Problems encountered with the introduction or movement of shrimp have recently become a major issue in world shrimp farming. This is particularly the case for introduction of non-indigenous species into new countries. The broad ecological risks and impacts of transboundary movements and introductions of shrimp are still poorly understood, but have to date centred on the unwitting introduction of pathogens/disease (particularly viruses) together with their shrimp hosts. These viruses have the demonstrated ability to spread rapidly through movements of live shrimp and cause serious losses to cultured and wild shrimp populations and severe socio-economic losses to the inhabitants of the countries involved.
Shrimp are particularly susceptible to viral pathogens since they are characterized by persistent viral infections which often produce no gross signs of disease or mortality, which may be unknown and/or difficult to detect, and which can cause severe losses of cultured and wild hosts, once they are unwittingly introduced. Shrimp may also be host to multiple viruses which may be tolerated in one host species or location, but which may cause serious problems in another.
Another problem is that the relative risks of viral transfer through many possible routes of infection remain largely unknown. Introductions of live PL and/or broodstock have been strongly implicated in the majority of cases, whilst frozen shrimp, although often infected, appears to be an unlikely source and are easier to control (Flegel and Fegan, 2002).
Additionally, there are also poorly researched and understood issues dealing with potential loss of biodiversity, the effects of escapes of alien shrimp and pathogens to the wild stocks, marketing issues and the triggering of trade barriers. These issues are discussed in sections 6.2 and 5.3.
These potential problems have resulted in many countries implementing bans or restrictions on the importation of live shrimp (and in some cases frozen shrimp) and in some cases other potential disease vectors such as fresh feeds. There has been much concern regarding the basis and implementation of these bans related to international trade agreements and difficulties in enforcing such limitations.
International trade issues are governed by the World Trade Organization (WTO), the legal and institutional basis for the international trading system. The main objectives of the WTO agreement were to ensure access to markets, promote fair competition and encourage development and economic reform. Aquacultural issues are covered specifically in the "Agreement on the Application of Sanitary and Phytosanitary Measures" (SPS, 1995) and the "Agreement on Technical Barriers to Trade" (TBT).
The SPS agreement attempts to prevent non-tariff trade barriers based on harmonized international standards, guidelines or recommendations where they exist. However, individual governments may take more stringent measures, over and above the guidelines provided by the standard setting authorities (in the case of animal health it is the World Organisation for Animal Health - OIE), provided they have scientific justification (i.e. following an import risk assessment), or if it is shown that international standards do not provide sufficient risk protection. Problems with harmonization of standards may arise if, for example, an importing country refuses permission to import a product from a country with a new or a viral disease listed in the OIE Aquatic Animal Health Code (OIE, 2003) list of viral diseases and the exporting country does not have the mechanism to ensure the product is free from the virus. Under these circumstances, the WTO has agreed to assist developing countries in building their capacities and improving their standards. Settlement of disputes bilaterally is encouraged, but the WTO has its own procedures and impartial bodies are available if this is not possible (Fegan, 2000).
The Paris-based World Organisation for Animal Health (OIE) sets the international standards for animal health measures. Since 1988, it has been involved in developing standards for aquatic animal health, through the Aquatic Animal Health Standards Commission (AAHSD). The OIE is responsible for informing governments of the worldwide aquatic disease situation, coordinating possible surveillance and control measures and harmonizing regulations for trade amongst member countries. The OIE regularly updates the two important documents for aquatic animal health: the Aquatic Animal Health Code (OIE, 2003a) and the Manual of Diagnostic Tests for Aquatic Animals (OIE, 2003b), which are available on the OIE website at http://www.oie.int.
A code of practice for introductions of non-indigenous marine organisms was set by the International Council for the Exploration of the Sea (ICES) in 1973 and revised in 1994 (ICES, 1995). These codes had recommendations in the following areas: recommended procedures for deciding on importations of new species; recommended actions once the introduction has been approved; encouragement for prevention of unauthorised introductions; and recommended procedures for introduced or transferred species already under commercial cultivation.
Member states of FAO adopted a voluntary Code of Conduct for Responsible Fisheries (CCRF) during the FAO Conference of 1995 (FAO, 1995). The CCRF was the result of four years of work following the International Conference on Responsible Fishing in Cancun, Mexico in May 1992. Although voluntary, the provisions of this Code are increasingly included in national legislature and regulatory frameworks, which indicates the national interest in compliance.
Article 9 of the code is on Aquaculture development and Article 9.3.3 states that: "States should, in order to minimize risks of disease transfer and other adverse effects on wild and cultured stocks, encourage adoption of appropriate practices in the genetic improvement of broodstock, the introduction of non-native species, and in the production, sale and transport of eggs, larvae or fry, broodstock or other live materials. States should facilitate the preparation and implementation of appropriate national codes of practice and procedures to this effect".
FAO further issued the "FAO Technical Guidelines for Responsible Fisheries No. 5: Aquaculture Development" in 1997 to provide general advice in support of Article 9 of the CCRF (FAO, 1997).
Based on Article 9.3.3 of the FAO CCRF, a set of regional guidelines were issued by FAO/NACA in 2000 called the "Asia Regional Guidelines on Health Considerations for the Responsible Movement of Live Aquatic Animals". These guidelines were developed through three years of awareness raising and consensus building and were adopted by 21 participating countries in the Asia-Pacific region in Beijing in June 2000. The guidelines were adopted by the ASEAN Fisheries Working Group in Bali in 2001 as an ASEAN (Association of South East Asian Nations) policy document and endorsed by the ASEAN/SEAFDEC (Southeast Asian Fisheries Development Centre) Millennium Conference on Fish for People in 2000 in Bangkok (FAO, 2000; NACA/FAO, 2001; SEAFDEC, 2001).
The USA has had a long history of introductions of transboundary shrimp species, including P. vannamei, P. stylirostris, P. monodon, and P. japonicus, amongst others. With the importation of these species from all over the world, non-indigenous and highly pathogenic viral diseases also entered the cultured and wild stocks of Penaeid shrimp.
For example, IHHNV was introduced to Hawaii with infected P. monodon from Asia (probably the Philippines) in 1981 (Tang et al., 2002). The TSV spread from South (1992) and Central (1994) America to arrive in the USA in Florida in 1994 and Texas and Hawaii in 1995 (Brock et al., 1997). In addition, WSSV arrived in Texas in 1995, at the time this was attributed to imports of frozen P. monodon from Asia, although this is still speculation (Lightner, 1996) (Section 6.3).
In 1988, the US Environmental Protection Agency (EPA) and the Joint Sub-committee on Aquaculture (JSA) (as a facilitator among the various agencies involved with aquaculture) held a Review and Risk Assessment Workshop to conduct a qualitative assessment of the risks associated with shrimp viruses, to evaluate the need for a more comprehensive risk assessment and to identify research needs. Results were passed to state management agencies to help develop regulatory mechanisms for the potential impacts of viral transmission to indigenous populations (Olin, 2001).
Further recommendations were made during the workshop held in 1996 by the specially organized Shrimp Virus Work Group of the JSA. The workshop was aimed at developing an interagency strategy to address the shrimp virus issue, beginning with an assessment of the effects on wild shrimp populations, and initiating the development of a risk assessment to determine actions aimed at averting further viral introductions (JSA, 1997). The viral risk assessment report arising from this was presented at the EPA/JSA Viral Assessment Workshop held in 1998, and concluded that more research is necessary to fully identify the risks of shrimp viruses on wild populations (US Environmental Protection Agency website).
In addition, the "Lacey Act" was formulated to help restrict the movement of named potential pathogens into the United States and into watersheds where the pathogen is not currently found. This act is overseen by the US Department of the Interior, Fish and Wildlife Service and is backed by individual state legislation according to needs and diagnostic assistance from USDA's Animal and Plant Health Inspection Service (APHIS) (US Congress, Office of Technology Assessment, 1996).
Due to the privileged position of Hawaii as being very isolated from most shrimp culture operations and the abundance of clean, essentially virus-free water, it was designated as a broodstock production area, concentrating on genetic improvement and health management (biosecurity), with funding from the USDA. From the late 1980s, the USDA Marine Shrimp Farming Program (MSFP) began the establishment of SPF lines of P. vannamei using the facilities of the Oceanic Institute in Hawaii (with assistance from the Waddell Mariculture Center, the Gulf Coast Research Laboratory and the University of Arizona Department of Medical Science). These SPF lines have subsequently been used by many commercial and government-run broodstock facilities in Hawaii (and Florida). The USA biosecurity strategy arising from deliberations of the USDA MSFP was published as Proceedings of the US Marine Shrimp Farming Biosecurity Workshop in 1988 (Moss, 1988). This document contains many of the regional criteria developed and implemented subsequently by the United States involving shrimp movements.
The broodstock facilities in Hawaii were cleared of their infected stocks and restocked with screened and quarantined stocks of various species from around the world which were confirmed as virus-free over a long period through multiple checking by PCR, according to the MSFP biosecurity code. These Hawaiian facilities then started producing SPF and SPF/SPR (mainly for TSV and IHHNV) stocks of P. vannamei (and also P. stylirostris, P. monodon and P. japonicus) for distribution to United States and worldwide production facilities. In 2004, however, WSSV was detected and reported in shrimp in Hawaii despite these stringent measures.
Broodstock of these SPF P. vannamei and P. stylirostris have been extensively introduced to Asia. The aim of these initiatives was to reduce dependence on imported shrimp (and thereby attendant viruses) by supporting the local industry, whilst reducing the national trade deficit (Olin, 2001). They have also, however, enabled Asian and other countries to have access to disease-free founder populations of various species with which to start their own industries. This is a new innovation in world shrimp farming, but unfortunately has not often been implemented with sufficient care to take full advantage of its benefits.
Several Central and South American countries immediately closed their borders to the importation of live, fresh and frozen shrimp after the introduction of WSSV to the region in 1999 from unknown sources. Most of those countries imposed new regulations in late 1999 (i.e. Mexico) or 2000 (i.e. Ecuador), which typically included specifying imports of only SPF stocks from certified, tested and enclosed facilities to certified and controlled facilities with quarantines in the respective countries. They also insisted on PCR testing of all imported material for WSSV and YHV. These regulations typically persist until today.
The WSSV episode in Latin America, however, caused major disruptions in the trade of shrimp within the region, particularly to live shrimp exporters - nauplii and PL suppliers in particular, which suddenly found their main markets closed to them. Fortunately for most of these countries, these problems gave the final incentive for the intensification of efforts to close the cycle and develop domesticated and selected SPR lines of P. vannamei and P. stylirostris. They were thus able to satisfy their own demands through the development of these lines, with the help of which the industry has managed to recover slightly over the ensuing three to four years.
Brazil began importing non-indigenous shrimp in 1980 and P. vannamei and P. stylirostris in 1983 from all over Latin America. This resulted in the introduction of various viral diseases including IHHNV, TSV and NHP. By 1998 the Brazilians began to invest more in captive breeding programmes for P. vannamei and therefore, once WSSV arrived in Latin America in 1999, Brazil immediately closed its borders to imports of live, fresh or frozen crustaceans (including Artemia) and polychaete worms (De Barros Guerrelhas, 2003). To date, the restriction has been successful, and Brazil remains free from WSSV and YHV. Because of this and successful genetic selection programmes (i.e. for TSV-resistant strains), Brazil has increased its production over 12-fold since 1998 to the present reaching 60 000 metric tonnes in 2002 and an estimated 90 000 metric tonnes in 2003 (De Paiva Rocha, 2003).
Both P. vannamei and P. stylirostris, along with many other Penaeid shrimp species, were imported to New Caledonia and French Polynesia in the 1970s, before many of the current viral diseases were known. They were all held in quarantine and proven free of diseases before being released for culture. The Pacific Islands are thus free of all major viral diseases currently recognized.
Stocks of most of these species still exist, so that subsequent importations have thus slowed considerably. Fiji has instituted an Animal Act regulating importations of alien species and New Caledonia has regulations that are self-imposed by producers cooperatives aimed at preventing the introduction of viral diseases (Ben Ponia, per. com.).
Thailand began importing P. vannamei in 1998 from diverse (non SPF) sources, but its commercial culture remained mostly experimental until 2001 when problems with a decrease in the growth rate of P. monodon encouraged farmers to search for alternatives. The possibility that this problem was actually caused by viruses brought in with P. vannamei is discussed in Section 6.3.
Although not officially permitted, farmers then began importing large numbers of broodstock and PL from Mainland China and Taiwan Province of China, and lobbying the government to allow P. vannamei importations. Due to these pressures the Thai government carried out a risk assessment regarding the possibility of interbreeding with native species and then finally allowed official importations of certified SPF broodstock only from March until the end of August 2002, but only to qualified and audited hatcheries with restrictions. At this time the first SPF broodstock were imported from Hawaii.
Due to increasing concerns about the smuggling of non-SPF P. vannamei from Mainland China and Taiwan Province of China to uncertified hatcheries, and the first signs of TSV in Thai P. vannamei and M. rosenbergii (Flegel, 2003), the Department of Fisheries then closed the border to all imports again in early 2003, and it remains officially closed to this date (April, 2004). However, due to the huge demand for PL, large quantities of P. vannamei from within Asia and from the Americas were still being smuggled into Thailand, despite the official ban. The Thai Department of Fisheries is currently considering what action, if any, to take regarding the importation of P. vannamei. Despite the importation ban, Thailand produced 10 000 metric tonnes of P. vannamei in 2002 and was expected to produce 120 000 metric tonnes in 2003, which would constitute nearly 40 percent of its total shrimp production (Table 3).
Malaysia implemented an indefinite ban on introduction of P. vannamei, operative from the 1 June 2003, in an effort to prevent the introduction of TSV and other viruses to Malaysia (The Wave website, 2 April 2003). This ban was late in coming since P. vannamei was imported into Peninsular Malaysia from Taiwan Province of China in 2001 and Thailand in 2002, and also to one farm in Sabah. However, these introductions were not technically illegal since Malaysia previously only had restrictions on certain named species, not including P. vannamei or P. stylirostris.
A continuing lack of information and education of the local farming community on the potential risks of P. vannamei farming, together with the absence of a system for monitoring the importation and farming of alien species in Malaysia, has resulted in limited impact on dissuading farmers from importing this species.
Malaysian farmers have already adopted the culture P. vannamei due to numerous perceived benefits, not least the mistaken idea that P. vannamei is resistant to WSSV. Malaysia already increased its production to 1 200 metric tonnes/year by 2002, with the potential to triple this in 2003 (Table 3). Despite importation of non-SPF stocks, in contravention of the ban, there are still no reports of TSV from Malaysia. Without strict enforcement, however, currently successful farmers could resort to illegal imports in order to maintain their production, which will inevitably bring TSV to Malaysia, if it is not already present.
The State of Sarawak has no P. vannamei farms since they have to obtain licenses from the Malaysian government to operate their farms and are concerned that these licenses could be revoked and their ponds destroyed if they are caught farming P. vannamei (Dato Mohamed Shariff, per. com.).
Registration of all farms culturing P. vannamei with the Department of Fisheries could permit a comprehensive monitoring programme and risk assessment to be undertaken to help ensure that practical, longer-term legislation could be introduced to help management of imported alien species and limit disease transmission (Dato Mohamed Shariff, per. com.).
The Philippines began legislation for shrimp importation at the height of the WSSV epidemic in Asia in 1993 to prevent the introduction and spread of alien disease agents. The Bureau of Fisheries and Aquatic Resources (BFAR) immediately implemented a ban on the importation of all live shrimp and prawn species of all stages except for scientific or educational purposes. Such importations required permission from the Secretary of Agriculture and a demonstrated capacity for biosecurity according to ICES and BFAR (import risk analysis panel) recommendations (Fred Yap, per. com.).
The first importation of P. vannamei was made before this law was passed in 1978, but was unsuccessful and the shrimp were all harvested and consumed. Despite private sector lobbying to lift the import ban, the fishery agency refused to approve any applications. The private sector then began illegal importation of P. vannamei due to disease problems with the culture of P. monodon. These illegal importations began in 1997 and comprised PL originating from Taiwan Province of China, labelled as milkfish fry to circumvent the importation law.
In 2001, due to these violations of the law, the Philippine government issued a new law to include prohibition of culturing imported shrimp and penalties for violations comprising a fine of up to US$ 1 500 and up to eight years in jail.
Subsequent high profile confiscations of illegally imported shrimp fry from Taiwan Province of China occurred in 2002 and 2003 and restrictions to limit all live fish imports to just one airport were applied to further strengthen the control over illegal importations. However, these regulations are known to have resulted in the dumping of PL P. vannamei into the wild in attempts to escape detection (Timothy Flegel, per. com.). Additionally, typhoons have also resulted in the liberation of P. vannamei from culture ponds into the surrounding sea. A population of P. vannamei in the wild therefore already exists in the Philippines, although it is still uncertain if this population is now breeding (Fred Yap, per. com.).
Plans to allow importation of only SPF P. vannamei broodstock and their subsequent sale only after reaching the F1 generation and confirmation of disease-free status were originally approved by the BFAR (Bureau of Fisheries and Aquatic resources), but later rescinded due to suspicions that the company requesting the importation permit was already producing PL P. vannamei for sale.
Despite all of the efforts of the BFAR, the culture industry for P. vannamei in the Philippines is growing and may produce as much as 5 000 metric tonnes in 2003 (Table 3). In addition, once pond reared broodstock become available the farmers will no longer have to rely on imports of PL or broodstock to maintain production. Although the culture of P. vannamei remains illegal, the ban is difficult to enforce (particularly since it is clearly now present in the country), and there are members of the government who are in favour of controlled importations of SPF animals. Legislation to allow this already exists and it may be the only way to ensure importation of disease-free stocks to the industry that is being driven by market forces despite official restrictions.
Viet Nam has had an official ban on the culture of P. vannamei since June 2002, but some importations and culture have been permitted. They are currently conducting culture tests with this species in (supposedly) biosecure facilities in order to evaluate its positive and possibly negative impacts. In order to conduct these trials, since 2001, the Ministry of Fisheries (MOFI) has granted nine licenses to commercial companies, permitting the importation of up to 48.5 million PL and 5 900 broodstock P. vannamei. These animals originated from the USA (Hawaii) and China, and are inspected by MOFI to ensure that they are disease-free before allowing their culture. However, current inspection protocols do not appear to be capable of definitively proving the disease status of the imported stocks (FAO correspondent, Viet Nam).
The PL imported or produced by these companies may be either cultured by these companies or sold to third parties to culture (after being given permission by MOFI). Of the nine companies, only one to date has passed the trial period and is officially allowed to disseminate its products for culture, and even then only within the Mekong river delta (FAO correspondent, Viet Nam).
However, while these quotas remain unfilled and open, P. vannamei appeared to be cultured in North Viet Nam using PL illegally imported from Mainland China. Additionally, it has been estimated that during the first six months of 2003 alone, two thousand million PL were imported from Mainland China for culture within Viet Nam (FAO correspondent, Viet Nam).
Despite official restrictions, it is estimated that approximately 10 percent of the countries' 479 000 ha of shrimp ponds are now being used to culture P. vannamei. This is driven by the scarcity and high price of PL P. monodon, together with the low cost of mostly Chinese PL and the good results obtained in grow-out. The result is that Viet Nam's estimated production in 2003 was 30 000 metric tonnes (FAO correspondent).
Indonesia has recently enacted a decree permitting imports of shrimp, including P. vannamei provided that the purpose is justified and an import certificate or a licence to import is obtained from the relevant national authorities. Importation from Taiwan Province of China is not permitted. It is certain that P. vannamei is present in Indonesia and there have been outbreaks of TSV in the country (Akhmed Rukyani, per. com.). It is suspected that TSV first occurred in Banyuwangi and Situbondo before spreading to other districts in East Java through movement of infected post larvae. Banyuwangi and Situbondo are two of Indonesia's shrimp (P. monodon and P. vannamei) production centres, producing both seed and marketable shrimp. Samples of P. monodon originating from Brebes (Central Java), Situbondo (East Java) and Bali islands have also been found to be PCR positive against TSV. TSV has also been found in P. vannamei from Maros (Sulawesi islands) and Sumbawa islands. All of the samples were confirmed by PCR-based methods (Agus Sunarto, per. com.)
Until recently, the only legislation relating to the importation of live animals into India was the 1898 Livestock Importation Act. However, this was neither designed nor effective in controlling importation of aquatic animals. Thus from 2001, the Department of Animal Husbandry and Dairying of the Ministry of Agriculture took responsibility for issuing (based on advice from the National Committee on Exotics) a Sanitary Import Permit, which must accompany any importation of fishery products. Subsequently, the Marine Products Export Development Agency (MPEDA) issued a public notice specifically warning against the illegal aquaculture of P. vannamei, with some success.
The first imports of P. vannamei were made prior to the recent legislation, with two farms importing PCRtested SPF broodstock from Hawaii via Taiwan Province of China in 2001. Subsequently, two more farms were granted permission by the government to import SPF broodstock, with provisions made for ensuring biosecurity from a specially appointed committee. These farms are being used to test the culture potential and risks involved with farming P. vannamei before any further measures are taken by the government. Other than these, there appear to have been no further introductions, suggesting that India may still be free of the viruses carried in with imported P. vannamei elsewhere in Asia.
Nonetheless, the initial successes of culturing P. vannamei have led to the more widespread culture of the species in India using locally reared broodstock. It is estimated that there now exist 3 hatcheries and 37 farms covering 120 ha, which may produce up to 1 000 metric tonnes in 2003 (FAO correspondent).
Currently, a national strategic plan to produce guidelines on importation and quarantine of aquatic aliens is being formulated by the government based on the FAO CCRF and the FAO/NACA Technical Guidelines (FAO/NACA, 2000). Once approved by the government, legislation can be drawn up to properly regulate the introduction of P. vannamei and other alien species.
Sri Lanka is one of the few Asian countries which has yet to import P. vannamei. Despite interest in the species due to its perceived tolerance to WSSV, the government has shown great caution due to the potential risks involved with such introductions. They have used existing legislation to prevent introductions, but intend to adopt the FAO and regional guidelines on introduction of alien species to help ensure any importations made are conducted according to established protocols (Sunil Siriwardena, per. com.).
Mainland China and Taiwan Province of China have not banned the importation of P. vannamei or P. stylirostris, but have guidelines in place for potential importers. Due to worries over importation of viruses, the Chinese government initiated an aquaculture disease surveillance system. Later, the Chinese Bureau of Fisheries of the Ministry of Agriculture implemented a regulation on aquatic animal epidemic disease prevention, based on the OIE guidelines. Subsequently, in 2001, China entered the WTO and thereby accepted the various relevant protocols and guidelines provided by the WHO, FAO, OIE and so on (Chen Aiping, per. com.).
Mainland China first imported P. vannamei in 1988 experimentally from the USA, and then commercially from 1996. They also imported P. stylirostris from the USA in 1999/2000. Taiwan Province of China began imports of P. vannamei in 1995 from USA (Table 2).
The existing regulations have proven difficult to enforce, and it has been estimated that Mainland China imported more than ten thousand million wild and cultured PL shrimp in 2002. The ineffectiveness or belated enactment of these regulations is also demonstrated by the appearance of viral pathogens including TSV, LOVV, REO III and BP by 1999, probably transferred through the importation of infected stocks of P. vannamei and now causing significant disease problems (Chen Aiping, per. com.).
Since there have never been outright bans on importation of alien shrimp, the industries for P. vannamei in Mainland China and Taiwan Province of China are longer established and more advanced than those of the other Asian countries, with an estimated 71 percent (300 000 of 420 000 metric tonnes) and 40 percent (8 000 of 19 000 metric tonnes) respectively of their production comprising P. vannamei (Table 3).
It seems likely that more countries will impose restrictions and/or more strictly enforce the existing restrictions on the import of non-indigenous shrimp species in an effort to control the currently unknown effects of such importations. This is particularly with regard to the importation of alien viruses that might compromise native shrimp populations, both wild and cultured, as well as other unknown effects on the genetic diversity and ecology of native fauna.
How effective these official bans will be remains to be seen, but past experience has shown that the private sector activities are difficult to control if the perceived rewards are great, as appears to be the case with P. vannamei.
Although the Asia-Pacific region has been working hard to improve safe transboundary movement of live aquatics and despite the various codes and guidelines that have been developed and agreed upon, most of those efforts have been largely ineffective at preventing the spread of alien shrimp and their viral pathogens. This scenario is not specific to the Asia-Pacific region but is also true for the Americas. The possible reasons behind this are varied and include the following:
In many cases, even though governments have guidelines and/or regulatory frameworks are in place to restrict movement of shrimp species, the private sector has gone ahead with such imports using illegal or illicit procedures. Thus, although there may be good reasons for limiting imports and to have regulations in place, these have little chance of success in limiting imports unless either the private sector can be convinced of their validity and importance or state law enforcement is improved. It is almost impossible to stop such imports, which are generally taken through illegal channels, unless strong public awareness programmes aimed at improving farmers and producer understanding of the risks involved are implemented.
The largely private sector introductions are done, whether or not official restrictions are in place, due to the perceived benefits offered by the introduced species. Thus, in the case of P. vannamei introductions into Asia, the current perceptions that: P. vannamei are more disease resistant than the indigenous species (P. monodon and P. chinensis), SPF broodstock can be purchased that are free from disease, and that they are more able to tolerate high density, often low-salinity culture, are the main driving forces behind their introduction. Whether these perceived benefits (as detailed in Section 4 and Table 4) are true or not is often irrelevant, particularly when Asian shrimp farmers are struggling to make money using their traditional native species. In this case, as has been seen in virtually all Asian shrimp-producing countries in the past few years, the perception of the private sector is that the potential advantages outweigh the disadvantages and so the importations are made.
Whether this perception is correct or not remains unproven. On the positive side, the Asian P. vannamei culture industry has seen a rapid expansion in the last few years, so that production of P. vannamei has surpassed that of traditional native cultured species in Mainland China, is rapidly approaching that level in Taiwan Province of China and Thailand, and is gaining increasing importance in Viet Nam and Indonesia (Table 3). The generally downward trend in Asian shrimp production during the 1990s, due largely to disease problems with P. monodon and P. chinensis, has thus now been reversed with the introduction of the relatively more tolerant P. vannamei.
On the negative side, the introduction of P. vannamei into Asia has been accompanied by the importation of various viruses, including TSV (already causing losses in Mainland China, Taiwan Province of China and Thailand) and LOVV (possibly responsible for the slowing growth rate of P. monodon) and probably others (Section 6.3). The long-term effects of these viruses is unknown, but precedents from introductions of shrimp and their viruses from Asia to Latin America (i.e. IHHNV in 1981 and WSSV in 1999) are known to have resulted in severe setbacks to the shrimp culture industry and the socio-economic status of many countries. Additionally, the associated impacts of transboundary introductions of shrimp have unknown, but possibly serious, consequences for wild shrimp populations and genetic diversity (Section 6.2).
The rapid expansion of culture of P. vannamei in the Asian region has more recently resulted in marketing problems. Increased production of the same species with a similar size range has led to recent price collapses. Low shrimp prices have led to disputes over alleged 'shrimp dumping'. This has further reduced confidence amongst producers who, in some cases, are shifting back to P. monodon in the hope that these will fetch a higher price.
Even where legislation on transboundary movements has been enacted, the extensive borders, lack of resources, lack of clear understanding and knowledge, and weak regulatory structures of many countries make enforcement very difficult, expensive and sometimes impossible. Although such measures for the establishment, enforcement and dissemination of laws and guidelines for the benefit of the aquaculture sector are expensive, the potential losses far outweigh the costs involved, as has been proven many times.
Additionally, many countries have officially regulated the importation of alien species, in harmony with existing international codes of conduct, but have then done little to enforce such limitations. This may be through a lack of desire or commitment, or reluctance (or pressure) not to interfere with the competitiveness of the commercial sector.
Effective enforcement requires cooperation between the state and private sector and this can only be achieved through improved dialogue, increased awareness and effective communication with concerned parties regarding the risks involved in introductions and movements. Only a few countries practice effective, science based risk assessment and analysis procedures for the movement of alien or other aquatic species in the Asia-Pacific region and the wider application of proper risk assessment procedures should be encouraged.
With P. vannamei and P. stylirostris (unlike P. monodon), SPF broodstock and PL are already commercially available (although limited). Despite the fact that this should be an advantage, where regulations have been made (based on published codes of conduct) to permit the importation of disease-free specimens, many problems may still arise.
Such difficulties stem partly from the fact that shrimp are characterized by persistent viral infections that often produce no gross signs of disease or mortality. Thus sophisticated procedures such as Polymerase Chain Reaction (PCR) methodologies may be required to detect these pathogens. Such techniques are fraught with error stemming from poor sampling techniques, sample size and preservation, analysis protocols, interpretation and lack of standardization and/or inter-calibration.
In addition, some viruses remain undetectable until after the shrimp are stressed, for example following spawning of broodstock. This means that stocks must be held in quarantine facilities until such time as they can be spawned, so that the virus can be detected. Such facilities are currently largely unavailable in Asia. Thus, even where facilities exist, competent and accurate analysis of the disease status of the imported shrimp is still not always possible.
The pathways of viral pathogen transfers in shrimp are still far from clear. With uncertainties regarding which sources carry high risk of viral transfer, it is difficult to design protocols for testing imported products. It seems clear that the importation of live animals offers the most high-risk route of infection, so that these should be the main focus of attention. However, the level of risk involved with other possible sources including green (fresh/frozen) shrimp for bait, processing or direct consumption, ballast water and hull fouling of ships, and aerial transfer through birds and/or insects is less clear. Meanwhile, fresh feeds used in shrimp culture such as Artemia and polychaete worms, cooked shrimp and shrimp head meal appear to carry little or no risk and may not need to be tested (Flegel, 2003).
Even where imported shrimp are subjected to thorough analysis for pathogens, there is a limit on the number of pathogens that are monitored. It is believed that there may exist many "hidden" or "cryptic" viruses within shrimp that are unknown and therefore impossible to detect. Certainly, the number of known viruses currently affecting cultured shrimp is growing every year.
In addition, viruses that may not infect or cause disease and mortality in one species, may have other effects in other species. Therefore, when planning which viruses much be checked, consideration must be made not just of the viruses that can infect the shrimp being imported, but also of those capable of infecting native species. Unfortunately, current understanding of the effects of many viruses on different shrimp hosts is limited, making such disease testing even more difficult.
A discussion of the recommendations proposed for controlling transboundary movements of shrimp and limiting viral spread are presented in the following Section.
A common perception amongst farms is that SPF and SPR shrimp are 'disease-free'. Although the original stock of SPF or SPR shrimp may be certified as clear of specific pathogens or resistant certain strains, the animals produced from this stock may not be so reliable, particularly if the biosecurity of the producing facility is poor. The sale of shrimp (either for use as broodstock or for stocking) that are claimed to be SPF or SPR because they were bred from SPF stocks is an example of how farmers may be misled regarding the quality of the animals they are buying and how disease can actually be spread by unsuspecting farmers.
The effective control and regulation of facilities that produce SPF/SPR shrimp and adequate diagnostic support for health testing are two areas where national effort can be concentrated to ensure that farmers get what they expect and that disease problems are solved rather than created by the use of SPF/SPR stocks.
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