Germany, and Denmark in the Wadden Sea. For clams,
about 30 different species are collected worldwide, but
two species, the surf clam Spissula solidissima and the
ocean quahog Arctica islandica, make up a large proportion of the clam landings; these are characteristic of offshore benthic marine habitats rather than estuaries. Clam
species found in USA estuaries include Mercenaria
mercenaria and Mya arenaria, and in Europe, one of the
dominant bivalves is the cockle, Cerastoderma edule,
found from mid-tide to low-tide level in sandy bottom
estuaries. Characterized by irregular recruitment, thought
to be a result of weather conditions, successful spatfall
occurs approximately once every 6 years. Scallops are
a high-value seafood, and natural populations have
fluctuated widely in estuarine regions as a result of
overharvesting, contaminants, and toxic algal blooms. At
present there appears to be low production values for
scallops in estuaries, contrasting with the successful
production of offshore species.
Aquaculture
Aquaculture depends on a ready supply of seed which can
be collected from the field or from brood stock held in
a hatchery. While the mussel industry depends mainly on
wild spat, clam and oyster aquaculture depends on
hatchery-produced seed. There may be problems with diseases in hatcheries, but this may be lessened once the
shellfish are in the field. During the grow-out phase trays,
ropes, platforms, and rearing nets must be cleaned of fouling organisms. Mussel culture systems include bottom
culture, poles, and rafts and suspended culture systems
using buoys as undertaken in New Zealand and Chile. In
Spain, Mytilus galloprovincialis is cultured on wooden
rafts in the rias of Galicia, where temperatures range
between 10 C and 20
C and the salinity is about 34 psu
(Gosling, 2003). Mussels reach harvestable size of
8–10 cm in about 15 months; growth is slowest in summer
probably because of a restricted food supply due to stratification of the water column. Mussels grown in Europe are
usually depurated before sale.
Oyster culture has expanded worldwide, with the
Pacific oyster Crassostrea gigas contributing most to the
4.6 million tons cultured annually. China, Japan, Korea,
and France are major producers. Seed supply is normally
from natural settlement onto artificial collectors, and
ongoing culture methods include bottom, rack, and hanging rope culture. Bottom culture involves sticks, mesh
bags, or ground culture, and after a pre-growing period,
the spat are scraped off, graded, and put back onto the
bottom for periods of up to 2 years (Gosling, 2003).
For all types of culture, the growing time to harvest ranges
between 2 and 4 years depending on local growth
conditions.
Clams total more than three million tons a year, including the Japanese carpet shell or Manila clam Ruditapes
philippinarum (which has been introduced to France, the
UK, and Ireland), hard clam Mercenaria mercenaria,
and blood cockle Anadara granosa. Spat are mostly supplied from hatcheries and once they reach 10 mm in length
are seeded into substrate where mesh is laid over the bed to
preclude crab and bird predation. The shellfish are usually
harvested when the shell length exceeds 20 mm, either by
hand or mechanical harvester (Gosling, 2003).
Scallops grown in estuaries provide options for aquaculture in many countries, through a combination of
natural seeding, collectors, or hatchery-produced spat
and ongrowing on the seafloor on rafts or in nets
(Shumway and Parsons, 2006). Scallop production in
estuaries over the last 10 years has been declining worldwide with mortalities and population fluctuations, especially in short-lived species. The Atlantic bay scallop
Argopecten irradians and Yesso scallop Mizuhopecten
yessoensis are cultured in a number of countries, and there
is increased interest in polyculture with seaweeds.
Shrimp and crustacean production
The whiteleg or Pacific white shrimp Litopenaeus
vannamei is the most productive of the internationally
introduced marine crustaceans. Native to Mexico, this
shrimp lives in the ocean down to 72 m, but juveniles
occur in estuaries where temperatures exceed 20
C all
year. Juveniles are captured in estuaries or cultured in
ponds, where they reach 30–35 g after about 7 months.
There are issues associated with the expansion of the
shrimp industry – the removal of mangroves to
construct ponds, pollution of coastal waters by pond
effluent, salinization of groundwater, and agricultural land
(Roth et al., 2008). Brood stock are sea caught, and one
eye is ablated, resulting in repeated maturation and
spawning. The hatcheries range from small backyard
hatcheries to large environmentally controlled purposebuilt buildings. Using natural productivity of the ponds,
this species costs less to produce than the more carnivorous Penaeus monodon where similar culture techniques
were used previously. In 2010 the catch was greater than
2.8 million tons. The main producers in 2004 were China
(700,000 t), Thailand (400,000 t), Indonesia (300,000 t),
and Vietnam (50,000 t). The freshwater prawn
Macrobrachium is targeted for ongrowing in some countries such as Bangladesh because prawns grown from wild
larvae are considered to be of high quality. Juveniles along
the Gulf coast recruit to sea grass and mangrove estuaries
where they grow in the rich productive estuarine waters
providing a refuge from predation.
Of the total catch from both wild capture and farms,
crabs comprise only a small proportion (20 %) of the
annual crustacean catch, and although the catch statistics
are dominated by the Japanese blue crab, the blue crab
Callinectes sapidus, the Dungeness crab Metacarcinus
magister, and the mud crab Scylla serrate each contributes
20,000 t annually. Although these landings might appear
low, they are important because of their high monetary
value compared with bivalves. Chesapeake Bay in the
USA once had a thriving industry for hard shell and soft
SHELLFISH PRODUCTION
575
about 30 different species are collected worldwide, but
two species, the surf clam Spissula solidissima and the
ocean quahog Arctica islandica, make up a large proportion of the clam landings; these are characteristic of offshore benthic marine habitats rather than estuaries. Clam
species found in USA estuaries include Mercenaria
mercenaria and Mya arenaria, and in Europe, one of the
dominant bivalves is the cockle, Cerastoderma edule,
found from mid-tide to low-tide level in sandy bottom
estuaries. Characterized by irregular recruitment, thought
to be a result of weather conditions, successful spatfall
occurs approximately once every 6 years. Scallops are
a high-value seafood, and natural populations have
fluctuated widely in estuarine regions as a result of
overharvesting, contaminants, and toxic algal blooms. At
present there appears to be low production values for
scallops in estuaries, contrasting with the successful
production of offshore species.
Aquaculture
Aquaculture depends on a ready supply of seed which can
be collected from the field or from brood stock held in
a hatchery. While the mussel industry depends mainly on
wild spat, clam and oyster aquaculture depends on
hatchery-produced seed. There may be problems with diseases in hatcheries, but this may be lessened once the
shellfish are in the field. During the grow-out phase trays,
ropes, platforms, and rearing nets must be cleaned of fouling organisms. Mussel culture systems include bottom
culture, poles, and rafts and suspended culture systems
using buoys as undertaken in New Zealand and Chile. In
Spain, Mytilus galloprovincialis is cultured on wooden
rafts in the rias of Galicia, where temperatures range
between 10 C and 20
C and the salinity is about 34 psu
(Gosling, 2003). Mussels reach harvestable size of
8–10 cm in about 15 months; growth is slowest in summer
probably because of a restricted food supply due to stratification of the water column. Mussels grown in Europe are
usually depurated before sale.
Oyster culture has expanded worldwide, with the
Pacific oyster Crassostrea gigas contributing most to the
4.6 million tons cultured annually. China, Japan, Korea,
and France are major producers. Seed supply is normally
from natural settlement onto artificial collectors, and
ongoing culture methods include bottom, rack, and hanging rope culture. Bottom culture involves sticks, mesh
bags, or ground culture, and after a pre-growing period,
the spat are scraped off, graded, and put back onto the
bottom for periods of up to 2 years (Gosling, 2003).
For all types of culture, the growing time to harvest ranges
between 2 and 4 years depending on local growth
conditions.
Clams total more than three million tons a year, including the Japanese carpet shell or Manila clam Ruditapes
philippinarum (which has been introduced to France, the
UK, and Ireland), hard clam Mercenaria mercenaria,
and blood cockle Anadara granosa. Spat are mostly supplied from hatcheries and once they reach 10 mm in length
are seeded into substrate where mesh is laid over the bed to
preclude crab and bird predation. The shellfish are usually
harvested when the shell length exceeds 20 mm, either by
hand or mechanical harvester (Gosling, 2003).
Scallops grown in estuaries provide options for aquaculture in many countries, through a combination of
natural seeding, collectors, or hatchery-produced spat
and ongrowing on the seafloor on rafts or in nets
(Shumway and Parsons, 2006). Scallop production in
estuaries over the last 10 years has been declining worldwide with mortalities and population fluctuations, especially in short-lived species. The Atlantic bay scallop
Argopecten irradians and Yesso scallop Mizuhopecten
yessoensis are cultured in a number of countries, and there
is increased interest in polyculture with seaweeds.
Shrimp and crustacean production
The whiteleg or Pacific white shrimp Litopenaeus
vannamei is the most productive of the internationally
introduced marine crustaceans. Native to Mexico, this
shrimp lives in the ocean down to 72 m, but juveniles
occur in estuaries where temperatures exceed 20
C all
year. Juveniles are captured in estuaries or cultured in
ponds, where they reach 30–35 g after about 7 months.
There are issues associated with the expansion of the
shrimp industry – the removal of mangroves to
construct ponds, pollution of coastal waters by pond
effluent, salinization of groundwater, and agricultural land
(Roth et al., 2008). Brood stock are sea caught, and one
eye is ablated, resulting in repeated maturation and
spawning. The hatcheries range from small backyard
hatcheries to large environmentally controlled purposebuilt buildings. Using natural productivity of the ponds,
this species costs less to produce than the more carnivorous Penaeus monodon where similar culture techniques
were used previously. In 2010 the catch was greater than
2.8 million tons. The main producers in 2004 were China
(700,000 t), Thailand (400,000 t), Indonesia (300,000 t),
and Vietnam (50,000 t). The freshwater prawn
Macrobrachium is targeted for ongrowing in some countries such as Bangladesh because prawns grown from wild
larvae are considered to be of high quality. Juveniles along
the Gulf coast recruit to sea grass and mangrove estuaries
where they grow in the rich productive estuarine waters
providing a refuge from predation.
Of the total catch from both wild capture and farms,
crabs comprise only a small proportion (20 %) of the
annual crustacean catch, and although the catch statistics
are dominated by the Japanese blue crab, the blue crab
Callinectes sapidus, the Dungeness crab Metacarcinus
magister, and the mud crab Scylla serrate each contributes
20,000 t annually. Although these landings might appear
low, they are important because of their high monetary
value compared with bivalves. Chesapeake Bay in the
USA once had a thriving industry for hard shell and soft
SHELLFISH PRODUCTION
575