2004; Troell et al., 1999).

To date, anthropogenic input of nutrients (not only by mariculture) have caused major changes in structure and functioning of phyto- and zooplankton, benthic and fish communities (Wu, 1999; Troell et al., 1999). For example, observations over a two-decade period show that long-term exposure to aquaculture effluents with high nutrient concentrations are a serious threat to coastal ecosystems along the whole Chinese coast, and in particular to seagrass meadows, which have largely disappeared (Thomsen et al., 2020). Areas with limited water exchange are at even greater risk. The effluents from fish farming have high N/P-ratios, which are considered a likely cause for the development of toxic algal blooms (Fig. 5), see also Harmful algal bloom.

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 produce toxic substances there is also a public health risk associated with them, mainly via human consumption of filter-feeding shellfish contaminated with biotoxins (Wu, 1995). The opposite of eutrophication may occur at intensive open ocean bivalve cultures: They take nutrients away from the marine foodweb. Excessive nutrient depletion limits the growth of other herbivores and phytoplankton and those that live off them. Apart from that, bivalves filter suspended particulate matter and change it into denser particles that fall to the bottom (faecal pellets). This can have an effect on benthic communities as well (CBD, 2004).

Polluting Chemicals

Another group of waste products from mariculture that is often released into the environment are certain chemicals, see Tab. 2.

Table 2: Chemicals used in mariculture practice that can become pollutants, their sources/uses and impact (CBD, 2004).

Spreading of parasites and diseases

Due to crowded and stressful conditions in intensive mariculture there are frequent outbreaks of diseases. The pathogens can be dispersed to previously disease-free regions by transport of hatchery products like shrimp-postlarvae. When animals with infections or parasites escape, the pathogens can be spread to wild stocks (CBD, 2004).

Escapes / Aliens / Biodiversity / Genetics

Non-native species resulting from escaped culture stocks can establish far from their home range. In some cases this may enrich biodiversity, but often they predate on or compete with native species and could eventually eliminate these (CBD, 2004). For example, surveys show the dispersal of alien invasive species from shellfish culture areas rafting on floating litter in the Venetian lagoon and the Portuguese Algarve region, including the notorious nuisance species H. sanctaecrucis (Rech et al., 2018). Fishery research in the sea around the Faroe islands showed that 20-30% of salmon there are escapees from farms (Read and Fernandes, 2003). It is estimated that escapes of non-native species from finfish farms threaten almost one third of the ocean ecosystems (Atalah and Sanchez-Perez, 2020). See also the article Non-native species invasions.

There is also concern that the escaped fish might lead to a decrease in intraspecific genetic variability via mixing of escaped cultured animals with wild stocks. Adaptive features of local fish populations can be lost by interbreeding with fish that was artificially selected with low genetic diversity (Miralles et al., 2016). Genetically modified fish can also become a problem in the future (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 practice raises two main issues. One is that less food left for marine predators like seals and seabirds and for commercially valuable predatory fish like cod (CBD, 2004). The other concern is human food security. Often 2-5 times more fish protein is put into the farmed species than is supplied by the farmed product. Such a concern does not exist for herbivorous filter feeders, who are net protein producers (Naylor et al., 2000).

Catching broodstock from the wild

This practice entails several threats to the environment. The natural stocks of the target specimen are depleted, leading to problems for species that normally feed on them (e.g. shrimp larvae are a food source for many organisms). 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

Depending on the cultivation method, mariculture can take a lot of space, which can perturb migratory routes, feeding patterns and reproduction of non-target species. One example is the conversion of mangroves into shrimp ponds. Once in operation, the effluents of these ponds pose a threat to adjacent mangrove ecosystems. Saltwater intrusion due to active pumping of groundwater into the ponds can cause additional problems (Páez-Osuna, 2001; CBD, 2004).

Acoustic devices

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 possible measures are discussed.

Use of enclosed recirculating systems for shrimp and finfish

Enclosed systems prevent 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 problem is that they require high initial investments (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). This is also called integrated (multi trophic) aquaculture, whereas the term polyculture is used only if no other trophic levels are included. This form of mariculture is a managed imitation of natural ecosystems. The effluents from intensively fed cultures 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 left-over 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). Integrated aquaculture includes the production of larvae in mariculture facilities instead of taking them from the wild (CBD, 2004).

Integrated aquaculture has many advantages:

• waste of one species can be converted into products that have an economic value, providing a higher income and diversification of the mariculture production while also reducing financial risks;
• the adverse environmental impacts of intensive culture of carnivores are reduced and sustainability can be reached, as seaweeds not only absorb the nutrient release from the fish and shrimp cultures, but also reduce other impacts related to dissolved oxygen, acidity and CO₂ ;
• access to the lucrative market of eco-friendly foods.

This system is considered a good solution by many scientists (Chopin et al., 2001; Neori et al., 2004; CBD, 2004; Troell et al., 2003)

Best site selection

An example is to choose sites with high water exchange rates and currents that dilute the waste (CBD, 2004). However, whether dilution is a long-term-solution, is questionable.

Better management to reduce nutrient input effect

This can be realized by a careful selection of farmed species and setting a limit to stocking density. The carrying capacity of the ecosystem to process waste products should also be taken into account (CBD, 2004). The effectiveness of these measures is difficult to assess, however, especially in coastal zones with cumulative pressure from other anthropogenic activities.

Another mitigation technique consists of 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). This may help to prevent degradation of benthic habitats.

Better feeding management: to reduce waste ...

One example is the improvement of feed composition by reducing N and P in the diets (N is often the limiting nutrient for phytoplankton growth in marine waters). Another example is the use of efficient strains of the farmed species. In shrimp ponds, natural feed items like zooplankton and benthic organisms can be used as a supplement to artificial diets (CBD, 2004). It is important to better inform farm workers and to raise their awareness on these issues.

... and improve food security:

Reducing fish meal in feed and improving feed efficiency are already priorities in the mariculture industry as feed is the largest cost item in many intensive culture systems and the prices of fish meal continue to rise (Naylor et al., 2000). Farming fish of low trophic levels and the reduction of fish meal and oils inputs in feed should be stimulated (Naylor et al., 2000).

Reducing disease outbreaks and transmission and the use of pesticides, piscicides and parasiticides and antibiotics

This can be achieved by establishing lower stocking densities and keeping 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 can 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).

See also

Internal Links

• Impact of fisheries on coastal systems
• Effects of fisheries on European marine biodiversity
• Harmful algal bloom
• ALGADEC - Detection of toxic algae with a semi-automated nucleic acid biosensor
• Non-native species invasions

External links

The State of World Fisheries and Aquaculture, FAO 2014

Assessment of Impacts of Mariculture, OSPAR Commission 2009

References

The main author of this article is Honnens, Hilke Please note that others may also have edited the contents of this article. Citation: Honnens, Hilke (2020): Mariculture. Available from http://www.coastalwiki.org/wiki/Mariculture [accessed on 20-04-2024] For other articles by this author see Category:Articles by Honnens, Hilke For an overview of contributions by this author see Special:Contributions/Hilke