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During the 1990s, the world's total production of fish and shellfish
(molluscs and crustacea) expanded rapidly from 99 million tonnes in 1990
to about 122 million tonnes by 1997. Whilst capture fisheries production
increased only slightly, output from aquaculture (farmed fish, shellfish
and algae) grew strongly from just over 13 million tonnes in 1990 to 28.3
million tonnes in 1997. Movement of live fish and shellfish between countries
has been a necessity for much of this expansion in aquaculture, but has contributed
significantly to the occurrence and spread of economically-serious diseases in
the industry.
Fish, shellfish and fishery products are widely traded and
the international trade has continued to grow at an accelerating
rate in recent years. In 1996, some 195 countries exported part
of their production and export volumes reached 22 million tonnes,
nearly three times the volume traded in 1976. This international
trade accounted for about 40% of total production from capture
fisheries and aquaculture and has been estimated to have a value
of US$52.5 billion, with developing countries achieving a net trade
surplus of US$16.6 billion (FAO). Aquaculture (the farming of fish,
shellfish and algae) contributed over 20% of the global fisheries
production (and 29% of food fish). Whilst output from capture fisheries
continues to flatten out, aquaculture production continues to grow at
approximately 9% p.a. It is estimated by FAO that global demand for
food fish by the year 2010 will be in the range of 110-120 million
tonnes from the level of 90 million tonnes in 1996. There is considerable
potential for further expansion of aquaculture and under favourable
conditions it is estimated that total aquaculture production could
rise from the level of 26 million tonnes in 1996 to 40 million tonnes by 2010.
In many countries, the rapid increase in aquaculture output has been based on
species diversification which has led to an increase in the demand for introduction
and transfers of live non-indigenous aquaculture animals. Although local pathogens,
inadequate farm-management, environmental factors and poor water quality continue to
be the most common causes of disease outbreaks in farmed fish and shellfish, pathogen
transfer due to international trade in live aquaculture animals and their products is
a major underlying reason for major epizootics. Disease outbreaks cause significant
losses in aquaculture production and trade and are affecting economic development of
some countries. An indication of the magnitude of economic loss is seen in the fact
that the value of losses in shrimp farm output in China in 1993 due to one introduced
disease has been calculated to be about US$400million, and that in recent estimates,
based on farm surveys in 16 Asian countries, annual losses due to disease in the
region total more than US$3.0 billion.
Probably the most striking example of spread of disease and consequential
major economic loss in aquaculture is white spot disease in farmed shrimp.
The disease first emerged in 1991 or early 1992 in a shrimp farm in Taiwan and
was observed later in 1992 in mainland China. There is some evidence that the
virus which causes the disease was introduced to the mainland with trade in live
post-larvae of shrimp for on-growing. From Taiwan, thereafter, the disease spread
rapidly along the coast of mainland China and in the same year appeared in Japan,
Korea and India. In 1994, the disease appeared in Thailand and subsequently
spread to all shrimp farming regions of the country and has since been reported
in most other shrimp farming countries of Asia, including Indonesia, Malaysia,
Sri Lanka, Bangladesh and Vietnam. Annual conomic losses have been estimated to
be in the range of more than US$ 400 million in China (1993), US$ 17.6 million
in India (1994), and over US$ 500 million in Thailand (1996). The Philippines
remain free of WSD and Australia continues to be free of the disease almost certainly
due to strict controls on importation of live shrimp and uncooked dead shrimp for use
as fish bait. In the mid-1990s the disease was detected in the USA and during 1999 and
2000 the disease flared up in all shrimp farming countries on the Pacific coast from
Mexico to Peru. The socio-economic effects of the disease in the region have been
dramatic - for example, in Ecuador there were losses of US$ 280 million and 150,000
jobs lost in the sector in 1999 alone. The source of the WSD introduction to the
Americas from Asia has not been identified with certainty, but trade in live shrimp
for hatcheries and dead shrimp for processing or for bait are thought to be the most
likely routes. The current global estimate of economic loss due to this disease is US$
3000 million/year.
Serious diseases of molluscs and fish have also been spread between countries
by international trade. Examples are the diseases Bonamiosis and Haplosporidiosis,
which are believed to have been introduced into France and the east coast of the USA
respectively via importation of live oysters for farming. These diseases have caused
major reductions in the production of the native species of oysters in the affected areas.
In fish, one of the most ecologically serious disease introductions has been the transfer of
the parasite Gyrodactylus salaris to Norway via importations of live juvenile salmon from
hatcheries in Sweden for stock enhancement of Norwegian rivers. The result was a rapid
and severe reduction in the highly sensitive local wild salmon populations in affected
rivers and draconian steps have been taken by the Norwegian authorities to eradicate
the parasite from almost 30 rivers by destruction of all resident fish stocks.
Such introductions of damaging diseases has inevitably led some countries to introduce
internal controls on transfers of live and dead aquatic species and also stricter
control on imports. Whilst quarantine and health certification programmes are a
valid part of a first line of defence against introduction or transfer of exotic
fish and shellfish diseases, they must be developed within the context of larger
national and international standards addressing this problem, and must not be used
as an unjustified barrier to trade. The OIE International Aquatic Animal Health
Code provides the only international standards recognised under the SPS Agreement
of the World Trade Organisation for health certification requirements for international
trade in fish and shellfish. These standards, along with national measures to reduce the
risk of importation of disease by means which are scientifically as well as politically
justified, will be briefly discussed.
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