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What it mariculture activities?
Mariculture is a specialized branch of aquaculture
involving the cultivation of marine organisms for food and other products in
the open ocean, an enclosed section of the ocean, or in tanks, ponds or
raceways which are filled with seawater. An example of the latter is the
farming of marine fish, including finfish and shellfish like prawns, or oysters
and seaweed in saltwater ponds. Non-food products produced by mariculture
include: fish meal, nutrient agar, jewellery (e.g. cultured pearls), and
cosmetics.
Similar to algae cultivation, shellfish can
be farmed in multiple ways: on ropes, in bags or cages, or directly on (or
within) the intertidal substrate. Shellfish mariculture does not require feed
or fertilizer inputs, nor insecticides or antibiotics, making shellfish
aquaculture (or 'mariculture') a self-supporting system. Shellfish can also be
used in multi-species cultivation techniques, where shellfish can utilize waste
generated by higher trophic level organisms.
Raising marine organisms under controlled
conditions in exposed, high-energy ocean environments beyond significant
coastal influence, is a relatively new approach to mariculture. Open ocean
aquaculture (OOA) uses cages, nets, or long-line arrays that are moored, towed
or float freely. Research and commercial open ocean aquaculture facilities are
in operation or under development in Panama, Australia, Chile, China, France,
Ireland, Italy, Japan, Mexico, and Norway. As of 2004, two commercial open
ocean facilities were operating in U.S. waters, raising Threadfin near Hawaii
and cobia near Puerto Rico. An operation targeting bigeye tuna recently
received final approval. All U.S. commercial facilities are currently sited in
waters under state or territorial jurisdiction. The largest deep water open
ocean farm in the world is raising cobia 12km off the northern coast of Panama
in highly exposed sites.
Enchanced Stocking (also known as sea
ranching) is a Japanese principle based on operant conditioning and the
migratory nature of certain species. The fishermen raise hatchlings in a
closely knitted net in a harbor, sounding an underwater horn before each
feeding. When the fish are old enough they are freed from the net to mature in
the open sea. During spawning season, about 80% of these fish return to their
birthplace. The fishermen sound the horn and then net those fish that respond.
In seawater pond mariculture, fish are raised
in ponds which receive water from the sea. This has the benefit that the
nutrition (e.g. microorganisms) present in the seawater can be used. This is a
great advantage over traditional fish farms (e.g. sweet water farms) for which
the farmers buy feed (which is expensive). Other advantages are that water
purification plants may be planted in the ponds to eliminate the buildup of
nitrogen, from fecal and other contamination. Also, the ponds can be left
unprotected from natural predators, providing another kind of filtering.
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.
Mariculture of finfish can require a
significant amount of fishmeal or other high protein food sources. Originally,
a lot of fishmeal went to waste due to inefficient feeding regimes and poor
digestibility of formulated feeds which resulted in poor feed conversion
ratios.
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. This impact is usually highly local, and depends
significantly on the settling velocity of waste feed and the current velocity
(which varies both spatially and temporally) and depth.
The impact of escapees from aquaculture
operations depends on whether or not there are wild conspecifics or close
relatives in the receiving environment, and whether or not the escapee is
reproductively capable. Several different mitigation/prevention strategies are
currently employed, from the development of infertile triploids to land-based farms
which are completely isolated from any marine 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.
One of the primary concerns with mariculture
is the potential for disease and parasite transfer. Farmed stocks are often
selectively bred to increase disease and parasite resistance, as well as
improving growth rates and quality of products. As a consequence, the genetic
diversity within reared stocks decreases with every generation – meaning they
can potentially reduce the genetic diversity within wild populations if they
escape into those wild populations. Such genetic pollution from escaped
aquaculture stock can reduce the wild population’s ability to adjust to the
changing natural environment. Also, maricultured species can harbour diseases
and parasites (e.g., lice) which can be introduced to wild populations upon
their escape. An example of this is the parasitic sea lice on wild and farmed
Atlantic salmon in Canada. 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.
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 concer. Globally, shrimp farming activity is a small contributor
to the destruction of mangrove forests; however, locally it can be devastating.
Mangrove forests provide rich matrices which support a great deal of
biodiversity – predominately juvenile fish and crustaceans. 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.
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. Mariculture development must
be sustained by basic and applied research and development in major fields such
as nutrition, genetics, system management, product handling, and
socioeconomics. One approach is closed systems that have no direct interaction
with the local environment. However, investment and operational cost are
currently significantly higher than open cages, limiting them to their current
role as hatcheries.
Sustainable mariculture promises economic and
environmental benefits. Economies of scale imply that ranching can produce fish
at lower cost than industrial fishing, leading to better human diets and the
gradual elimination of unsustainable fisheries. Maricultured fish are also
perceived to be of higher quality than fish raised in ponds or tanks, and offer
more diverse choice of species. Consistent supply and quality control has
enabled integration in food market channels
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