Beyond the intertidal zone, epifaunal species can be
grown on line systems attached to rafts or a variety of surface floats (long-line culture) and may extend many
meters below the surface. Mussels attach directly to the
line systems, with intermediate supports to keep the crop
from sliding off. Oysters and scallops may be attached to
lines but are more typically placed in cages that are then
attached to lines. Fouling control is an important maintenance procedure. Cages maintained on the surface can
simply be inverted to allow the top to dry and the fouling
organisms to die, while submerged cages must be cleaned
or exchanged on a regular basis.
Infaunal species are typically planted in prepared areas
(beds) in the intertidal or very shallow subtidal. Bed preparation may be rudimentary or elaborate. In most cases, protective mesh is stretched over the bed and its edges
imbedded in the bottom to reduce predation. In areas of
low predation, or when large seed are planted, the mesh
may be eliminated. Mesh size is based on seed size. The
mesh may be placed on the sediment surface and the seed
allowed to dig through or the seed planted and the mesh
placed over the seed. In the former, the seed must be smaller
than the mesh, while in the latter, the seed are larger than the
mesh. In both cases, the beds must be in areas of low wave
energy or the mesh can be covered by moving sediments.
Beds of shallow-burrowing species are typically mesh covered for the duration of the grow-out cycle (2–3 years)
except in areas where ice can cause severe damage. In such
areas, mesh may be removed from large seed (after the first
summer’s growth) in late fall and replaced in early spring.
In ice-prone areas, meshes are maintained over small seed
because of predation from ducks. Some high-value species
such as the geoduck may be planted in tubes (PVC)
implanted in the intertidal area and covered, individually
or en masse, with mesh that may remain for several years.
When the clams become larger and are deeply burrowed,
the tubes and mesh are removed for final grow-out. In some
areas, flats are bisected with low earthen berms being
seeded to grow Solen without mesh. These berms retain
the water for a longer portion of the tidal cycle.
Environmental effects
Environmental impacts of bivalve aquaculture have been
shown to be relatively small and isolated because no food
is added to the system. Further, bivalves filter the water,
increase the biodeposition rate, and increase the rate of
nutrient recycling, including denitrification. Exclusive of
the potential for the importation of unwanted species,
which has been reduced by importation regulations, the
environmental impact of bivalve culture can be divided
into three major categories: aesthetic, water column, and
benthic. Aesthetic effects have caused delays in obtaining
permits for farms because property owners do not want to
see culture gear or hear noise associated with gear maintenance and harvest. Proper siting and education of nearby
property owners and culturist usually result in
compromises that satisfy both parties. Water column
effects are generally positive because water clarity is
improved by removal of inert particles and microalgae.
Too many bivalves placed in the water column can reduce
growth rates because the local carrying capacity is
exceeded. In temperate areas, annual periods of low temperature plus low growth may add substantially to the
length of the culture cycle. Studies documenting where
ecosystem carrying capacity has been exceeded have
recently been reviewed (Burkholder and Shumway,
2011). The culture of infauna and the bottom culture of
epifauna typically results in fewer water column effects
than the epifaunal culture on long lines or rafts because
bottom culture is conducted in a single layer and results
in less biomass per square meter than water column
methods.
The biggest environmental change caused by bivalve
aquaculture is benthic due to the accumulation of
biodeposits on the sea floor that in turn can affect the other
benthos. For animals cultured in the water column,
biodeposits can greatly exceed normal deposition by animals living in or on the bottom. This accumulation and its
effects were documented over a half century ago (Ito and
Imai, 1955; see also Norkko and Shumway, 2011), and
effects can be reduced by proper siting or site rotation.
For infaunal and epifaunal benthic culture, the biodeposits
are limited by food supply and resuspension/erosion rates.
If the food supply is too low, growth decreases and deposition of feces and pseudofeces decreases. If food supplies are
not limiting, siting the culture in an area of moderate currents can reduce excessive buildup of biodeposits. This
scouring effect is particularly evident in intertidal or shallow subtidal culture areas where both currents and waves
serve to clear the bottom. In spite of this natural sediment
movement, the increased density of cultured organisms
causes an increase in the fine particle content of the sediments. This change plus the physical presence of the cultured species can alter the infaunal community. Protective
structures such as mesh increase epibiota and may emulate
the structure and function of nearby reef or seagrass areas.
In general, except for the increased density of the cultured
species and effects associated with harvesting, bottom culture of bivalves has relatively little ecosystem level effect
(Dumbauld et al., 2009). Studies on effect of the adding
structural components (PVC tubes) for geoduck culture
on the US west coast and screening for clam culture on
the US east coast have found that these culture operations
do not significantly alter the ecosystem processes
(Kraeuter et al., 2013; Van Blaricom et al., 2013).
Breeding
Selective breeding has been conducted on a few bivalve
species. Oysters have received the most attention
because of the need to develop strains that resist diseases.
Stocks of Crassostrea virginica have been developed that
are resistant to MSX (Haplosporidium nelson)
BIVALVE AQUACULTURE
87
grown on line systems attached to rafts or a variety of surface floats (long-line culture) and may extend many
meters below the surface. Mussels attach directly to the
line systems, with intermediate supports to keep the crop
from sliding off. Oysters and scallops may be attached to
lines but are more typically placed in cages that are then
attached to lines. Fouling control is an important maintenance procedure. Cages maintained on the surface can
simply be inverted to allow the top to dry and the fouling
organisms to die, while submerged cages must be cleaned
or exchanged on a regular basis.
Infaunal species are typically planted in prepared areas
(beds) in the intertidal or very shallow subtidal. Bed preparation may be rudimentary or elaborate. In most cases, protective mesh is stretched over the bed and its edges
imbedded in the bottom to reduce predation. In areas of
low predation, or when large seed are planted, the mesh
may be eliminated. Mesh size is based on seed size. The
mesh may be placed on the sediment surface and the seed
allowed to dig through or the seed planted and the mesh
placed over the seed. In the former, the seed must be smaller
than the mesh, while in the latter, the seed are larger than the
mesh. In both cases, the beds must be in areas of low wave
energy or the mesh can be covered by moving sediments.
Beds of shallow-burrowing species are typically mesh covered for the duration of the grow-out cycle (2–3 years)
except in areas where ice can cause severe damage. In such
areas, mesh may be removed from large seed (after the first
summer’s growth) in late fall and replaced in early spring.
In ice-prone areas, meshes are maintained over small seed
because of predation from ducks. Some high-value species
such as the geoduck may be planted in tubes (PVC)
implanted in the intertidal area and covered, individually
or en masse, with mesh that may remain for several years.
When the clams become larger and are deeply burrowed,
the tubes and mesh are removed for final grow-out. In some
areas, flats are bisected with low earthen berms being
seeded to grow Solen without mesh. These berms retain
the water for a longer portion of the tidal cycle.
Environmental effects
Environmental impacts of bivalve aquaculture have been
shown to be relatively small and isolated because no food
is added to the system. Further, bivalves filter the water,
increase the biodeposition rate, and increase the rate of
nutrient recycling, including denitrification. Exclusive of
the potential for the importation of unwanted species,
which has been reduced by importation regulations, the
environmental impact of bivalve culture can be divided
into three major categories: aesthetic, water column, and
benthic. Aesthetic effects have caused delays in obtaining
permits for farms because property owners do not want to
see culture gear or hear noise associated with gear maintenance and harvest. Proper siting and education of nearby
property owners and culturist usually result in
compromises that satisfy both parties. Water column
effects are generally positive because water clarity is
improved by removal of inert particles and microalgae.
Too many bivalves placed in the water column can reduce
growth rates because the local carrying capacity is
exceeded. In temperate areas, annual periods of low temperature plus low growth may add substantially to the
length of the culture cycle. Studies documenting where
ecosystem carrying capacity has been exceeded have
recently been reviewed (Burkholder and Shumway,
2011). The culture of infauna and the bottom culture of
epifauna typically results in fewer water column effects
than the epifaunal culture on long lines or rafts because
bottom culture is conducted in a single layer and results
in less biomass per square meter than water column
methods.
The biggest environmental change caused by bivalve
aquaculture is benthic due to the accumulation of
biodeposits on the sea floor that in turn can affect the other
benthos. For animals cultured in the water column,
biodeposits can greatly exceed normal deposition by animals living in or on the bottom. This accumulation and its
effects were documented over a half century ago (Ito and
Imai, 1955; see also Norkko and Shumway, 2011), and
effects can be reduced by proper siting or site rotation.
For infaunal and epifaunal benthic culture, the biodeposits
are limited by food supply and resuspension/erosion rates.
If the food supply is too low, growth decreases and deposition of feces and pseudofeces decreases. If food supplies are
not limiting, siting the culture in an area of moderate currents can reduce excessive buildup of biodeposits. This
scouring effect is particularly evident in intertidal or shallow subtidal culture areas where both currents and waves
serve to clear the bottom. In spite of this natural sediment
movement, the increased density of cultured organisms
causes an increase in the fine particle content of the sediments. This change plus the physical presence of the cultured species can alter the infaunal community. Protective
structures such as mesh increase epibiota and may emulate
the structure and function of nearby reef or seagrass areas.
In general, except for the increased density of the cultured
species and effects associated with harvesting, bottom culture of bivalves has relatively little ecosystem level effect
(Dumbauld et al., 2009). Studies on effect of the adding
structural components (PVC tubes) for geoduck culture
on the US west coast and screening for clam culture on
the US east coast have found that these culture operations
do not significantly alter the ecosystem processes
(Kraeuter et al., 2013; Van Blaricom et al., 2013).
Breeding
Selective breeding has been conducted on a few bivalve
species. Oysters have received the most attention
because of the need to develop strains that resist diseases.
Stocks of Crassostrea virginica have been developed that
are resistant to MSX (Haplosporidium nelson)
BIVALVE AQUACULTURE
87