Effects of Aquaculture on World Fish
Supplies
by Rosamond L. Naylor, Rebecca J. Goldburg, Jurgenne Primavera, Nils Kautsky,
Malcolm C. M. Beveridge,
Jason Clay, Carl Folke, Jane
Lubchenco, Harold Mooney, and Max Troell
SUMMARY
Global production of farmed fish,
shrimp, clams, and oysters more than doubled in weight and value during the
1990s while landings of wild-caught fish remained level. Many people look to this growth in
aquaculture to relieve pressure on ocean fish stocks, most of which are now
fished at or beyond capacity, and to allow wild populations to recover. Production of farmed fish and shellfish
does increase world fish supplies.
Yet by using increasing amounts of wild-caught fish to feed farmed shrimp
and salmon, and even to fortify the feed of herbivorous fish such as carp, some
sectors of the aquaculture industry are actually increasing the pressure on
ocean fish populations.
The available scientific evidence indicates that some types of
aquaculture are on a destructive path that poses a threat not only to wild fish
stocks but also to the industry’s own long-term potential. One of the most disturbing trends is the
rapid expansion and intensification of shrimp and salmon farming and culture of
other high-value carnivorous marine fish such as cod, seabass, and tuna. Production of a single kilogram of these
species typically uses two to five kilograms of wild-caught fish processed into
fish meal and fish oil for feed.
Besides this direct impact on wild fish stocks, some aquaculture as
currently practiced degrades the marine environment and diminishes the
ecological life support services it provides to fish, marine mammals, and
seabirds, as well as humans. These
impacts include
• Destruction of
hundreds of thousands of hectares of mangrove forests and coastal wetlands for
construction of aquaculture facilities
• Use of wild-caught
rather than hatchery-reared finfish or shellfish fry to stock captive
operations, a practice that often leads to a high rate of discarded bycatch of
other species
• Heavy fishing
pressure on small ocean fish such as anchovies for use as fish meal, which can
deplete food for wild fish such as cod, as well as seals and
seabirds
• Transport of fish
diseases into new waters and escapes of non-native fish that may hybridize or
compete with native wild fish
As aquaculture production continues to expand and intensify, both its
reliance and its impact on ocean fisheries are likely to increase. The balance
between farmed and wild-caught fish, as well as the total supply of fish
available for human consumption, will depend on future trends in aquaculture
practices. If the goal of aquaculture is to produce more fish for consumers than
can be produced naturally, then it will become increasingly counterproductive to
farm carnivores that must be fed large amounts of wild-caught fish that form the
foundation of the ocean food chain.
Indeed, non-carnivorous species such as marine mollusks and carps account
for most of the current net gain in world fish supplies from
aquaculture.
Without clear recognition of its dependence on natural ecosystems, the
aquaculture industry is unlikely to develop to its full potential or continue to
supplement ocean fisheries. We
recommend the adoption of four priority goals for
aquaculture:
• Encourage farming
of species lower on the food web – that is, fish with herbivorous or omnivorous
diets or filter feeders such as oysters
• Improve feed
management and efficiency in industrial aquaculture systems and develop
substitutes for fish-derived feed ingredients
• Develop
integrated fish farming systems that use multiple species to reduce costs and
wastes while increasing productivity
• Promote
environmentally sound aquaculture practices and resource management
Governments have a key role
to play in developing regulations to protect coastal ecosystems and in
reexamining subsidies to unsustainable marine fisheries. Development agencies are strategically
placed to help in developing and implementing sustainable production practices
and in financing otherwise economically and socially unattainable reforms in
developing countries. If public and private interests act jointly to reduce the
environmental costs generated by fish farming, present unsustainable trends can
be reversed and aquaculture can make an increasingly positive contribution to
global fish supplies.
INTRODUCTION
Global production of farmed fish and shellfish has more than doubled in
weight and value during the past 15 years, growing from 10 million metric tons
or megatons (Mt) in the late 1980s to 29 Mt in 1997. Meanwhile, harvests of ocean fish have
remained at around 85 to 95 Mt, and there is wide acknowledgment that most wild
fish stocks are either over-fished or fished at maximum capacity. Today aquaculture — the farming of fish,
shrimp, clams, and oysters — supplies more than one-fourth of all fish that
humans eat. Many people believe
continued growth in aquaculture will relieve pressure on deteriorating wild fish
stocks, allowing their populations to recover while supplying an ever-increasing
demand for protein to nourish a growing human population.
Current trends in the aquaculture industry, however, do not support that
belief. As practiced today,
aquaculture is a mixed blessing for the sustainability of ocean fisheries. The
diversity of production systems leads to an underlying paradox: aquaculture is a
possible solution, but also a contributing factor, to the collapse of fisheries
stocks worldwide.
The farming of carnivorous species such as salmon and shrimp, for
example, requires vast quantities of wild-caught fish to feed confined stocks —
indeed, the norm is that two to five kilograms of wild fish biomass are required
to produce a single kilogram of these high-market-value species. Confining large numbers of fish in
coastal waters, especially in mangroves and wetlands, can also degrade the
marine environment and threaten wild species by destroying nursery habitat,
generating large quantities of nutrients and other wastes, importing diseases
that can spread to wild fish, or allowing exotic species to escape and thus
compete or hybridize with wild fish.
In contrast, the farming of species such as carp and tilapia that can eat
aquatic plants, or oysters, clams, and mussels that filter plankton from the
water, can make a large contribution to global fish supplies and food
security. However, the trend toward
industrial-scale production of carp and other herbivores — and omnivores such as
tilapia, catfish, and some varieties of shrimp — has led to increasing use of
manufactured feed that incorporates fish meal and fish
oil.
Despite the surge in production of farmed fish, the tonnage of wild fish
harvested has not declined.
Moreover, as catches of large, valuable carnivorous fish such as cod and
haddock have decreased, there has been a gradual shift to harvest of smaller,
less valuable species such as anchovy — species destined, in fact, to be ground
into fish meal or fish oil for use in manufacturing feed for livestock and
farmed fish. Between 1986 and 1997,
four of the top five, and eight of the top 20 wild species harvested from the
ocean were small fishes used in production of animal feed: anchoveta, Chilean jack mackerel,
Atlantic herring, chub mackerel, Japanese anchovy, round sardinella, Atlantic
mackerel, and European anchovy.
As aquaculture production continues to increase and intensify, both its
reliance and impact on ocean fisheries are likely to expand even further. The future balance between farmed and
wild-caught fish, the total supply of fish available for human consumption, and
the very health of the marine environment will depend on trends in aquaculture
practices.
AQUACULTURE IS A DIVERSE ACTIVITY
Three-fourths of global aquaculture production by weight involves finfish
and shellfish; the other fourth is seaweed. Worldwide, more than 220 species of
finfish and shellfish are farmed.
The range of species includes giant clams that obtain most of their
nourishment from symbiotic algae, mussels that filter plankton from the water,
carps that largely graze on plants, and salmon that prey on smaller fish (Figure
1). Typically, the farmed species
are enclosed in a secure system such as a pond or floating pen in which they can
be raised under suitable conditions, sheltered from predators and competitors,
and sometimes fed and medicated with antibiotics and other drugs. As the intensity of an aquaculture
operation increases, fish are confined at higher densities, supplied with all
nutritional requirements, and managed more heavily. The more intensive the operation, of
course, the larger the volume of wastes generated and the greater the
possibilities for the spread of disease.
From one aquaculture operation to another, the intensity of culture
practices and their impacts on marine ecosystems vary widely (Figure 2). Clams, oysters, and other mollusks are
generally farmed along coastlines, with wild-caught or hatchery-reared seed
grown on the sea floor or on suspended nets, ropes, or other structures. The animals feed entirely on ambient
supplies of plankton and organic particles in the water. Finfish may be farmed in ponds, tanks,
or cages. Most marine fish and
species such as salmon that migrate between fresh and salt water are reared in
floating net cages near shore, and all their nutrition is supplied by formulated
feeds. Carp, catfish, and other
freshwater finfish are usually grown in ponds, often integrated within
agricultural settings.
Crustacean farming is dominated by shrimp, which are grown in coastal
ponds. Farming of both shrimp and
freshwater finfish varies greatly from one operation to another in intensity and
in reliance on formulated feeds.
In the past decade, two distinct sectors have emerged within this diverse
industry. The first includes commercial farms that rely on intensive and
semi-intensive methods to produce commodities for regional or global
markets. The second encompasses
family and cooperative farms that rely on less intensive practices to produce
low-value species for household subsistence or local markets. The line between these sectors is
growing more blurred, however. In
China and other parts of Asia, for example, many small-scale farming operations
are intensifying as land and water resources become increasingly scarce and
valuable.
Asia produces roughly 90 percent of global aquaculture output, and China
alone contributes more than two-thirds of the total. Although Europe, North
America, and Japan together produce just over one-tenth of the global total,
these regions consume the bulk of farmed seafood that is traded internationally.
Various species of carp dominate the tonnage of farmed fish produced
worldwide, and carp production for local or regional use by relatively
low-income households has increased dramatically in Asia (mainly China). In
contrast, increased volumes of salmon, shrimp, and other high-value species are
marketed mainly in industrialized countries. Farmed output and markets for other
lower-value species such as tilapia and milkfish have increased in both
developing and industrialized countries. Most farmed mollusks are still
consumed locally and regionally in China and in other developing countries. However, production of certain species
for global markets has increased in several developed countries. These species include the Pacific cupped
oyster, blue mussel, New Zealand mussel, and Yesso
scallop.
FEEDING FISH TO FISH
Many intensive and semi-intensive aquaculture systems use two to five
times more fish protein, in the form of fish meal and fish oil, to feed the
farmed animals than is produced in the form of farmed fish. By contrast, so-called extensive or
traditional aquaculture systems use little or no fish meal or fish oil, although
operators often add nutrient-rich materials such as crop wastes to the water to
stimulate growth of algae and other naturally available organisms on which the
fish feed.
Worldwide, about 80 percent of carp and 65 percent of tilapia are farmed
without the use of modern compound feeds – that is, feeds formulated from
multiple ingredients. In China,
however, farmed production of carp and other omnivorous species is intensifying,
and new commercial feed mills are being developed to serve this industry. China is also the largest importer of
fish meal in the world. Such
intensive systems, including U.S. catfish farms, must rely heavily on added
feeds because fish are stocked at higher densities than can be supported by
natural food sources. Generally
these operations use compound feeds that contain high percentages of protein
supplements from soybean meal, cottonseed meal, and peanut meal. But compound
feeds for herbivorous and omnivorous fish can also contain low to moderate
levels of protein obtained from fish and terrestrial animals.
By contrast, fish meal and fish oil are dominant ingredients in compound
feeds for carnivorous fish and shrimp. These two ingredients supply
essential amino acids (that is, lysine and methionine) that are deficient in
plant proteins and fatty acids (eicosapentaenoic acid [EPA] and docosahexaenoic
acid [DHA], known as n-3 fatty acids) not present in vegetable oils. The fish oil and protein also provide
energy, which is important because fish tend to be poor at using carbohydrates
for energy.
All fish, whether omnivorous, herbivorous, or carnivorous, require about
the same quantity of dietary protein per kilogram. But freshwater herbivores and omnivores
such as carp, tilapia, and catfish are better than carnivores at using
plant-based proteins and oils, and consequently, they need only minimal
quantities of fish meal to supply essential amino acids. Nevertheless, compound feeds for tilapia
and other omnivorous fish often contain about 15 percent fish meal — much more
than required. Indeed,
manufacturers often over-formulate feeds, in part because information on the
dietary requirements for particular fish species is inadequate.
Because of these high levels of fish meal and fish oil in aquaculture
feeds, it takes more fish biomass to raise some farmed species than those
species produce. For the ten types
of fish most commonly farmed, for instance, an average of 1.9 kilograms of wild
fish are required for every kilogram of farmed fish produced using compound
feeds (Figure 3). The highest
inputs of wild-caught fish — more than five kilograms for each kilogram produced
— are used in raising marine fish such as flounder, halibut, sole, cod, hake,
haddock, redfish, seabass, congers, tuna, bonito, and billfish. Many salmon and shrimp operations use
roughly three kilograms of fish biomass for each one produced (Figure 4).
Only three of the ten types of fish most commonly farmed — catfish,
milkfish, and carp — use less fish as inputs than is ultimately harvested. (Marine mollusks and many filter-feeding
carp are not fed compound feeds at all.)
Aquaculture is not the world’s largest consumer of fish meal. That distinction belongs to the poultry
and swine industries. Aquaculture,
however, has the fastest growing demand for fish meal and fish oil. Its share of
fish meal supplies rose from 10 percent in 1988 to 17 percent in 1994 and 33
percent in 1997. Also, the
proportion of fish meal in aquaculture feeds is much higher than in poultry and
livestock feeds, which contain an average of only 2 to 3 percent fish meal as a
protein supplement. The production
of a kilogram of pork or poultry typically uses large amounts of plant proteins,
but only a few hundred grams of fish, whereas production of a kilogram of
carnivorous fish can use up to five kilograms of wild
fish.
Some aquaculture proponents argue that even if farmed fish production
requires more wild fish biomass than is ultimately harvested, it is still more
efficient than the making of big fish from little fish in the wild. In other
words, even if it takes several kilograms of wild-caught fish to grow one
kilogram of salmon or cod in captivity, these and other carnivorous fish species
would consume at least that amount of smaller fish if they grew to maturity in
the wild. Whether natural predation
or captive feeding is more energy efficient is an unsettled scientific question
that involves calculations of energy flows in wild food webs. It is reasonable
to believe that farmed fish operations are somewhat more efficient since captive
fish are protected from some types of mortality as they grow. Regardless of the outcome of the
efficiency debate, however, it is clear that the growing aquaculture industry
cannot continue to rely on finite stocks of wild-caught fish, many of which are
already classified as fully exploited, overexploited, or depleted. Taking ever-increasing amounts of small
fish from the oceans to expand the total supply of commercially valuable fish
would clearly be disastrous for marine ecosystems and, in the long term, for the
aquaculture industry itself. If the goal of aquaculture is to produce more fish
for consumers than can be produced naturally, then it will become increasingly
counterproductive to farm carnivores that must be fed large amounts of
wild-caught fish that form the foundation of the ocean food
chain.
NET INCREASE IN FISH SUPPLIES FROM
AQUACULTURE
Clearly, the feed requirements for some types of aqua-culture systems
place a strain on wild fish stocks.
But does farmed fish production overall represent a net gain to global
fish supplies? Our calculations
indicate it does, but most of that gain in fish supplies from aquaculture comes
from carps, marine mollusks, and other mostly herbivorous
species.
Global harvest of wild fish and aquatic plants removes 123 Mt from seas
and lakes each year, and 27 Mt of this is directly discarded as bycatch (Figure
5). Without the bycatch, fisheries
landings amount to 96 Mt, of which 65 Mt of whole fish and 1 Mt of seaweeds are
consumed by humans. The remaining
30 Mt of fish catch plus another 2 Mt of processing scraps from aquaculture and
fisheries are used for fish meal production.
(The fish meal industry has proposed that fishing vessels be encouraged
to retain the currently discarded bycatch for sale to producers of fish meal and
fish oil. Sale of bycatch could
prove undesirable, however, if it undermines efforts to reduce bycatch rates or
decreases the return of bycatch to the waters from which it was taken.)
One-third of the fish used to make fish meal, about 10 Mt, is currently
converted to aquaculture feeds, while the remaining 22 Mt goes into fish meal
for chicken, pig, and other livestock feeds. The use of these wild-caught fish for
feeds reduces supplies of wild fish that could potentially be consumed directly
by people. In Southeast Asia, for
example, small open ocean fishes such as mackerel, anchovy, and sardines supply
an important protein source for local people. Although some fish utilized for fish
meal and fish oil, such as menhaden, are distasteful to humans or are worth more
as fish meal and oil than as food for consumers, the demand for small ocean fish
for direct human consumption is likely to increase with population growth in the
developing world.
Finally, total aquaculture production of finfish, crustaceans, and
mollusks amounts to 29 Mt. However,
after the 10 Mt of wild-caught fish going into fish feed is subtracted, the net
volume of fish provided for human consumption via aquaculture is 19 Mt.
Carps and marine mollusks account for more than three-fourths of current
global aquaculture output, and tilapia, milkfish, and catfish contribute another
5 percent. These species, fed
mostly herbivorous diets, account for most of the 19 Mt gain in fish supplies
from aquaculture.
ECOLOGICAL IMPACTS OF
AQUACULTURE
The use of wild fish to feed farmed fish directly impacts ocean
fisheries. But aquaculture can also
diminish wild fisheries indirectly by habitat modification, collection of wild
seedstock, changes in ocean food webs, introduction of non-native fish species
and diseases that harm wild fish populations, and nutrient pollution (Figure 6).
The magnitude of such impacts
varies considerably among different types of aquaculture systems, but it can be
severe.
Habitat Modification
Hundreds of thousands of hectares of mangroves and coastal wetlands
around the world have been transformed into milkfish and shrimp ponds (Figure
7). This transformation results in
direct loss of essential ecological services that mangroves provide, including
nursery habitat for juvenile fish and shellfish, protection of the coast from
battering storms and typhoons, flood control, trapping of sediments, and
filtering and cleansing of nutrients from the water.
Mangrove forests provide food and shelter to many juvenile finfish and
shellfish that are later caught as adults in coastal and offshore
fisheries. In Southeast Asia,
mangrove-dependent species account for roughly one-third of yearly wild fish
landings, excluding trash fish. In Indonesia, Malaysia, and the Philippines,
catches of finfish and shrimp increase with mangrove forest area. Healthy mangroves are also closely
linked to the condition of coral reefs and seagrass beds. As mangrove forests are lost, more
sediment runoff is carried onto and can smother downstream coral reefs and
seagrass beds. The degradation of
these biologically rich systems, in turn, affects fish harvest: fish caught from
reefs contribute about 10 percent of fish humans consume globally, and the
proportion is much higher in developing countries.
Conversion of coastal habitats into shrimp farms can lead to large losses
in wild fisheries stocks. In
Thailand, where shrimp farms have been carved out of mangrove forests, we
estimate that a total of 400 grams of wild fish and shrimp are lost from
nearshore catches for every kilogram of shrimp farmed. In addition, if other
fish and shellfish species caught from waterways adjoining mangrove areas are
considered, the total reduction increases to 447 grams of wild fish biomass per
kilogram of shrimp raised. If
the full range of ecological effects associated with mangrove conversion is
taken into account, including reduced mollusk productivity in mangroves and
losses to seagrass beds and coral reefs, the net yield from these shrimp farms
is low — even without considering the use of fish meal in aquaculture feeds for
shrimp. Moreover, building
aquaculture ponds in mangrove areas transforms fisheries from a common property
resource available for use by numerous local people — including subsistence
fishermen — into a privatized farm resource that benefits a small number of
investors.
Use of Wild-Caught Seedstock
Many aquaculture operations, especially extensive ponds, stock
wild-caught rather than hatchery-reared finfish or shellfish fry. Examples include farming of milkfish in
the Philippines and Indonesia, tuna in South Australia, shrimp in South Asia and
parts of Latin America, and eels in Europe and Japan. In these systems, aquaculture is not a
true alternative to wild harvests, but rather a means to raise wild fish to
marketable size in captivity by reducing the high mortality rates characteristic
of wild populations.
Collection of seed-stock for aquaculture operations can have very large
consequences for wild fisheries if it results in high bycatch rates. For example, milkfish constitute only 15
percent of total finfish fry collected inshore by seine net — the remaining 85
percent of fry are discarded and left to die on the beach. Thus the capture of
the 1.7 billion wild fry stocked annually in Philippine milkfish ponds results
in destruction of more than 10 billion fry of other finfish species. In India and Bangladesh, up to 160 fish
and shrimp fry are discarded for every fry of giant tiger shrimp collected to
stock shrimp ponds. The magnitude
of annual fry bycatch has been estimated at somewhere between 62 million and 2.6
billion in three collecting centers in West Bengal, India.
Changes in Ocean Food
Webs
Stocks of some small ocean fish exploited for fish meal are over-fished,
and their populations fluctuate sharply during the climate shifts brought on by
El Nino-Southern Oscillation events.
In seasons when these stocks are depleted, available food supplies for
commercially valuable marine predators such as cod and also marine mammals and
seabirds decline. In the North Sea,
for example, over-exploitation of many capelin, sandeel, and Norway pout stocks,
largely for production of fishmeal, has been linked to declines of other wild
fish such as cod and also changes in the distribution, populations sizes, and
reproductive success of various seal and seabird colonies. Similarly, off the coast of Peru,
scientists have documented a strong interaction between anchoveta stocks and the
size of sea bird and mammal populations.
Introduction of Non-Native Fish and
Pathogens
Aquaculture can also affect stocks of wild fish by allowing escapes of
non-native species and by spreading diseases among both farmed and wild
fish. Scientists call these
introductions of non-native organisms “biological pollution.”
Atlantic salmon — the dominant salmon species farmed worldwide —
frequently escape from net pens. In
some areas of the North Atlantic Ocean, as much as 40 percent of Atlantic salmon
caught by fishermen is of farmed origin.
In the North Pacific Ocean, more than a quarter million Atlantic salmon
have reportedly escaped since the early 1980s, and Atlantic salmon are regularly
caught by fishing vessels from Washington to Alaska. Increasing evidence suggests that farm
escapees may hybridize with and alter the genetic makeup of wild populations of
Atlantic salmon, which are genetically adapted to their natal spawning
grounds. This type of genetic
pollution could exacerbate the decline in many locally endangered populations of
wild Atlantic salmon. In the
Pacific Northwest, there is evidence that escaped Atlantic salmon now breed in
some streams, perhaps competing for spawning sites with beleaguered wild Pacific
salmon.
Movement of captive fish stocks for aquaculture purposes can also
increase the risk of spreading pathogens.
The relationships between farmed and wild fish and disease transfer are
complex and often difficult to disentangle. In Europe, however, serious epidemics of
furunculosis and Gyrodactylus salaris in stocks of Atlantic salmon have
been linked to movements of fish for aquaculture and
re-stocking.
Since the early 1990s, the Whitespot and Yellowhead viruses of shrimp
have caused catastrophic, multimillion-dollar crop losses in shrimp farms across
Asia. Both pathogens have recently
appeared in farmed and wild shrimp populations in the United States, and the
Whitespot virus has been reported in several countries in Central and South
America. In Texas shrimp farms, the Whitespot virus has caused high mortalities,
and the disease may also kill wild crustaceans. This virus is thought to have
been introduced into a Texas shrimp farm by release into nearby coastal waters
of untreated wastes from plants processing imported Asian tiger shrimp, and also
by shipping of contaminated white shrimp larvae throughout the
Americas.
Nutrient Pollution from Aquaculture
Wastes
Untreated wastewater laden with uneaten feed and fish feces may
contribute to nutrient pollution near coastal fish ponds and cages, especially
when these are situated in or near shallow or confined water bodies. Such
pollution also can be severe in regions where intensive aquaculture systems are
concentrated. In many such areas,
buildup of food particles and fecal pellets under and around fish pens and cages
interferes with nutrient cycling in seabed communities. And when quantities of nitrogen wastes
such as ammonia and nitrite are greater than coastal waters can assimilate,
water quality can deteriorate to a level that is toxic to fish and shrimp.
Aquaculture managers clearly have a stake in regulating nutrient
pollution since poor water quality and high stocking densities often promote
outbreaks of disease and lead to declines in farmed fish production. While waste
problems have been widely discussed, however, current management solutions are
largely limited to controlling the intensity of fish production by reducing
stocking and feeding levels rather than treating wastes.
TOWARD SUSTAINABLE AQUACULTURE
Production of farmed fish and shellfish currently adds to net global fish
supplies, although many types of aquaculture result in a net loss of fish. Rapid
growth in this net-loss sector is severely limiting the potential contribution
of aquaculture to future world food supplies. The benefits of aquaculture, and indeed
the potential growth of the industry itself, are diminished by escalating
production of species fed carnivorous diets and by aquaculture practices that
lead to coastal habitat destruction, biological pollution, and discharge of
untreated fish wastes into some of the world’s most diverse and productive
marine habitats. Continued
expansion of aquaculture will require healthy coastal and freshwater
ecosystems. Without clear
recognition by the industry of its dependence on natural ecosystems, aquaculture
is unlikely to develop to its full potential or continue to supplement ocean
fisheries. We therefore
suggest that governments and development agencies, as well as the aquaculture
industry and its trade organizations, adopt four major priorities: 1) expansion of the farming of
non-carnivorous fish; 2) reduction of fish meal and fish oil inputs in feed; 3)
development of integrated farming systems that use multiple species to reduce
costs and wastes and increase productivity; and 4) promotion of environmentally
sound aquaculture practices and resource management.
Farming Lower on the Food Web
Farmed fish species fed mainly on herbivorous diets account for most of
the 19 Mt gain in fish supplies that aquaculture now provides to the world. Carps and marine mollusks make up 75
percent of current global aquaculture output, and tilapia, milkfish, and catfish
contribute another 5 percent. But
market forces and government policies in many countries favor rapid expansion in
production of high-value, carnivorous species, such as salmon and shrimp.
Globally, these species represent only 5 percent of farmed fish by weight, but
almost 20 percent by value.
In
addition, fish meal and fish oil are increasingly being added to carp and
tilapia feeds to boost weight gain, especially in Asia where farming systems are
intensifying as a result of the increased scarcity and value of land and
freshwater resources. Given the huge volume of farmed carp and tilapia in Asia,
significant increases in the fish meal and fish oil content of feed would place
even more pressure on open ocean fisheries, resulting in higher feed prices as
well as harm to marine ecosystems.
We
believe new initiatives by governments and international donor agencies are
needed to further encourage farming of species lower on the food web — that is,
fish with herbivorous diets. At the
same time, we believe more scientific research on the feed requirements of
herbivores and omnivores is required to lessen the drive to add fish meal and
fish oil to their feeds.
Reducing Fish Meal and Fish Oil in Fish
Feed
The cost of purchasing feed is the largest production expense for
commercial aquaculture, including most farming of salmon, other marine finfish,
and shrimp. Moreover, the price of
fish meal relative to other protein substitutes has risen in real terms in the
past few decades and is likely to continue to escalate as demand grows. Increases in the prices of fish meal and
fish oil could undermine the profitability of many aquaculture enterprises. For
these reasons, research to improve feed efficiency in industrial systems is
already a priority in the aquaculture industry.
Efforts to develop substitutes for fish-derived feed ingredients are now
focused on commodities such as oilseeds (especially soybeans), meat byproducts
(such as blood meal and bone meal), and microbial proteins. Already the fish
meal content of some feeds — for example, feed for salmon — has been reduced
considerably, albeit largely by substituting cheaper fish oil for fish
meal. Nevertheless, severe barriers
exist to complete replacement of fish meal and fish oil in aquaculture feeds,
especially for carnivorous fishes, because vegetable proteins have inappropriate
amino acid balance and poor protein digestibility.
We
believe more scientific research is also needed on the feed requirements of
herbivores and omnivores in order to reverse the trend toward adding fish meal
and fish oil to their feeds.
Substituting vegetable oils for fish oils in freshwater fish diets is
technically possible since the n-3 fatty acids found in fish oil are not
essential in the diets of these species.
However, some herbivorous fish appear to have more robust immune systems
when fish oil is included in their diet.
In
addition, substitution of fish oil with cheaper vegetable oil in aquaculture
feeds may also affect the fatty acid profile and thus flavor and marketability
of the fish to consumers. Evidence
suggests that the ratio of n-6 to n-3 fatty acids in human diets is already too
high. There are, however,
alternatives to finfish as sources of n-3 fatty acids for humans, including
mollusks and other types of seafood, and research is underway to increase the
n-3 fatty acid content in poultry products and in oilseeds used for feed.
A
move toward partial substitution of plant and terrestrial animal proteins for
fish proteins now used in feed is widely accepted as necessary within the
aquaculture industry, yet there is disagreement over the urgency of such a move.
Because over-exploitation of ocean fisheries has negative ecological and social
consequences, developing a strategy to replace fish meal and fish oil in feeds
should be a priority for governments and development organizations as well as
industry.
Integrating Production
Systems
The farming of multiple species in a single pond — polyculture — was
practiced for centuries before the advent of industrial-scale aquaculture. Even
today, four of the most widely cultivated fish species are sometimes produced
together in the same ponds in China: silver carp (a phytoplankton filter
feeder), grass carp (a herbivore that grazes aquatic plants), common carp (an
omnivorous bottom feeder that eats detritus), and bighead carp (a zooplankton
filter feeder). This type of system
efficiently uses food and water resources from all levels of the pond ecosystem,
thereby reducing costs and wastes while increasing
productivity.
Integrated systems can also be used for high-value fish, such as salmon
and shrimp, in order to reduce waste outputs, diversify products, and increase
productivity. Some studies show
that seaweed and mussels grow well in wastewater from intensive and
semi-intensive aquaculture systems, and as a result, reduce nutrient and
particulate loads to the environment.
In Chile, for example, salmon can be farmed along with a type of red alga
that removes large amounts of dissolved nitrogen and phosphorous wastes from
salmon cages. The effluent output
from salmon farming is thus used to nourish a seaweed crop, and the added
revenue from the sale of the seaweed more than pays for the extra infrastructure
needed for the integrated system.
If government policies required fish farms to internalize the
environmental costs of waste discharges — that is, by making sewage treatment
mandatory — then integrated systems that reduce the waste stream would be even
more profitable. Some caveats
apply: Human health considerations
now limit the marketability of mollusks raised in the waste stream from
intensive fish farming areas, and such concerns must be addressed in order to
make these types of integrated systems economically viable.
Promoting Sustainable Aquaculture
Long-term growth of the aquaculture industry depends on both ecologically
sound practices and sustainable resource management. Governments can encourage
such practices by stringently regulating the creation of new farming facilities
in mangroves and other coastal wetlands, establishing fines to minimize escapes
of fish from aquaculture pens, enforcing strict disease control measures for the
movement of stock, and mandating effluent treatment and in-pond recirculation of
wastewater. Many aquaculture
operations have adopted such practices even in the absence of strict government
policies, especially with the heightening of environmental concerns in recent
years. In poor countries, however,
such policies are often neither politically enforceable nor economically and
socially feasible.
Despite significant improvements in the industry, many ecologically sound
technologies remain on the shelf and underused in the field. This is an arena where external funding
agencies such as development banks can play a strategic role by encouraging the
development and financing the implementation of sustainable aquaculture
technologies, the rehabilitation of ecosystems degraded by aquaculture, and the
protection of coastal ecosystems.
Whether aquaculture depletes or enhances net fish supplies in the future
will depend to a large extent on how markets for resources are managed. The absence of regulations or price
disincentives on coastal pollution by fish farms, for example, limits mollusk
farming and slows the adoption of non-polluting technologies by other marine
aquaculture systems. Furthermore,
government subsidies to the ocean fisheries sector often prevent farmed fish
from undercutting the market for wild-caught fish, at least until ocean
fisheries are fully depleted. Whether farmed fish can replace or provide market
alternatives for ocean catches will depend significantly on the economics and
policies of fisheries in various nations.
High fixed costs of fishing fleets, labor considerations, and continued
subsidies to the ocean fisheries sector — subsidies that currently approach 20
to 25 percent of gross fisheries revenue globally — may prevent increased
aquaculture production from lowering catches of wild fish in the short
term. In the case of salmon, for
instance, increased farm production did not result in reduced capture levels
despite 30 to 50 percent declines in the international prices for four of the
five main species of wild salmon (chinook, coho, pink, and chum) during the
1990s. Salmon catches worldwide
actually rose by 27 percent between 1988 and 1997. Similarly, despite rapid growth in
alternative farmed fish such as tilapia, wild capture of hake and haddock has
remained relatively stable during the past decade.
Finally, perhaps the largest unknown for both the private and public
sectors is the future availability of freshwater sites for aquaculture
production. Increasing scarcity of
freshwater resources could severely limit the farming of herbivorous fish such
as carps and tilapia. This
constraint on the future growth of freshwater systems makes it even more urgent
to develop marine aquaculture systems that are both ecologically and socially
sound.
Mandate for the Future
Aquaculture is an industry in transition, and we will continue to
evaluate trends as the field develops.
Already it is clear, however, that if aquaculture is to fulfill its
long-term potential to enhance global fish supplies and provide food for the
world’s growing population, both public and private sectors must embrace a
shared vision of a sustainable industry.
On the public side, governments can support research and development on
environmentally benign aquaculture systems, eliminate implicit subsidies for
ecologically unsound practices, and establish and enforce regulations to protect
coastal ecosystems. At the same
time, the private sector must alter its course and recognize that current
practices that lead to further pressures on ocean fish stocks, destruction of
coastal habitats, water pollution, and introductions of pathogens and non-native
fish run counter to the industry’s long-term health. If public and private interests act
jointly to reduce the environmental costs generated by fish farming, present
unsustainable trends can be reversed and aquaculture can make an increasingly
positive contribution to global fish supplies. Without this shared vision,
however, an expanded aquaculture industry poses a threat, not only to ocean
fisheries, but also to itself.
ACKNOWLEDGMENTS
The authors thank the David and Lucile Packard Foundation for funding and
M. Williams, W. Falcon, V. Spruill, M. Drew, N. Wada, R. Kautsky, K. Jauncey, C.
Tirado, R. Hoguet, R. Tatum, and R. Mitchell for helpful comments and
assistance.
SUGGESTIONS FOR FURTHER
READING
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About the Panel of
scientists
Dr. Rosamond L. Naylor, Panel Chair, Institute for
International Studies, Stanford University, Stanford, CA,
94305
Dr. Rebecca J. Goldburg, Environmental Defense, 257
Park Avenue South, New York, NY, 10010
Dr. Jurgenne Primavera, Aquaculture Department,
Southeast Asian Fisheries Development Center, Tigbauan, Iloilo, 5021,
Philippines
Dr. Nils Kautsky, Department of Systems Ecology,
Stockholm University and the Beijer Institute, Stockholm,
Sweden
Dr. Malcolm C. M. Beveridge, Institute of
Aquaculture, University of Stirling, Stirling, FK9 4LA, UK
Dr. Jason Clay, World Wildlife Fund, 1250
24th Street NW, Washington, DC 20037
Dr. Carl Folke, Department of Systems Ecology,
Stockholm University and the Beijer Institute, Stockholm,
Sweden
Dr. Jane Lubchenco, Department of Zoology, Oregon
State University, Corvalles, OR, 97331
Dr. Harold Mooney, Department of Biological Sciences,
Stanford University, Stanford University, Stanford, CA,
94305
Dr. Max Troell, Department of Systems Ecology,
Stockholm University and the Beijer Institute, Stockholm,
Sweden
About the Science
Writer
Yvonne Baskin, a science writer, edited the report of the panel of
scientists to allow it to more effectively communicate its findings with
non-scientists.
About Issues in
Ecology
Issues in Ecology is designed to report, in language
understandable by non-scientists, the consensus of a panel of scientific experts
on issues relevant to the environment.
Issues in Ecology is supported by a Pew Scholars in Conservation
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53706
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22203.
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and Evolutionary Biology, Princeton University, Princeton, NJ
08544-1003
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Zoology, Oregon State University, Corvallis, OR
97331-2914
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University of Georgia, Athens, GA
30602-2202
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University of Washington, Seattle, WA
98195
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Frostburg, MD
21532
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