There are many reasons for the introduction of P. vannamei and P. stylirostris into areas where they are not indigenous. Despite the presence of various international, regional and country-specific regulations, the private sector (and/or the state sector) will often attempt to initiate introductions due to problems that they face with the culture of their indigenous species and the perceived (rightly or wrongly) production benefits of the alien species. There may also exist marketing advantages and a desire to expand, intensify and/or diversify aquaculture practices. The improved transportation efficiency available recently has also removed some old limitations and encouraged international movement of alien species.
The advantages and disadvantages of P. vannamei and P. stylirostris as compared to native species, specifically P. monodon, are shown in Table 4. Data on the productivity of P. vannamei compared to P. monodon are shown in Table 5.
The reasons behind the introductions of these alien species and the possible risks involved are described below:
Penaeus vannamei has the potential to grow as fast as P. monodon (at up to 3 g/wk) up to 20 g (the maximum size of P. vannamei usually cultured) under intensive culture conditions (up to 150/m2). Although it will keep growing beyond 20 g, its growth may slow (particularly males) to 1 g/wk once above 20 g in weight (Wyban and Sweeny, 1991).
Under commercial conditions in Asian earthen ponds, however, typical growth rates of 1.0–1.5 g/wk (with 80–90 percent survival) are common in the high-density pond system (60–150/m2) currently in use in Thailand and Indonesia. In contrast, the growth (and survival) rate of P. monodon has been declining in recent years from 1.2 to 1 g/wk (and 55 percent to 45 percent survival) over the last five years in Thailand (Chamberlain, 2003) due perhaps to disease load and/or genetic inbreeding (Table 5). Penaeus stylirostris can also grow equally fast and to a larger size than P. vannamei.
Penaeus vannamei are amenable to culture at very high stocking densities of up to 150/m2 in pond culture, and even as high as 400/m2 in controlled recirculated tank culture. Although such intensive culture systems require a much higher degree of control over environmental parameters, it enables the production of high numbers of shrimp in limited areas, resulting in better productivity per unit area than that currently achievable with P. monodon in Asia.
Both P. monodon and P. stylirostris can be aggressive, have high protein requirements, and may be more demanding of high water quality, making them difficult to culture as intensively as P. vannamei.
Penaeus vannamei tolerates a wide range of salinities, from 0.5–45 ppt, is comfortable at 7–34 ppt, but grows particularly well at low salinities of around 10–15 ppt (where the environment and the blood are iso-osmotic) (Wyban and Sweeny, 1991). This ability makes it a good candidate for the newer inland farms that have become common in Asia and Latin America in the past few years. For example, a high percentage of Chinese P. vannamei are cultured in inland, freshwater sites, where production is much higher than with the indigenous species.
Summary of advantages and disadvantages of the culture of P. vannamei and P. stylirostris over P. monodon in Asia
|Growth rate||P. vannamei and P. stylirostris can grow as fast as P. monodon up to 20g and typically grows faster (1–1.5 g/wk) than P. monodon (1 g/wk) currently in Asia. Size range on harvest generally smaller.||Growth rate of P. vannamei slows after reaching 20g, making production of large-sized shrimp slower.|
|Stocking density||P. vannamei is easier to culture in very high densities (typically 60–150/m2, but up to 400/m2) than P. monodon and P. stylirostris which can be aggressive.||Very high stocking densities require high control over pond/tank management practices and are high-risk strategies.|
|Salinity tolerance||P. vannamei are tolerant of a wide range of salinities (0.5–45 ppt) and more amenable to inland culture sites than P. monodon or P. stylirostris.||None|
|Temperature tolerance||P. vannamei and particularly P. stylirostris are very tolerant of low temperatures (down to 15 °C) enabling them to be cultured in the cold season.||None|
|Dietary protein requirements||P. vannamei require lower protein feed (20–35%) than P. monodon or P. stylirostris (36–42%), resulting in a reduction in operational costs and amenability for closed, heterotrophic systems. Food Conversion Ratios (FCRs) are lower at 1.2 compared to 1.6.||None|
|Disease resistance||Although P. vannamei is susceptible to WSSV, Asia is not currently experiencing problems from this virus; P. stylirostris is highly resistant to TSV. Both species have been selected for resistance to various diseases. Survival rates with P. vannamei are thus currently higher than with P. monodon in Asia and production is more predictable.||P. vannamei is highly susceptible to and a carrier of TSV, WSSV, YHV, IHHNV and LOVV. P. monodon is refractory to TSV and IHHNV. There is currently no ability to select P. monodon for disease resistance.|
|Ease of breeding and domestication||Availability of pond-reared broodstock; ability to conduct domestication and genetic selection work; SPF and SPR lines already available; elimination of problems associated with wild broodstock and/or PL collection; source of cheap broodstock from ponds; and small sized broodstock mean faster generation times.||SPF animals sometimes have high mortality in disease-laden environments. Broodstock rearing and spawning more technical and complicated than use of wild P. monodon spawners.|
|Larval rearing||Higher survival rates in hatchery of 50–60% for P. vannamei and P. stylirostris compared to P. monodon (20–30%).||Ninguna|
|Post-harvest characteristics||If treated with ice, P. vannamei are resistant to melanosis.||Handling, transportation and processing of P. monodon is easier.|
|Marketing||White shrimp generally preferred in United States market over tiger shrimp due to taste. Strong local demand for white shrimp in Asia. Meat yield is higher for P. vannamei (66–68%) than for P. monodon (62%).||P. monodon and P. stylirostris can grow to larger size, commanding higher price than P. vannamei. High competition on international markets for P. vannamei as production is world-wide.|
|Origin||None||P. vannamei and P. stylirostris are alien to Asia and their importation may cause problems with import of new viruses and contamination of local shrimp stocks.|
|Government support||None||No support from most countries since they remain undecided on ban imports and farming of P. vannamei. Supply of broodstock and seed problematic in face of bans, leading to smuggling of sub-optimal stocks and disease introduction.|
Production, survival and cost data for P. vannamei and P. monodon in Asian countries and the Pacific
|Country/Region||Total production area (ha)||P. vannamei production Area (ha)||P. vannamei production (t/ha/cycle)||P. vannamei survival (%)||P. monodon production (t/ha/cycle)||P. monodon survival (%)||P. vannamei production cost (US$/kg)||P. monodon production cost (US$/kg)|
|China||246 275||68 837||7 a 11||7||<7.5||7||2.00||2.00|
|Taiwan Province of China||8 160||3 053||?||?||?||?||1.95||3.50|
|Thailand||80 000||32 000||6 a 7||80||3||45||2.14||3.10|
|Viet Nam||479 000||48 000||4a7||80||4a 5||?||?||?|
|Philippines||1 58 920||700||4||90||5 a 8||80||1.89||3.40|
|Indonesia||350 000||1 000||3 a 5||65||1 a 3||7||7||7|
|Malaysia||7 260||200||5 a 12||85||1.5 a 9||45||2.63||4.27|
|Sri Lanka||1 300||0||N/A||N/A||7||7||N/A||4.13|
|Total||1 518 125||153 910||Average|
4 a 7
3 a 5
Note: All data is for 2002
This trend is likely to continue due to concerns over coastal development including biosecurity, land cost and conflicts with other users of common resources in coastal zones. In addition, farmers in Thailand have been prohibited from farming P. monodon in freshwater areas, while no such restrictions currently apply to P. vannamei. Penaeus stylirostris and P. indicus are not as able to tolerate low salinities, so are less suitable for this purpose.
Although P. vannamei will tolerate a wide range of temperatures, it grows best between 23–30 °C, comprising the majority of the tropical and subtropical world, with the optimum for growth being 30 °C for small (1 g) and 27 °C for larger (12–18 g) shrimp. They will also tolerate temperatures down to 15 °C and up to 33 °C without problems, but at reduced growth rates (Wyban and Sweeny, 1991). Penaeus vannamei (and P. stylirostris) can thus be profitably cultured during the cool season in Asia (October–February). This is traditionally the low season for P. monodon farmers in this part of the world, meaning that increased yearly harvests may be possible using these alien species. This greater temperature tolerance of P. vannamei may also be a reason why farmers have perceived this species to be more resistant to WSSV relative to P. monodon. However, recent experience in Thailand, Ecuador and elsewhere has shown that when water temperatures decline to less than 30 °C, increased problems with viral diseases such as WSSV and TSV occur not just with P. monodon, but equally with P. vannamei.
Penaeus stylirostris can tolerate even colder temperatures than P. vannamei, P. monodon or P. indicus but require higher oxygen levels (Rosenberry, 2002).
Compared with other species, P. vannamei requires a lower protein (and hence cheaper) diet (20–35 percent) during culture than P. monodon, P. chinensis or P. stylirostris (36–42 percent), and are more able to utilize the natural productivity of shrimp ponds, even under intensive culture conditions (Wyban and Sweeny, 1991). In Thailand for example, current grow-out feeds for P. vannamei contain 35 percent protein and cost 10–15 percent less than the 40–42 percent protein feeds for P. monodon. Additionally, feeding efficiency is better with P. vannamei, which yield an average FCR of 1.2, compared to 1.6 for P. monodon (Dato Mohamed Shariff, per. com.). These factors, together with higher growth and survival rates are responsible for the 25–30 percent lower production costs for producing 20 g of P. vannamei than P. monodon (US$2.33 compared to US$3.41/kg across Asia, Table 5).
Recent commercial results from Indonesia have shown that P. vannamei growth, survival and production rates all slightly increased using 30–32 percent compared to 38–40 percent protein diets in intensive (60/m2) culture (Taw et al., 2002). Additionally, results from recycled, heterotrophic systems originating from Belize and now also being used in Mainland China, Indonesia and elsewhere have shown that even lower protein levels of 20 percent or less can be used successfully with P. vannamei if the natural bacterial productivity of the ponds is correctly stimulated (McIntosh, Drennan and Bowen, 1999).
Both P. vannamei and P. stylirostris are open thelycum species, meaning that they can be induced to mate and spawn easily in captivity (unlike the closed thelycum P. monodon) which enables the culturist to close the life cycle of the shrimp, facilitating genetic selection (i.e. for improved growth rate and disease resistance) and domestication programmes. This feature permits much more control and enhancement of the cultured stock and allows the development of SPF and SPR stocks, which are already commercially available. This in turn relieves the expense, disease implications, environmental concerns, unpredictability and waste of relying on wild broodstock.
Despite the ease of obtaining pond-reared broodstock and subsequently spawning them, these techniques are by no means simple. Many Asian farmers have no experience with these techniques, which is leading to difficulties with seed production in Thailand, Indonesia, Malaysia and other countries. This, in turn, results in farmers importing PL and broodstock of often unknown health status into the country for stocking their ponds. This practice is a major risk for bringing viral and other pathogens into once-clean areas. These risks could be reduced through approved and well designed and run SPF-maturation and broodstock centres in each country wanting to culture these new species.
The extent of maturation and larval culture facilities in Asia is shown in Table 6. Apart from Mainland China and Taiwan Province of China, which have relatively well-established industries for P. vannamei, the other countries in Asia have very few maturation and larval culture facilities for this species. More facilities can be expected, once these other nations perfect their broodstock production and hatchery techniques for P. vannamei and the demand for PL grows.
This ability to produce high-quality, fecund domesticated stocks can also be seen as an advance in the sustainability and environmental friendliness of shrimp farming since it precludes the necessity of catching large numbers of wild post-larvae and wild broodstock (and the wastage associated with the bycatch from these activities). Production of pond-reared broodstock is also much cheaper than buying wildcaught animals from fisherfolk and is also economically advantageous.
Work on the domestication of P. monodon has been going on for some time in the United States of America, Australia and Thailand, but as yet without commercial success. However, it is expected that, from 2004, for the first time, SPF domesticated broodstock of P. monodon have been made commercially available from Hawaii (Wyban, per. com.) and also probably from Thailand within the next couple of years. Thailand's National Science and Technology Development Agency (NSTDA), together with the National Centre for Genetic Engineering and Biotechnology (BIOTEC), have continued their previous work with P. monodon domestication with a US$4 million government grant and have already developed sixth generation animals SPF for WSSV and YHV. If successful, this development will allow the same degree of control over the life cycle of P. monodon as is currently available for P. vannamei and P. stylirostris.
Hatchery and PL production for all shrimp and P. vannamei in Asian countries and the Pacific
|Country/Region||P. vannamei maturations||P. vannamei hatcheries||Other Shrimp hatcheries||Total shrimp PL production|
|P. vannamei PL production|
|China||7||1 959||1 893||56 375||9 900|
|Taiwan Province of China||20||150||250||754||644|
|Thailand||20||26||2 000||3 700||1 200|
|Viet Nam||9||9||4 800||1 600||90|
|Total||54||2 157||9 661||63 451||11 886|
Note: All data are unofficial figures, based on industry estimates for 2002.
However, minimum spawning size for P. monodon females is 100 g, which will take at least 10–12 months under commercial pond conditions, while P. vannamei and (less so) P. stylirostris can be spawned at only 35 g, which can be achieved easily in 7 months. This has obvious advantages over P. monodon in terms of generation times and the expense involved in producing captive broodstock.
Larval survival rates during hatchery rearing are generally higher (50–60 percent) with P. vannamei and P. stylirostris than with P. monodon (20–30 percent) (Rosenberry, 2002).
Penaeus vannamei is generally considered to be more disease resistant than other white shrimp (Wyban and Sweeny, 1991), although it is in fact highly susceptible to WSSV and TSV (can cause high mortality) and a carrier of IHHNV (results in runt deformity syndrome - RDS) and Lymphoid Organ Vacuolization Virus (LOVV). Mostly owing to its perceived disease tolerance, it is replacing the less virus-tolerant P. chinensis in southern Mainland China (Rosenberry, 2002). Nonetheless, uninformed farmers throughout Asia recently began farming P. vannamei in the belief that it was resistant to WSSV and YHV, encouraged by traders and salespeople involved in this business.
To date, Thailand, Malaysia and Indonesia have not suffered major WSSV or YHV-related epidemics with P. vannamei, despite the presence of these pathogens in the environment. This has translated into current survival rates of 80–90 percent with P. vannamei on some farms, compared to just 45–60 percent with P. monodon (Table 5). The disease resistant view of P. vannamei is no longer held by many farmers in Mainland China, Taiwan Province of China and Thailand, where disease epidemics within P. vannamei farms have started, but are typically blamed on TSV.
Injection of WSSV into P. vannamei and P. stylirostris was shown to result in 100 percent mortality within 2–4 days, proving its infectivity and pathogenicity was similar to that found with P. monodon, P. japonicus and P. chinensis (P. orientalis) (Tapay et al., 1997). The WSSV has also been identified as the prime cause of major mortalities of P. vannamei and P. stylirostris in Latin America since 1999. However, some unpublished work has suggested that WSSV alone may have only 30 percent of the effect of a mixture of viruses on mortality of P. vannamei fed infected shrimp tissue in Ecuador (Matthew Briggs and Neil Gervais, per. com.). Additionally, the generally higher water temperatures experienced in tropical Asian countries may help to limit mortalities due to WSSV in P. vannamei (compared to Latin America) since WSSV has been shown repeatedly to lose its virulence in water over 30°C in temperature.
Penaeus monodon is generally regarded as being highly susceptible to both WSSV and YHV, but not to IHHNV or TSV, although Macrobrachium rosenbergii, another important cultured prawn in Southeast Asia, is sensitive to TSV (Rosenberry, 2002; Flegel, 2003). Penaeus stylirostris from the wild are highly susceptible to the IHHN virus, leading to their falling out of favour with Latin American farmers in the late 1980's. However, the ability to domesticate and selectively breed for disease resistance confers a big advantage on P. vannamei and P. stylirostris until domesticated lines of P. monodon becomes available. Domesticated lines of both P. vannamei and P. stylirostris have been shown to gain resistance to both IHHN and TSV. Penaeus stylirostris have been injected with TSV and were not found to get infected, so are refractory, rather than resistant (Timothy Flegel, per. com.). This trait has promoted a resurgence in the farming of P. stylirostris in Mexico and interest in P. vannamei culture in Asia where the lack of domesticated P. monodon precludes the possibility of selection for disease resistance (Rosenberry, 2002).
Penaeus monodon are highly resistant to IHHNV, but do act as carriers, so farmers must be careful to avoid cultivating P. monodon together with P. vannamei in maturation, hatchery or grow-out facilities, as cross contamination of viruses may result (Timothy Flegel, per. com.).
It is believed that the current declines in growth rate and survival of cultured P. monodon in Asia are due to the stress of high IHHN viral loading in the broodstock and the passing of these viruses to their offspring. Due to the coincidence in dates, it is even possible that these problems with P. monodon resulted from the introduction of viral pathogens carried by P. vannamei. A recently (December 2002, by Lightner) discovered Ribonucleic Acid (RNA) viral pathogen, very similar to LOVV in P. vannamei, has been detected in Thailand in the lymphoid organ of P. monodon. This new type of LOVV (temporarily named LOVV2) might be the causative agent of this slow growth phenomenon. This slow-growth problem was estimated to have resulted in US$5–10 million in lost earnings in 2002 (Timothy Flegel, per. com.). Additionally, recent research in Thailand has shown that even apparently healthy shrimp in culture ponds have a high prevalence of one to four different viral pathogens (Flegel, 2003).
One of the main advantages of culturing the shrimp species P. vannamei and P. stylirostris is that both species are commercially available as high health animals from SPF stocks. Penaeus monodon have very limited availability from SPF stocks, but this may well change in the near future as such stocks are currently under development. Nevertheless, at this time, the availability of domesticated strains of SPF P. vannamei and P. stylirostris offer great advantages over P. monodon and other native Asian shrimp, which must still be collected from the wild.
The status of Specific Pathogen Free should signify that the shrimp have passed through a rigorous quarantine and disease screening process that determined them to be free from specified pathogens of concern to culturists. This characteristic means that countries or regions which still do not have this species can be reasonably sure that the importation of SPF animals will not result in the introduction of the specified pathogens for which the animal is declared “free”. This does not, however, guarantee against the animal being infected with unknown pathogens or known pathogens which are not screened against.
There is significant confusion in Asia regarding the exact meaning of SPF. For example, a widely held belief is that SPF animals are resistant to and cannot become infected by any viral pathogens that they encounter during cultivation. This is most certainly not the case. SPF means that the animals have been assured of being free from specific pathogens. Whether a particular animal or strain is genetically resistant to a specific pathogen is independent of its present status. SPF refers only to the present pathogen status for specific pathogens and not to pathogen resistance or future pathogen status (Lotz, 1997).
Genuine SPF shrimp are those which are produced from biosecure facilities, have been repeatedly examined and found free of specified pathogens using intensive surveillance protocols, and originate from broodstock developed with strict founder population development protocols. These founder populations are generated by extensive quarantine procedures that result in SPF F1 generations derived from wild parents (Lotz, 1997). Only when raised and held under these conditions you can have true SPF stocks. There is not yet an internationally agreed protocol for the development of SPF shrimp and certainly some variation in the quality of different SPF stocks exists. Once the animals are removed from the SPF production facilities, they should no longer be referred to as SPF, even though they may remain pathogen free. Once outside the SPF facility, the shrimp may be designated as High Health (HH) as they are now subject to a greater risk of infection, but only if they are placed into a well-established facility with a history of disease surveillance and biosecurity protocols. If the shrimp are put anywhere else, for example into a non-biosecure maturation unit, hatchery or farm, they can no longer be called SPF or HH as they are now exposed to a high risk of infection.
The primary goal of SPF facilities is to produce strains of shrimp that are disease-free, domesticated and genetically improved for aquaculture. Since, for P. vannamei and P. stylirostris, such SPF lines are available, it makes sense to use them to begin breeding programmes in those countries which are introducing these species for the first time. This is because even if the SPF lines are not resistant to major pathogens, they are not infected with them. Additionally, they are already domesticated and possess growth and behavioural characteristics that make them preferable to their wild counterparts. It is important to note here that the health aspect of a proposed introduction is only one part of the full risk assessment that should be undertaken prior to introduction. Other important aspects are the issue of whether the imported alien species is likely to be invasive and the likely impacts of escapees on wild populations and the environment.
Recent research work by some state and private companies has focused efforts on the development of SPF strains that are also resistant to specific pathogens (SPF/SPR). This is a long process, and usually focused on one pathogen at a time. Thus, although the development of pathogen resistant strains is a long-term goal of SPF breeding programmes, it is unlikely that they will ever result in strains that are unaffected by all disease organisms (Lotz, 1997).
One potential drawback of SPF animals is that they are only SPF for the specific diseases for which they have been checked. Typically this will consist of the viral pathogens which are known to cause major losses to the shrimp culture industry, including WSSV, YHV, TSV, IHHNV, BPV and HPV as well as microsporidians, haplosporidians, gregarines, nematodes and cestodes. Despite this screening, new, hidden or “cryptic” viruses may be present, but because they are as yet unrecognized, may escape detection. Thus, it is believed that SPF shrimp shipped from Hawaii resulted in the contamination of shrimp in Brazil and Colombia with TSV (Brock et al., 1997). This was because, at the time, TSV was not known to have a viral cause and therefore went unchecked in SPF protocols.
Additionally, new diseases may emerge from mutations of previously non-pathogenic organisms - i.e. the highly mutable RNA viruses. Hence, it remains a possibility that importation of SPF shrimp may not rule out simultaneous importation of pathogens. Another possibility is that if SPF shrimp are stocked into facilities with high viral loads, substantial mortality can result as they are not necessarily more resistant to these diseases than non-SPF shrimp, and in some cases, less so. They may thus be more suited to culture in biosecure systems, which may explain the reliance of the big, non-biosecure pond farms of Latin America on SPR, rather than SPF shrimp.
In any case, the use of SPF stocks is only one part of a complete plan for minimizing disease risks in shrimp culture. The development of SPF strains is really designed to help ensure that PL stocked into grow-out ponds are free of disease, one of, if not the most serious source of contamination. Other areas of this strategy that must be implemented include: strategies to ensure broodstock, eggs, nauplius, larvae and juveniles derived from SPF stock remain SPF such as: farm biosecurity, early warning surveillance and rapid response to disease outbreaks. Recommended management strategies for maintaining biosecurity and disease surveillance are given in Annexes 2 and 3.
In response to disease problems due largely to IHHNV (the causative agent of runt deformity syndrome [RDS] in the United States of America in the late 1980s), a programme to develop SPF P. vannamei was started in 1989 in the United States Department of Agriculture (USDA)-funded Oceanic Institute in Hawaii (Wyban and Sweeny, 1991). This programme continues to this day and has been expanded by a number of commercial ventures, mostly located in Hawaii.
This initial work with SPF P. vannamei has been extended in the private sector to include work with P. stylirostris, P. monodon, P. japonicus and P. chinensis (principally in Hawaii but also in Florida and Mexico), P. indicus, P. merguiensis and P. semisulcatus (in Iran) and SPF stocks of P. vannamei with resistance to TSV (in the United States of America). Some of these lines are now more than ten generations SPF. Current suppliers of SPF (and SPR) strains of shrimp are shown in Table 7. Despite the declaration of SPR status, it is important to note that this resistance is only to some specific strains of TSV, not all of them, and even this is subject to proper confirmation7.
Once outside an SPF facility, maintenance of high-health (HH) status requires that all SPF shrimp must be quarantined, isolated and reared away from those that may be infected for their entire life cycle to prevent the spread of pathogens to the clean stock. Once the initial SPF stock has been established, new HH stock can be produced locally, using specific rearing techniques that avoid contamination. These techniques, although known, are not easy to fulfil and so far have only been achieved in the United States of America (and possibly Iran).
Another point to consider when buying SPF stocks with which to begin domestication programmes in other countries, is that such stocks may have been deliberately in-bred and consist entirely of siblings. This means that future generations of animals based only on such lines will probably lead to inbreeding within a few generations. Such inbreeding has been noted in stocks of P. stylirostris bred in Tahiti for 22 generations (Bierne et al. , 2000). It has also been noted in captive stocks of P. vannamei, which were characterized by a diminished ability to tolerate TSV challenges compared to a more diverse, heterozygous wild control population (Jones and Lai, 2003).
There are many problems involved with the use of non-SPF broodstock. The first and foremost has already been discussed which is the possibility of importing novel pathogenic viruses and other diseases into new or clean areas. This has already been seen in Asia with the introduction of P. vannamei into Mainland China, Taiwan Province of China and Thailand. The problem here is that non-SPF shrimp tend to be cheaper and more easily available (pond-reared broodstock in Asia currently sell for US$8–10, while SPF broodstock from Hawaii cost US$23–25 delivered) and are hence initially attractive, but may have long-term negative consequences.
In addition, without strict biosecurity and disinfection protocols for treating non-SPF broodstock, eggs and nauplius (which are largely unknown and unused in Asia), any pathogens infecting the broodstock tend to be passed to the larvae which increases the possibility of serious disease problems during on-growing. Another problem is that it is extremely difficult to ascertain whether the stocks bought in are really SPF or not. Often competent testing facilities do not exist in Asian countries and many unscrupulous dealers will sell supposedly SPF stocks with false certificates to unwary farmers. A final problem is that while SPF stocks are almost certainly domesticated lines which have been selected for growth and disease resistance over a long period, non-SPF stocks may not have been selected and are of often uncertain parentage. This makes their use as founder populations for genetic selection and domestication programmes undesirable.
SPR is another term that is often misconstrued and is short for Specific Pathogen Resistant. It describes a genetic trait of a shrimp that confers some resistance against one specific pathogen. SPR shrimp usually result from a specific breeding programme designed to increase resistance to a particular virus. SPF and SPR are independent characteristics. Not all SPR shrimp are SPF and vice versa.
Much work has been done on the selective breeding of P. vannamei and P. stylirostris for increased growth rate and resistance to a variety of diseases, with many positive results. Such work was initiated in Tahiti by “Aquacop” in the early 1970s with a variety of species, and by the Oceanic Institute and commercial companies using their original SPF lines since 1995.
In fact, recent research work by some state and private companies has focused efforts on the development of SPF strains that are also resistant to specific pathogens (SPF/ SPR). These strains are typically resistant to only one pathogen, currently largely either TSV or IHHNV, but some work has indicated that strains with multiple resistance to TSV and WSSV (at up to 25 percent survival to challenge tests) may be possible (Jim Wyban, per. com.). This is accomplished by challenging sub-lots of shrimp families to a particular pathogen (or combination of pathogens) and then selecting the most resistant families as broodstock for the next generation. Some recent work with SPF/SPR strains of P. vannamei challenged with different isolates of TSV has shown survival rates of 55–100 percent in the lab and 82 percent in ponds (Jim Wyban, per. com.; James Sweeney, per. com.).
A selective breeding programme for P. vannamei was initiated in 1995 in the Oceanic Institute in Hawaii. Original work was based on a selection index weighted equally for growth and TSV resistance (the major disease problem in the Americas at that time). Confirmation that growth and survival (to TSV challenge) responded well to selection was obtained, but there appeared to be a negative genetic correlation between these traits. Further investigation revealed that the shrimp selected only for growth were 21 percent larger than unselected shrimp (24 vs. 20 g) after one generation, with a realized heritability (h2) of 1. Females were 12.7 percent larger than males at about 22 g, but it was not possible to select for a higher percentage of females. Meanwhile, shrimp selected on an index weighted 70 percent for TSV resistance and 30 percent for growth showed an 18 percent increase in survival to a TSV challenge (46 vs. 39 percent) after one generation, with a realized heritability (h2) of 0.28. However, selected shrimp were 5 percent smaller than control shrimp, revealing a negative genetic correlation between mean family growth and mean family survival to a TSV challenge. This negative correlation between growth and disease resistance must therefore be taken into account when developing breeding plans for these shrimp (Argue et al., 2002).
However, recent work in progress in a United States-based facility producing SPF and SPR P. vannamei has reportedly achieved a growth rate potential of 2 g/week with families of shrimp selected for resistance to TSV, with no negative correlation between growth and survival. Additionally, they have seen an 18 percent/generation average improvement in growth rate in families selected only for growth (Edward Scurra, per. com.).
SPR strains of shrimp, however, do not necessarily have to be SPF. Latin America is now almost exclusively using pond-grown and (often) disease checked and quarantined SPR P. vannamei due to their better performance in maturation, hatcheries and grow-out ponds. A recent survey conducted by FAO revealed that there were close to 100 maturation units (mostly in Ecuador and Mexico), producing 15 billion nauplius/ month, stocking close to 400 hatcheries, mostly of SPR P. vannamei (and P. stylirostris in Mexico) (FAO, 2003).
The Latin American SPR strains of P. vannamei have high genetic diversity, coming from multiple sources (both SPF and non-SPF), and have been selected from the survivors of multiple disease outbreaks in grow-out ponds, in some cases for five years or more (i.e. in Panama, Ecuador, Colombia and Brazil). They may thus have more resistance to a combination of diseases (i.e. WSSV, TSV and IHHNV) than their purely SPF counterparts and be uniquely adapted to the culture conditions and diseases encountered in their respective countries. Commercial results have indicated that such selection procedures can enhance both maturation attributes (i.e. behaviour, time to spawning and spawning rate) and growth rate (10 percent increase/generation) and survival (disease resistance) during pond on-growing (Matthew Briggs and Neil Gervais, per. com.).
Suppliers of SPF and SPR shrimp (source: first author)
|High Health Aquaculture Inc.||Hawaii||M, V, S, J||B, N, PL||Yes||to TSV1|
|Shrimp Improvement Systems||Florida||V||B, N, PL||Yes||to TSV1|
|Molokai Sea Farms Intl.||Hawaii||V||B, N, PL||Yes||to TSV1|
|The Oceanic Institute||Hawaii||V||R, N, PL||Yes||to TSV1|
|Ceatech USA Inc.||Hawaii||V||R, N, PL||Yes||to TSV1|
|Kona Bay White Shrimp||Hawaii||V||R, N, PL||Yes||No|
|AFTM||Iran||I, Me, Se||R, N, PL||Yes||7|
|Xiamen Xinrongteng ATD||China||V, J||PL||No||?|
|Seajoy S.A.||Ecuador, Honduras||V||R, N, PL||No||7|
|Pacific Larval Centre, Inc.||Panama||V||R, N, PL||No||7|
|Acuacultura de La Paz S.A.||Mexico||V||R, N, PL||No||?|
|Tincorp S.A.||Ecuador||V||R, N, PL||No||?|
|C.I. AquaGen S.A.||Colombia||V||PL||No||?|
|Supershrimp Group||California||S||R,N, PL||Yes||to IHHN|
|Farallon Aquaculture S.A.||Panama||V||PL||Yes||to TSV1|
SPF/SPR status: “Yes” indicates the claims of the supplier, however, detailed information is not available to the authors regarding the actual pathogens that the stock supplied is claimed to be free of, or resistant to.
Specific pathogen resistance to TSV is only for certain TSV strains, not all. To date, SPR status is only confirmed for P. stylirostris strain resistant to IHHNV. Some P. vannamei stocks exist with limited resistance to TSV strain 1 but not to strains 2 and 3. There are no stocks available that are resistant to WSSV.
Species: M = P. monodon, V = P. vannamei, S = P. stylirostris, J = P. japonicus, I = P. indicus, Me = P. merguiensis, Se = P. semisulcatus
Life stage: B = Broodstock, N = Nauplius, PL = Postlarvae
TSV can cause significant losses in farms stocked with P. vannamei and can be transmitted easily through insect or avian vectors between ponds. Because of this, the use of TSV-resistant strains combined with biosecurity measures to reduce infections with other viruses such as WSSV, IHHNV and YHV could greatly assist the development of the new culture industry for P. vannamei in Asia. Such a protocol was adopted by the United States of America industry that, as a result, has seen a 50 percent growth rate per year over the last three years (Wyban, 2002).
Some work has also recently been done developing a strain of P. chinensis that is SPR for WSSV. Improvement in survival rate from 0–0.8 percent to 12–45 percent was recorded from ponds stocked with PL produced from survivors of a WSSV epidemic, while lab challenge tests revealed 30–60 percent improvements in survival rates for 3rd and 4th generation survivors. That this was due to resistance was proven by polymerase chain reaction (PCR) testing which showed both control and selected animals to have an average 60 percent infection rate with WSSV (Jie et al., 2003).
The development of WSSV-resistant lines of P. vannamei is a possibility. Because WSSV remains the biggest disease problem in Asian shrimp culture, this would provide a much-needed impetus for the Asian shrimp culture industry as a whole. The recent applications of quantitative genetics to shrimp breeding, including the identification of various molecular markers (particularly microsatellites) associated with disease resistance and growth, offer a method through which the selection of fast-growing, disease resistant strains might soon become much more efficient. It may also shed some light on invertebrate antiviral immunity, about which currently nothing is known. Such disease related markers have already been identified for IHHNV in P. stylirostris (Hizer et al., 2002).
The selected line of £stylirostris, commercially known as “supershrimp”, have been shown to be 100 percent resistant to an infectious strain of IHHNV fed to juveniles during laboratory challenge tests. The shrimp remained free of the disease over the 30 day trial period and so were really refractory rather than resistant since the virus did not replicate within the shrimp (Tang et al., 2000).
After harvest, if well treated with plenty of ice, P. vannamei are particularly resistant to melanosis and keep a good appearance three to four days after defrosting. However, P. monodon tend to have a longer shelf life and are easier to handle, transport and process than P. vannamei.
7 To date, SPR status is only confirmed for a line of P. stylirostris resistant to IHHNV. There are some P. vannamei stocks with limited resistance to TSV strain 1, but this does not extend to strains 2 and 3. There are no stocks available that are resistant to WSSV.