As Asia’s aquaculture production continues to rise, so have the losses associated with disease outbreaks. The drive to intensify production and to maximise short-term profits may mean that investment in appropriate biosecurity measures is reduced, thereby exposing aquaculture enterprises to an increased risk of disease. In this article, we consider some of the losses associated with current pathogen events that impact on Asia’s crustacean and tilapia industries and focus on selected viral, bacterial, fungal, and parasite infections as examples.
An impressive growth but…
Over the period from 2001 to 2013, Asia’s aquaculture industry has grown at an impressive average 6.85% year-on-year, and increasing its output 2.37 times from 37.59 million tonnes in 2001 to 88.90 million tonnes in 2013. The production increased 8.64% annually over the 2012-2013 period.
Although, there has not been a suggested drop in total output since 1961, the production, sustainability and economic viability of each aquaculture enterprise, can be significantly impacted upon by a broad spectrum of obligate or opportunistic aquatic pathogens. This is appropriately demonstrated by the historical growth of the shrimp industry, which has been characterised by cycles of booms followed by busts caused by disease outbreaks.
First were viral agents throughout the 1990s (e.g. by yellow head virus (YHV), Taura syndrome virus (TSV) and white spot syndrome virus (WSSV). Since 2010 to present, it is the bacterial agents (e.g. isolates of Vibrio parahaemolyticus with a toxin gene bearing plasmid responsible for acute hepatopancreatic necrosis disease (AHPND), and most recently the fungal pathogen - the microsporidian Enterocytozoon hepatopenaei which is the causative agent of hepatopancreatic microsporidiosis (HPM).
For many pathogens, their occurrence and impact on production can be unpredictable/sporadic, whilst others may be predictable/ regular. While these infections may result in the direct loss of stock and incur costs associated with the control and management of infections once established, for predictable infections there are also costs associated with the implementation and maintenance of biosecurity/preventative measures, prophylactic treatment, regular screening and management.
Calculating the true cost of each disease outbreak therefore is complicated by an intricate interplay of a diverse array of environmental and management based factors that can have repercussions through the entire production pipeline from the direct losses in stock, through the associated downstream industry, and on the livelihoods of those affected.
Impact of crustacean and tilapia diseases in Asia
So what is the current picture of disease outbreaks across Asia? Well, while it is not possible to address the impacts of each pathogen within this short article, we provide a summary of pathogen presence across the 42 Southeast Asian aquacultureactive states over the last three years (Table 1).
For certain pathogens like Streptococcus, the figures cited are most likely underestimated. From this summary, some of the losses currently incurred within Asia’s crustacean and tilapia industries are considered by focusing on selected viral (e.g. WSSV), bacterial (e.g. V. parahaemolyticus; Streptococcus spp.), fungal (e.g. microsporidian E. hepatopenaei), and parasite infections as examples.
Losses within the shrimp industry
AHPND
The current Asian outbreak of AHPND, which first appeared in China in 2009, Vietnam in 2010, Malaysia in 2011 and, then Thailand in 2012 has proven to be devastating. If the situation in Thailand is considered and assuming that the Thai production of Litopenaeus vannamei, in the absence of other disease events, had been maintained at its last peak of 603,227 tonnes in 2011, then the current AHPND outbreak has cost the Thai industry in excess of USD 5.01 billion to date (based on the value of lost tonnage).
Elsewhere, the current status of AHPND in the Vietnamese Mekong Delta suggests that 2,318 ha for Penaeus monodon culture and 2,309 ha for L. vannamei have had infections this past year (i.e. throughout 2015). If the following are assumed:
- 38% and 52.7% of the total area was used for semi-intensive culture for P. monodon and L. vannamei respectively; the remaining area was extensive production;
- P. monodon sites were stocked at 15 post larvae (PL)/m2 (semi-intensive) and 8 PL/m2 (extensive); the L. vannamei sites at 100 PL/m2 (semi-intensive) and 70 PL/m2 (extensive);
- one crop was lost from each site within 40 days of transfer (i.e. a T50 of 20 days is used); and,
- apply a labour rate of USD 5.77 ha/day
Then the losses, including other factors (e.g. feed costs etc), can be simply estimated at USD 1.84 million for P. monodon which translates into a loss of USD 796/ha. In the case of L. vannamei, losses were calculated at USD 8.93 million which was USD 3,867/ ha because of the higher stocking densities typical in L. vannamei production systems.
WSSV
Robust data on the losses due to WSSV also exist for the Mekong Delta with 2,510 ha of ponds used for P. monodon culture and 1,397 ha of ponds for L. vannamei reported as having had infections in 2014. By applying the same assumptions and a 2% daily loss over a 109 day grow-out period, then losses of P. monodon were in the region of USD 2.37 million which was USD 943/ha. For L.vannamei, it was USD 5.65 million or USD 4,041/ha.
EHP
There are, of course, levels of complexity to consider in modelling and estimating the costs of each disease, and figures are based on the quality of datasets available. In light of this, the impact of EHP on the Asian shrimp industry serves as an appropriate example. This enigmatic, fungal pathogen, that only infects the tubule epithelial cells of the hepatopancreas, reputedly does not cause mortality but results in the severe retardation of growth.
The current economic impact of this parasite within China, India, Malaysia, Thailand and Vietnam requires establishing. Estimates can be supported through the provision of national statistics regarding the area of ponds affected and detailed farm production data from infected sites. Some reports from farm producers, however, are emerging. L. vannamei growth is reported to arrest at around 12 g, capping production at approximately 9 tonnes/ha as opposed to the nominal target of 12 tonnes/ha. The decision to harvest early means that farm gate prices for the smaller size shrimp are a third lower at USD 3.50/kg instead of USD 5.30/kg for 18 g shrimp. The net result is that production costs are not covered. There is a loss of USD 4,500/ha or a loss of USD 32,000/ha over normal production economics if the shrimp are grown on to the planned harvest size. In the absence of pathogens and, assuming the normal growth of stock and a survival of >70%, then a food conversion ratio (FCR) of 1.5 might be expected. If EHP is present and slowed growth from 12 g onwards is noted, then FCRs can rise – the final value of which will be dictated by the severity of infection in the stock, the monitoring frequency and by management decisions regarding when to harvest (i.e. when FCRs rise outside the acceptable range to the point where decisions regarding harvesting have to be made).
Losses within the tilapia industry
If some of the disease costs within the tilapia industry are considered, then there are both evident and, perhaps, ignored losses. Streptococcal-based infections, with significant resultant mortalities, have been reported from a large number of Asian countries. Thai sites, for example, reported losing 20% of production in the hot season, with a minimum 7.5% total loss across the industry. This is 16,270 tonnes valued at USD 26.57 million. In Bangladesh, mortalities attributable to Streptococcus were estimated at >26% at certain sites. If these are representative nationally, then losses can be estimated at 20,910 tonnes valued at USD 36.67 million. Elsewhere, in China the reported mortality range was 80-90% at sites within Leizhou Bay, where production is estimated at 394,600 tonnes. An undisclosed site elsewhere in Asia reported losing 50,000 tonnes of stock valued at USD 81.65 million.
If we extrapolate from the production in 2014 by applying a 10.68% year-on-year increase, then Asia’s tilapia industry looks set to top 4.11 million tonnes for 2015. If a 7.5% mortality across Asia is assumed, except for countries such as Bangladesh and the Philippines, where smaller size fish are harvested, and where a loss of 3.75% can be applied, then the losses to streptococcal infections could be in the region of 289,440 tonnes valued at USD 480 million (applying an average Asian price of USD 1,657/ tonne).
Impact of sporadic mortalities
Alongside some of the major disease events discussed above, many of the smaller magnitude, sporadic mortality events that can be attributed to less specific diseases, which can result from low water quality or poor fish handling and welfare that might be ameliorated through improved husbandry practices, are also worth considering. Although most of these losses are ignored and/or unreported, either because of the smaller scale of these losses or because of the general acceptance that they fall within the typical, accepted margins of loss in production, their collective impact on aquaculture production can be very significant. The value of stock lost may arguably exceed the economic impact of many major pathogens.
As an example, the losses of juvenile Nile tilapia due to parasitic infections were studied at four Thai commercial farms over a 12-month period. Assuming the following survival rates at each stage of production:
- 77.5% egg hatch rate;
- 77.8% survival from hatched eggs to the swim-up fry stage;
- 78.9% survival of swim-up fry to 21 day post-mono-sex fry and,
- 83.3% survival of the 21 day mono-sex fry to 2.5 cm nursery sized fish.
By applying a 20% loss due to parasitic agents to each life cycle stage, the total loss to Asia’s tilapia industry is in the region of USD 13.5-18.5 million annually. This is the total of USD 4.1-5.7 million loss at the nursery stage; USD 5.0-6.8 million loss at the mono-sex stage; and USD 4.4-6.0 million loss at the swim-up stage.
Concluding remarks
Estimating the costs of disease events within the shrimp and tilapia industries is complicated, not only by the magnitude of each industry and the number of animals/sites involved, but also by the frequency of disease reporting and the access to quality farm data. Minimal investment in preventative measures, lapses in biosecurity rigour, lack of regular farm health assessments, inconsistent disinfection practices and/ or complacency, consequentially exposes sites to higher risks of pathogen introduction that may result in frequent disease episodes and loss.
For intensive systems, even occasional lapses in biosecurity can result in devastating losses. Although Asia’s L. vannamei shrimp and tilapia industries continue to rise, averaging 7.93% and 10.82% year-on-year over 2009-2013 period, the long-term trends suggest that rate of growth for both is decreasing. While this slowing growth may be imposed by the availability of new sites for culture and access to resources etc, the correlation of disease risk and loss with the intensification of each industry cannot be ignored and must be addressed through appropriate biosecurity, health management and control programs if Asia is to maximise its aquaculture potential by minimising losses.
This article first appeared in Aqua Culture Asia Pacific magazine
