TECHNICAL PAPER # 52
 
                       UNDERSTANDING AQUACULTURE
 
                                  By
                            Ira J. Somerset
 
                          Technical Reviewers
                          Marilyn S. Chakroff
                            Robert Bettaso
                            Martin Vincent
 
                                 VITA
 
                             Published By
                  VOLUNTEERS IN TECHNICAL ASSISTANCE
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                                PREFACE
 
This paper is one of a series published by Volunteers in Technical
Assistance (VITA) to provide an introduction to specific
state-of-the-art technologies of interest to people in developing
countries. The papers are intended to be used as guidelines to
help people choose technologies that are suitable to their
situations. They are not intended to provide construction or
implementation details. People are urged to contact VITA or a
similar organization for further information and technical
assistance if they find that a particular technology seems to
meet their needs.
 
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteers technical experts on a purely
voluntary basis. Some 500 volunteers were involved in the
production of the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff included Margaret
Crouch as Executive Editor, Suzanne Brooks handling typesetting,
layout, and graphics, and James Butty as technical writer/editor.
 
The author of this paper, VITA Volunteer Ira J. Somerset, is a
sanitary engineer working for the US Food and Drug Administration
as an evaluator of shellfish sanitation programs in the northeast
states. The reviewers are also VITA Volunteers. Marilyn S.
Chakroff, a technical writer and fishery trainer, is the author
of Fresh Water Fish Pond Culture and Management, published by
VITA; Robert Bettaso is an agricultural scientist with specialty
in fish culture; and Martin Vincent is a self-employed fisheries
expert.
 
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries. VITA
offers information and assistance aimed at helping individuals
and groups to select and implement technologies appropriate to
their situations. VITA maintains an international Inquiry
service, a specialized documentation center, and a computerized
roster of volunteer technical consultants; managers long-term
field projects; and publishes a variety of technical manuals and
papers.
 
                       UNDERSTANDING AQUACULTURE
 
                   By VITA Volunteer Ira J. Somerset
 
I. INTRODUCTION
 
Aquaculture is the production of protein-rich foods through the
controlled cultivation and harvest of aquatic plants and animals.
Using inexpensive equipment and simple techniques, aquaculture
can supply more protein than normally produced through conventional
agriculture such as dairy, poultry, and cattle farming;
and traditional fishing.
 
Aquaculture is not new. More than 2,500 years ago the sticky eggs
of some fish were collected on mats and bundles of reeds or wood
attached to posts in streams. Oyster and clam eggs were also
collected and transferred to other waters to hatch. This was the
first form of aquaculture.
 
In the 11th and 12th centuries, pond culture developed. Carp were
moved through a series of ponds where they reared young fish and
grew to harvest size. Later, other fish were cultured in a
similar manner. Today, several types of fish and shellfish are
grown in high density aquaculture operations throughout the
world.
 
The techniques of animal husbandry improve the chances of
survival of the plants and animals being raised and speed up
their growth so that the food yield is quick and large. Almost
any type of aquatic organism can be raised from its youth to a
healthy, marketable adult. However, this paper in restricted to
fish and shellfish culture. The reader is presented with only
general considerations and approaches to aquaculture, since it
requires specialization to address each possible cultural
species.
 
ADVANTAGES AND DISADVANTAGES OF AQUACULTURE
 
Systematic aquaculture operations have a number of advantages
over fishing for the production of protein foods. Some of these
are:
 
    o      Economics (employment, new industry and support
          services, and increased foreign and domestic ex
          change);
 
    o      No need for expensive fishing craft and gear;
 
    o      Low operating and maintenance costs;
 
    o      Low capital investment (unless Ponds must be constructed;
 
    o      Reasonably predictable yields;
 
    o      Less time lost due to bad weather or breakdowns;
 
    o      Fewer equipment malfunctions and injuries;
 
    o      Reduced health risks to consumers.
 
Aquaculture operations do have drawbacks, however. These include:
 
    o      Water is necessary, in predictable quantity and
          quality;
 
    o      Large land area on which to construct ponds or access
          to large shallow area of water is required;
 
    o      Knowledge of culture conditions may not be generally
          available.
 
TYPES OF AQUACULTURE
 
There are five major types of aquaculture:
 
    1.   Transplantation:   The movement of a species to a
        suitable location. This method in also used to introduce
        species into new environments.
 
    2.   Hatchery and Stocking:   The spawning, hatching, and
        rearing of a cultural species that will be transplanted
        to suitable or desirable areas. This method is
        used to supplement or replace the natural stock, or
        for transplantation.
 
    3.   Enbayment Culture:   The use of enclosures, such as
        ponds, cages, baskets, and strings, for aqua
        culture in natural waters.
 
    4.   Ponds with Supplemental Feed and Fertilizer:  Aquaculture
        in natural or artificial ponds with food and
        fertilizer provided to maintain algae and species
        at desirable levels. In some systems, animal
        manures are used to provide fertilizer and some food.
 
    5.   Ponds without Supplemental Feed and Fertilizer:
        Aquaculture in natural or artificial ponds with the
        cultured species subsisting an natural available food
        in the pond water. This requires a high rate of
        exchange of water for high growth rates.
 
As can be seen, the basic theory of aquaculture is to obtain
small animals and provide them with an environment that allows
for their rapid, healthy growth. A desirable-sized fish can be
harvested in a short period of time.
 
II. CULTIVATION REQUIREMENTS
 
The most commonly cultivated species of fish are carp and
tilapia. Shellfish such as oysters and mussels, which are low on
the food chain, are also farmed extensively. While culture
techniques must be adapted to the needs of specific species and
to local needs and conditions, some general rules apply:
 
    1.      The species must be Suitable for cultivation under
           the proposed conditions.
 
    2.      The program must develop the best method of cultivating
           the identified species from physiological, geographical,
           and market points of view.
 
    3.      Adequate support must be available. This includes
           changing and aerating the water, feeding the fish,
           maintaining equipment, marketing, and so on. Experimentation
           is often necessary to improve yields substantially.
 
    4.      Predators must be controlled.
 
    5.      Cannibalism must be controlled.
 
    6.      The species life cycle must be understood, and good,
           inexpensive feed must be available.
 
A dense population of animals demands abundant food and oxygen
and a means of removing metabolic wastes. There is a limit to the
size of the biological community that can be supported before
growth is limited by competition for food, oxygen, and space. The
high density of cultured animals makes them susceptible to
disease and predation. To prevent juveniles from being attacked
by these diseases, drained ponds must be thoroughly dried to
destroy parasites and disease-causing organisms. The water and
stocking animals should be free of parasites and disease-causing
organisms. Feed and feed supplements should not introduce
parasites or disease-causing organisms.
 
On the positive side, the fertile fish and shellfish wastes can
be used in the production of leaf crops requiring nitrogen.
Shellfish wastes are best used on fruit trees.
 
OVERALL OPERATION AND MAINTENANCE
 
Aquaculture systems can be operated and maintained in three ways:
 
    1.   Communal:  This is subsistence cultivation that is some
        times publicly funded. The conditions are often mediocre,
        and production is poor because duties are attended
        randomly.
 
    2.   Family:  This can range from subsistence cultivation to a
        very sophisticated operation, depending on the skill and
        energy of the owners. Under the worst of conditions, it
        can be more variable than communal; at its best, it can
        exceed the standards of a dedicated system. The key to a
        successful operation is the family's commitment to putting
        forth the effort necessary to produce a quality
        product.
 
    3.   Dedicated:  This operation is designed to produce food
        for market, and is usually well-regulated with high
        yields.
 
Each of these types of operation can be run as extensive or
intensive culture.
 
Extensive Culture provides little or no control over the environment.
Placing shellfish on a site and allowing them to grow on
their own, or trapping fish and invertebrates in special enclosures
and holding them until they reach market size, are examples
of extensive culture. In extensive culture, the fish depend upon
the natural food supply in the water. Only 20 to 50 percent of
the stocked animals survive in this uncontrolled environment.
 
Intensive Culture on the other hand provides full control, over
the environment. An indoor culture of shellfish, in which
temperature, salinity (salt/water ratio), flow rate, feed type,
amount of feed, and light are fully controlled, is an example of
intensive culture.
 
No matter which type of operation or which method of culture is
selected, sufficient food and oxygen must be provided. Oxygen
levels of 4 to 5 milligrams per liter (parts per million) are
satisfactory. Water can be aerated by spraying it out at least
0.6m (2 feet) in droplet form. Food requirements are discussed in
a later section.
 
There is one other general consideration in aquaculture that is
extremely important: The size of the animals. The animals stocked
in the aquaculture system must be large enough to grow to market
size in the desired time. Some preliminary experimentation is
needed to determine the minimum desirable size. only healthy
animals should be chosen for stocking the aquaculture system.
 
SITE SELECTION
 
An aquaculture system can be operated on a shore, in an intertidal
region (zone between the high and low tides levels), in a
sub-tidal region (zone below the low tide level), on a water surface,
in mid-water, or on a seabed. Certain culture systems are
better-suited to certain sites. A shore facility is usually used
for fish and shrimp production. Full control (intensive culture)
of the environment is characteristic of shore sites, and pumps
may be needed to provide the water supply.
 
Controlled Pond Facilities
 
These are either man-made or natural areas that can be isolated
from the water source. Water flows by gravity into the pond or is
pumped in. Ponds are suitable for such fish as tilapia or carp,
or even game fish such as salmon.
 
Intertidal Facilities
 
Intertidal facilities take advantage of the movement of the tides
to replenish food and water. They are used for shellfish culture
and spat (larval shellfish) collection and can be controlled if
properly constructed. The incoming high tides are let into an
area that can then be closed off. The high water, with its load
of baby fish or shellfish, is dammed off and is held until the
fish reach marketable size. Pumps may be necessary to provide the
water supply.
 
Subtidal Facilities
 
Subtidal facilities have extensive culture (little environmental
control) characteristics. No water pumps are needed, but detailed
water quality analysis in required to ensure adequate circulation.
Fouling organisms must be regularly removed from your stock
and equipment.
 
Surface Floating Facilities
 
In this case, floating cages and rafts are used, which can be
moved to protected areas if necessary. This is extensive culture
and usually does not require pumping of water. However, fouling
organisms may restrict the flow of water, creating supply and
feeding problems. Supplemental feeding may be necessary, and
fouling organisms must be removed regularly.
 
Mid-Water Culture Facilities
 
Mid-water culture facilities consist of strings of mollusks
(shellfish) suspended through the water column. Since this is
extensive culture, restricted flow may create fooding problems.
Fouling organisms must be removed periodically.
 
Seabed Culture Facilities
 
These are also extensive culture sites and may be subject to
fouling organisms that restrict water flow and cause feeding
problems. Because of natural flow restrictions along the bottom,
oxygen and food supply may be reduced.
 
For all of those sites, you must evaluate the exposure to
pollution from land runoff (pesticides or siltation), and f rom
sewage and industrial wastes. Ways of protecting a site from high
winds and waves must also be evaluated.
 
MATERIALS
 
Enclosures are needed to keep predators away and to prevent the
loss of stock through sluice gates and other outlets. Materials
used in aquaculture must:
 
    1.   Have a long visible life.
    2.   Be resistant to fouling.
    3.   Be easily cleaned.
    4.   Be nontoxic.
 
Structures supporting enclosures within the intertidal zone must
be rigid to allow for the rise and fall of the tide. Floating
rafts, nets, and cages must be anchored to allow for wind and
waves. Wind and waves cause wear and abrasion of the materials.
The structure may also need fine-mesh nets for protection from
predators and coarse-mesh nets for protection from trash and
floating objects.
 
 
Surface floating units, consisting of a timber structure on
floatation barrels or floats, require much maintenance. The
condition of the floatation and framework should be checked
often, especially when used in salt water. Before using any
material in water, especially salt water, the effects of marine
predators on the material should be evaluated by installing test
pieces for at least one growing season.
 
Organisms growth on equipment and shellfish can be removed by
brushing, hand picking, or high-velocity water jet. Growth may be
prevented in some cases by periodically removing the material
from the water. In removing growth, care must be exercised to
ensure that the underlying material (rope, net, and shell) is not
damaged.
 
FOODS AND FEEDING
 
Fish or shellfish cultured must be limited in number so that each
animal can obtain enough food to grow. Insufficient food will
result in slow growth, or even shrinkage, small animals (dwarfism),
and a high potential for disease. Harvest has been found
to increase as much as 1,000 percent when animals are fed
regularly. Figure 1 shows how the growth rate can be graphed.

ua1x7y.gif (540x540)


 
The conversion rate from feed to flesh varies with fish species,
food type, temperature, individual fish, and food availability.
Generally, it in between 10 to 1 and 20 to 1. Cultured fist and
shellfish should not be overfed, since unconsumed feed sinks to
the bottom, decays, and aids the development of algae growth,
while reducing oxygen levels through the decomposition process.
Although some of this fertilization is good, too much growth
creates low oxygen levels. Fish should be fed 6 days a week at
the same time and place each day, ideally, 2 to 3 hours after
sunrise or before sunset. To empty the digestive tract and
produce better-quality fish, don't feed them on the day before
harvest.
 
Feed only what will be eaten daily. If a floating feed is used,
feed what in eaten in 10-15 minutes. Observe the response to
feeding: If the fish do not appear hungry, there may be logical
reasons (abundant natural food available, low dissolved oxygen,
poisons, etc.) and feeding should be discontinued until the
reason in found and corrected. If sinking food in used, check the
feeding response by placing a 1.2m x 1.2m (4 feet x 4 feet) tray
on the bottom in the feeding area. After 1 hour, raise the tray
slowly and carefully. Look for feed on the tray. If the feed has
not been completely consumed, reduce the amount of feed. Generally,
fish will eat one tenth to one half their own weight per day.
 
Both natural and artificial foods say be used. Controlled
fertilization of ponds in order to increase their productivity
and providing more natural food to the cultured species are
established practices. Artificial foods (those that will be
consumed directly without conversion to algae) consist of plants,
processed food, and certain industrial wastes. Examples of plant
foods are the leaves of the cassava (tubers and peelings are not
suitable), sweet potatoes, eddoes, banana, paw paw, maize, and
canna plants. Processed foods include meal waste, cassava bran,
flour, rice chips and balls, corn flour, and cotton and groundnut
oil cakes. Industrial wastes such as decomposed fruit, brewery
sediment, coffee pulp, and local beverage wastes have also been
used successfully.
 
Fertilizer is added to a pond to ensure that there are minimum
amounts of nitrogen, phosphorous, and potassium in the water to
support algae growth. The requirements will vary with the water
quality and fish population. Fertilizer should be added before
the fish-growing season and repeated at ten-day intervals to
produce the desired algae population, known as a bloom. After the
bloom, add fertilizer as necessary to maintain a light bloom. The
density of the bloom must be adjusted for different seasons,
since too much algae will cause a reduction in the dissolved
oxygen levels and could kill fish. A desirable bloom will shade
out a bright object 0.3 - 0.5m (12-18 inches) below the surface.
If 3 to 5 applications of fertilizer are made and a bloom is not
observed, there may be other problems, such as filamentous algae
or other plants using the fertilizer. These must be killed before
phytoplankton algae can grow, unless the aquaculture system uses
filamentous algae or large plants. If filamentous algae or larger
plants are consistent problem, you should consider adding species
of fish that can eat then, thus converting then into useful
protein, rather than staying in a constant battle to remove them.
 
The pond can be fertilized in three ways: by spreading the
fertilizer over the water surface; by placing perforated bags at
intervals around the pond edge to allow the wave action to
dissolve the fertilizer; or by placing the fertilizer on sub
merged floating or stationary platforms off the bottom. That last
method provides the best results with the least fertilizer.
Although agricultural runoff may help by providing nitrogen and
phosphorous from the fields, pesticide and herbicide residues may
destroy all of the fish in the pond. The direct application of
animal manure has been shown to be effective in producing algae
bloom, but it does have two potential dangers. Oxygen may be used
up and ammonium (a reduced form of nitrogen) may reach too high a
level. Both of these problems can be avoided if manures are used
in moderation or if they are held in a pretreatment aeration
pond. In general, if used carefully, animal manures may be an
excellent, inexpensive, source of fertilizer for the fish pond.
The aquaculturist should, of course, be aware of any religious or
cultural taboos against such use that may affect marketing. (If
taboos exist, the fish can be hold in "clean" ponds, or use of
the manure can be suspended, for a week or two prior to harvest.
III.  CULTURAL METHODS
 
Fish can be grown in open ponds or in cages in ponds. Shellfish,
on the other hand, often do better in what is called suspension
culture. These three methods are described below.
 
POND FISH CULTURE
 
Types of Pond Culture
 
There are four general types of pond fish cultures:   mixed age
groups, temporary age group mixing, separated age groups, and
controlled reproduction.
 
The Mixed Age Groups Method. This method produces all sizes of
fish in great quantity. The level of production is maintained by
catching some fish while the fish are growing.   This may be done
with a hook and line or a limited number of traps. At the end of
the growth period, the pond is drained and all fish are harvested.
Some are selected for restocking the pond when it is
refilled. This method provides a high production rate if the fish
are well-fed. Fish from a different source should be put if the
pond periodically to improve the fish quality.
 
The Temporary Age Group Mixing. This culture produces a large
portion of equal-sized fish. The pond in stocked with young fish
of approximately the same size, which are fed and allowed to grow
and reproduce once. When the largest of the fish spawned in the
pond are large enough to use for restocking, the pond is drained
and the fish harvested. All adults are sold or used for food; the
smaller fish are used for restocking. In this method, the weight
per fish is usually small. A mixed size fishery usually evolves
from temporary size mixing.
 
Separated Age Groups Method. In this method, two ponds and heavy
feeding are used to produce table or market-size fish as rapidly
as possible. Adults of a single species are introduced into a
reproduction pond. When the young spawned in the reproduction
pond are large enough to survive in a larger growing pond, they
are transferred to the larger pond.
 
The "Natural" Predation Method. This method attempts to balance
the fish's growth and reproduction through the introduction of a
predator. The results of this method are uncertain, since
over-predation will reduce or even eliminate the population,
leading to too many fish that are too small (dwarfing).
 
Controlled Reproduction Methods. These methods control the sizes
and numbers of fish in the growth ponds by controlling reproduction
within a laboratory. Fish stock in the ponds do not reproduce
because conditions in the pond are not favorable for the
species used or because something is done in the laboratory to
prevent fertility. One method that has been used with some
success if separation of fish by sex. Males and females are
simply placed in separate ponds. However, this is a very difficult
method to use, because a small number of males in the
female pond (or vice-versa) will cause reproduction in the female
pond (and in the male pond to a lesser extent).
 
Other methods include production of sterile hybrids, operating on
fish to sexually denature them; or treating the fish to reduce
fertility.
 
Construction and Operation of Fish Ponds
 
Once pond cultivation has been decided on, the technical considerations
must be addressed. A suitable location with an
adequate water supply must be chosen. The soil must be able to
contain the water in the pond. The water quality must be adequate
for the species, and the quantity must fill the pond in less than
one month and replace losses due to seepage and evaporation.
 
Water Supply. There are several sources of water for pond
culture, including rainfall, surface water, springs, and wells.
Surface water often contains unwanted fish, pollution, parasites,
and disease, and is the least desirable water source. It is often
necessary to aerate to remove undesirable gazes and raise the

ua3x13.gif (600x600)


oxygen level. Springs may also contain unwanted fish and can dry
up at the time water is most needed. Rainfall may be even more
undependable and low in nutrients. But it will generally be free
of pollutants and high in oxygen.
 
Well water in usually the highest quality (especially when it
comes from covered wells). It does not contain unwanted fish or
suspended material, and is protected from flood water. But it
also may need aeration to remove undesirable gases and raise the
oxygen level. If the well's water source is of uncertain quantity
or quality, test wells should be constructed first.
 
The minimum pond water depth depends on the air temperature,
seepage rates, and the dependability of the water supply. In an
area dependent on seasonal rains, the water should be at least 3m
(10 feet) deep over at least 25 percent of the pond. In warm
areas with low seepage or sufficient water supply, the minimum
depth may be as little as 1m (3 feet). If the pond will be ice
covered for one month or more, the pond will have to be at least
6m (20 feet) depth to prevent winter-kill.
 
Woods may grow in shallow water. Since this may be beneficial,
removal will depend on whether the benefits outweigh the problems
associated with the additional use of nutrients, loss of pond
volume, and potential oxygen use when the plants decay. Shallow
areas with weeds are favorite brooding areas for mosquitoes. It
is recommended that the pond be not less than 12 (3 feet) deep to
minimize weed and mosquito growth, or herbivorous fish, such as
grass carp, should be among the species stacked in the pond.
 
Pond construction.  The pond should be constructed with side
slopes in a ratio of 2.5 to 1 and a gentle bottom slope of at
least 6.4cm per 30m (2 1/2 inches per 100 feet). (see Figure 2.)

ua2x11.gif (600x600)


To stabilize side slopes, grass should be planted as soon as
possible after construction. If the bottom material consists of
good stable soil, put in a drain well, or harvest basin. Although
most fish are harvested by netting, some will escape and be
easily caught in the drain well. The drain should be approximately
1/10 of the size of the production area and 0.7m (2 feet)
deeper than the surrounding area.
 
It may be necessary to build a dam to trap the water for the
pond. If so, assistance should be gained from a qualified
engineer, as a break in the dam can have serious consequences. An
emergency spillway that prevents water from flowing over the top
of the dam should be constructed when the pond is created. The
spillway must keep the flow shallow enough or must have a barrier
so that large fish stay in the pond and unwanted fish cannot
enter. A vertical overflow from the spillway of 0.7m to 1m, (2 to
3 feet), or a turndown pipe, will keep out unwanted fish.
 
A drainpipe large enough to drain the pond in less than five days
should be placed in the bottom of the pond through the dam. A
trickle tube--a small adjustable-height pipe that allows excess
water to flow out without going over the spillway--may be
connected to the drain pipe. The trickle tube should be small
enough to prevent small fish from swimming out. It can also be
used to regulate the depth of the water behind the dam.
 
To prevent decaying material from reducing the oxygen levels and
to allow harvesting with nets, all trees, bushes, rocks, and
stumps should be removed from the pond bottom and sides. Any
trees within 9m (30 feet) of the edge of the pond may have to be
cleared to reduce leaves, which can discolor the water and
promote algae growth. Algae and decaying leaves cause oxygen
depletion, which may endanger the fish. On the other hand, both
can be a source of food and might be desirable depending on the
species chosen for culture.
 
Operation. Unwanted fish must be prevented from entering the pond
wherever possible. Incoming water should be filtered and the pond
located so that the overflow from streams does not enter. This
will also exclude disease-carrying organisms and parasites. To
keep birds from landing and taking off in the pond, you may have
to stretch crossed wires across the pond.
 
It is critical in pond operation that an adequate amount of
oxygen be dissolved from the air into the water. Without enough
dissolved oxygen, the fish will die. To maintain adequate levels,
do not make the pond too deep and provide a means to aerate the
water if necessary (Figure 4). Unless there is good circulation

ua4x15.gif (540x540)


from the top to bottom, the bottom sediments will become anaerobic
without oxygen) and produce hydrogen sulfide. This will
interfere with the ability of fish to use the available oxygen,
without which they may die. Decay from dead fish also requires
oxygen, which reduces the oxygen available for the live fish,
thus creating a deadly cycle. The pond must be filled with good
water, ever-fertilization must be avoided, and dissolved oxygen
levels should be checked frequently, especially at daybreak.
 
Harvesting
 
Harvesting the fish may be done by partially draining the pond

ua3x13.gif (600x600)


and netting the fish. Make the not large enough to let undersized
fish escape. Do not drain the pond down so far that the
undersized fish are killed. The water level should be reduced
slowly enough to allow the fish to move to deep water to prevent
their death from stirred-up sediment and a lack of oxygen.
Harvesting is best done in cool weather, but can be done at any
time. After drying the pond and performing any necessary maintenance,
refill and restock the pond.
 
Salt Water Ponds
 
Although most of the information in this section has related
primarily to freshwater fish ponds, the same approach can be used
to grow salt water fish in ponds. With a salt water pond, the
tide circulates new water through the pond frequently enough to
prevent low dissolved-oxygen levels. Predatory fish and crabs
must be kept out of the pond. Crabs entering the pond can be
trapped, but it is best to keep them out in the first place. Any
starfish and crabs that are found in weekly inspections should be
picked up and used for crop fertilizer, eaten, or grown in
another pond and used for human or animal food.
 
CAGE FISH CULTURE
 
Fish can be confined to cages anchored in ponds, lakes, or salt
water bodies. This method of growing fish is most often used when
the desired species is not spawned in captivity, and the young
can be caught in the wild and placed in cages to restrict their
movement. They must be checked frequently for disease and
parasites, but should be handled as little as Possible. Oxygen
levels must be kept high enough for the fish Species. Regardless
of which method of cage culture is Used, the water must have
enough oxygen to prevent suffocation of the cultivated fish.
Competing organisms must be removed with brushes, picks, or
high-velocity water jets.
 
Cages
 
It has been found that unprotected metal cages rust quickly.
Therefore, it is advisable to use plastic-coated metal whenever
possible. Other materials, such as plastics and bamboo may be
satisfactory. Cages should be anchored firmly, with the top of
the cage high enough to retain food when the fish are being fed.
The cage top should extend down about 20 cm (8 inches), and about
5-10cm (2 to 5 inches) below the water. Rigid or floating netting
may be substituted for the top. At least 30cm (1 foot) must be
left between the bottom of the cage and the bottom of the pond or
ocean to keep predators from entering and to prevent wave action
from bumping the cage on the ocean or pond bottom. Fish in cages
must be fed if they are not plankton eaters. The outside of the
cages must be cleaned periodically to remove fouling organisms
and restore water flow through the cages.
 
Raceways
 
Raceways are long narrow artificial channels in which fish are
raised. Water is usually recirculated in this type of system.
The ends are secured to prevent the escape of the fish. A raceway
system requires a water supply pond, a method of regulating the
depth of the water in the channels, a settling basin to remove
dirt and deposits, an auxiliary water supply, and a pump. This is
a very complex, energy-using system.
 
Shrimp Ponds
 
Shrimp are often cultured in ponds where post-larval shrimps are
washed into the ponds at high tide. Shrimp ponds must have a hard
bottom consisting of sandy-silt, or the pond bottoms may become
anaerobic. This is critical with shrimp, since they burrow, into
the bottom of the pond during the day. Shrimp ponds are constructed
with gates that allow the water and shrimp to enter at
high tide when the gate is open. The opening in screened on the
ebb tide to prevent the loss of the shrimp. Shrimp culture
requires circulating water to keep the bottom oxygen levels high.
 
Shrimp are harvested by placing a not at the pond outflow at
night on an ebb tide. Do not harvest shrimp by draining the pond
as those in their borrows will be lost. The pond should be
drained and baked in the sun for 3 or 4 days once a year.
 
SUSPENSION CULTURE
 
Oysters and other mollusks grow
better with fewer deaths in
suspended culture. Shellfish may
be cultured on the bottom, on
stakes or racks, in cages or
nets, from rafts, or from long
lines. They must be grown in the
intertidal or subtidal zones.
 
Shellfish culture begins with
the collection of the seed,
called spat. Spat are the spawned
animals that are ready to set
on a hard object. Many shellfish
do not move once they attach to something, so a proper material
is essential. Collection units consist of shells on strings laid
over or tied to racks, sticks, plastic disks, ceramic tiles, mesh
bags of shells, or any other hard rough surface. Mussels prefer
fibrous material such as coarse fiber ropes. These are placed in
the water when shellfish are ready to attach at the time of
spawning (to reduce fouling).   After about one month, the
collectors are moved to hardening racks where they are exposed
only at low tide. They are raised gradually until they are
exposed f or 4 to 5 hours per tidal cycle. This helps produce a
thicker shell and stranger animal that can survive the first
hibernation period (spawning usually occurs in the spring and
fall). Mussels are transferred directly to the growing area and
are placed on posts or strings to grow since they have the
ability to reattach themselves once removed from a surface.
 
Suspended culture of shellfish is practiced because it allows the
use of all depths of water and helps control predators. Off-bottom
culture provides a better quality product with no pearls,
better meat yield, good meat color, and no foreign particles
within the shell. The highest yields are obtained in the early
spring, before the shellfish spawn, then again in late summer
before the fall spawning.
 
The ABC's of Suspension Culture

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       *      Anchorage - making sure the shellfish stay where they
             are put.
 
       *      Buoyancy - keeping the strings from touching the
             bottom.
 
       *      Cultivation Materials - making sure the materials are
             sound.
 
Shellfish spat may be collected on racks in shallow water 2 to 4m
(6 to 12 feet) at low tide. A rigid frame structure of poles
planted vertically with horizontal ties are placed in the
collection area. The collectors are arranged so there are 6 to 10
collector plates 20cm (8 inches) apart on strings 1.5m (5 feet)
long. Twenty units are hung in every 3.3 sqare meters (10 square
foot) area. Mussel collectors are best made from woven grasses,
1.5cm (3/4-inch) square wood pegs 25cm (10 inches) long are 40cm
(2-foot) intervals.
 
Oysters are generally cultured by suspending them from rafts or
long lines.  Rafts are usually made of cedar or bamboo poles tied
together in two perpendicular layers. Styrofoam cylinders, drums
or floats are usually used for floatation. Additional floatation
must be added as the shellfish grow. Rafts are usually 8 x 16m
(26 by 50 feet), and contain 500 to 600 vertical strings of spat.
Rafts are often tied together end to end and anchored at the ends
of the row. They are placed in rows 102 (35 feet) apart. Production
will vary depending on the amount of spat collected,
disease, predation, available food, and water temperature.
 
In long-line culture, lines about 70m (225 feet) long are buoyed
by wood or styrofoam floats or glass balls. Floatation is
initially 3m (9 feet) with more added as the shellfish grow. The
lines are placed 10m (35 feet) apart and anchored at each and and
in the center. Usually it is leas expensive to construct and
maintain long lines, which withstand wind and waves better than
do rafts. The vertical strings of spat are placed 45cm (18
inches) apart. They can be of any manageable length, but are
usually in multiples of 5m (16 feet).
 
In areas where predators, waves, or winter storms are a concern,
shellfish can be cultured in floating net cages. These are
usually 10m (35 feet) square, 3 to 5m (9 to 16 feet) deep. They
consist of floats, nets, and an anchored rectangular frame. These
small rafts with the shellfish enclosed in cages can be moved to
sheltered areas in winter, when storms approach, or for maintenance.
Several cages can be Joined together to form a large
raft.
 
It is extremely important to recognize that the strings and cages
require maintenance to remove fouling organisms. The strings must
be removed from the water periodically and washed with a high
pressure spray. A barge-mounted crane will be necessary for raft
or long line culture.
 
The large volume of waste produced by cultured shellfish creates
special problems. A 60 square meter (600 square foot) bed can
produce between 1/2 and 1 ton (dry weight) of organic material.
Decay of this material can cause anaerobic conditions close to
the bottom, killing the shellfish on the bottom of the strings.
 
Monitoring Growth Rates
 
Shellfish have highly variable length-weight relationships that
must be determined before the culturer can decide how long the
shellfish must be grown and whether shellfish culture has a
reasonable return for the time spent.
 
Probable growth can be determined by suspending about 25mm (1
inch) long shellfish in containers about 1m (3 feet) bellow the
water surface. The container must have a good water circulation;
the holes should be about 1cm (1/2 inch) in diameter. Inspect the
shellfish monthly, brushing them clean, measuring them, and
recording lengths and weights. Average the measurements and graph
them, with the length (or weight) on one axis and the month on
the other. This will provide a good guide to time of growth and
feeding.
 
The shellfish cultures in the wild will suffer a higher mortality
due to fouling organisms. The size at harvest should be deter
mined by the use. Reference to the size-month chart will give the
minimum length of time needed to culture the shellfish to that
size. In practice, it is usual to allow one additional growing
season for all shellfish to reach that size.
 
Mussels are slightly different from oysters in that they will
attach themselves to a secure place after being harvested and
replanted. Mussel seed can be placed in very coarse cotton tubes
and fastened in a spiral around ropes or thick poles driven into
the ground. By the time the cotton has decayed, the mussels
should be attached to the rope or pole. They can be harvested,
cleaned, and graded with the smallest ones returned to the water
in now tubes. They should be kept out of water for the shortest
time possible.
 
To harvest mussels from strings, a collecting basket must be
placed under the string when it in lifted to catch those mussels
that drop off.
 
IV. DECISION MAKING FACTORS
 
The management of a high density aquaculture operation is
complex, requires hard work, and is subject to the whims of
nature. An difficult as it might appear, aquaculture has continued
for thousands of years and is the source of food for many
people today. Even though there will always be problems, the
beginner aquaculturist is encouraged to start on a small scale,
allowing the aquaculture operation to grow an the product does,
in a controlled manner.
 
Researchers are working an improving aquaculture techniques.
Specifically, they are working toward identifying additional
species suitable for culture, producing industrial fish (for fish
meal), and improving methods of managing various aspects of
aquaculture such as seed supply availability and disease,
predator, and water quality control. other areas of research
include genetic improvement, manipulating water temperature, and
treating fish with hormones to promote spawning, and identifying
new protein sources (e.g., agriculture wastes and yeasts grown on
petroleum products or wood pulp) to replace fish meal in feed
formulations and to reduce the cost of feeding fish.
 
Some of the problem the aquaculturist will likely face include
the effects of corrosion, fouling, weather, and climate. The
aquaculturist will also encounter conflicting complaints and
demands from those concerned about land and coastal areas, water
use, and pollution. Aquaculture risks may be natural (adverse
weather, disease), economic (price and market changes), or human
improper care).
 
ECONOMICS
 
One major constraint on aquaculture development has been the
limited supply and high cost of juvenile animals obtained from
nursery areas. This can be solved locally by raising animals and
producing juveniles, or by harvesting juveniles from their
natural habitat. Once the basic problem of mating, spawning, and
raising the juvenile stages have been solved, the hatchery
production of large numbers of juveniles becomes routine and
inexpensive. It does not require large or expensive facilities.
By contrast, many variables make reliance on the harvest of wild
juveniles a very risky long-term undertaking.
 
In evaluating the economics of aquaculture, it must be remembered
that the price of the product is very important and will decrease
as the fish supply increases. The price must exceed the cost if
the project is to succeed. The cost of the right to use the
property or the right of access to the culture area must be
considered in addition to the equipment, maintenance, and labor
costs.
 
MARKETING FACTORS
 
In marketing your aquaculture products, you need to:
 
       o    Develop a marketing system, including disseminating
           product information and identifying products that
           consumers will want to buy.
 
       o    Set or adhere to quality control standards.
 
       o    Consider transportation and marketing facilities.
 
       o    Preserve your fish products to prevent their spoilage
           before they can be sold.
 
SOCIAL FACTORS
 
Social factors that may affect your decision to pursue aquaculture
include:
 
       o    The williness of your community to respond to changes
           in technology (e.g., from the technology of ocean
           fishing to that of aquaculture).
 
       o    Acceptance of your aquaculture products. For example,
           traditional food preferences and religious or
           cultural taboos may impede the acceptance of your
           products.
 
ENVIRONMENTAL FACTORS
 
Establishing an aquaculture operation may cause degradation of
the environment through dredging and filling, pond effluent
discharges, increased mosquito population, and exploitation of
natural resources.
 
Care must be exercised when a new or foreign species is being
considered for culture. A new species could escape into the wild
and, without natural predators, multiply rapidly with disastrous
consequences for the overall ecological balance.
 
LEGAL FACTORS
 
Consult your local authorities to find out whether there are any
laws or regulations that may prohibit you from developing an
aquaculture system or using an aquaculture area.
 
                      GLOSSARY
 
Anaerobic - Without free available oxygen
 
Aquaculture - The controlled cultivation and harvest of aquatic
plants and animals.
 
Filter Feeders - Shellfish that food by filtering food particles
from the water through their gills.
 
Food Chain - Transfer of food energy through a series of
organisms with many stages of eating and being eaten.
 
Invertebrates - Lower animals, without backbones.
 
Larval Stage - An immature stage of an invertebrate animal. The
animal in this stage is called larva (plural, larvae).
 
Mollusk - Invertebrate characterized usually by a hard,
limy, one or more part shell that encloses a soft, unsegmented
body.
 
Parasitic Organisms - Organisms growing on cultured organisms
and competing for the available food and oxygen.
 
Predation - The act of an animal eating another animal, usually
smaller and of a different species.
 
Spat - Young mollusks past the free-swimming stage and ready to
settle and attach to a hard object.
 
                               BIBLIOGRAPHY
 
Burrill, G., and Lynch, K. An Evaluation of the Aquaculture
  Extension Project at Goddard College:  Report to the ARCA
  Foundation. Bennington, Vermont:  Goddard College, 1975.
 
Chakroff, M.  Freshwater Fish Pond Culture and Management.
  Arlington, Virgina: Volunteers in Technical Assistance (VITA),
  1976.
 
Conklin, D.E.  "The State of Aquaculture,"  The professional
  Nutritionist, Vol. 8, 1976, pp. 3-7.
 
Cramer, D. L., Slabji, B.M., True, R.M.   "Seasonal Effects on
  Yield, Proximate composition, and Quality of Blue Mussels,
  Mytilus Adults, Meats obtained from Cultivated and Natural
  Stock," Marine Fisheries Review (Volume 40, August 1978), pp.
  18-23.   U.S. Department of Commerce, National Oceanic and Atmospheric
  Administration. Washington, D.C.
 
Cuyvers, L. Aquaculture 1980. Newark, Delaware: University of
  Delaware Sea Grant College Program, 1981.
 
Gates, J.M. "Aquaculture in Less Developed Nations, Some
  Economic Considerations."  Presented at the Conference of the
  Marine Technical Society, Washington, D.C., 1971.
 
Grinzell, R.A.; Dillon, O.W., Jr.; and Sullivan, E.G. Catfish
  Farming.   Farmers Bulletin, No. 2260. Washington, D.C.:
  U.S. Department of Agriculture, 1975.
 
Imai, T. Aquaculture in Shallow Seas:   Progress in Shallow Sea
  Culture. Now Delhi, India: Amerind Publishing Co., 1971.
 
Jensen, J.  Home-Grown Fish from Cages.  Circular ANR-269.
  Auburn, Alabama: Alabama Cooperative Extension Service, Auburn
  University University, 1981.
 
Landis, R. C. A Technology Assessment Methodology. Mariculture
  (Sea Farming). McLean, Virginia: The Mitre Corporation,
  1971.
 
Lutz, R.A, Bivalve Molluscan Mariculture: A Mytilus Perspective.
  Contribution No. 138. Walpole, Maine: Ira C. Darling Center,
  University of Maine, 1978
 
Meyers, E.  The Husbandry of Mussels in a Maine Estuary:  An
  Approach to a Commercial Enterprise (Publication No. UNH-SG-164).
  U.S. Department of Commerce, National Oceanic and Atmospheric
  Administration, University of New Hampshire/University
  of Maine Sea Grant College Program. Washington, D.C. 1981.
 
Milne, P. H.  Fish and Shellfish Farming in Coastal Waters.
  London, England: Fishing News Ltd., 1972.
 
Missouri Conservation Department.   Fish Farming:  What You Should
  Know. Jefferson City, Missouri: Missouri Conservation Department,
  1981.
 
Ouasim, S.Z.  "Sea Farming: An Appropriate Technology for
  Generating Sea Food,"  Appropriate Technology, Volume 6, 1979,
  pp. 26-28.
 
Shapiro, S.  Our Changing Fisheries.  Washington, D.C.:  U.S.
  Government Printing Office, 1971.
 
Tyther, J.H.  "Mariculture:  Potential Protein for the Third
  World,"   The Commercial Fish Farmer.   Little Rock, Arkansas:
  Catfish Farmers of America.
 
U.S. Department of Agriculture.   The Yearbook of Agriculture.
  Washington, D.C.:  U.S. Department of Agriculture, 1978.
 
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