Visitors to grouper farms are often curious as to why the fish gather lazily at the bottom of ponds or fry tanks, moving very little. It turns out that stillness is the norm for bottom fish. If instead the fish appeared stir crazy, turning hither and thither and swimming often to the surface, the behavior, rather than suggesting fish that were brimming with vitality, would probably mean that the young groupers were sick. If the fish also lost their appetite and began to change color, then one could be certain that they had come down with a virus.

NNV, source of the greatest headaches among grouper farmers in recent years, mostly afflicts 3-6-cm fingerlings in the 14 days immediately after hatching. The iridovirus hits 3-9-cm fingerlings. Both of these viruses attack groupers when they are immature and their immune systems are weak. Because there are no drugs to combat the viruses, 80-100% of groupers that fall victim to them will die.

To take effective preventative measures, it is first necessary to know how a fish comes down with a virus. What is the path of transmission? Chou Hsin-you, a professor of aquaculture at National Taiwan Ocean University, explains that most diseases that afflict fish are transmitted through water-borne organisms or food. "The gills of fish have numerous small capillaries, and the viruses enter the bloodstream through small openings in the capillaries." They then go on to infect internal organs or the entire body. The iridovirus thus infects fry via the bloodstream.

The tangle of effluent and intake pipes climbing the dykes along the Pingtung coast from Fangliao to Jiadong were installed by fish farmers themselves. Inefficient and leading easily to the intake of polluted seawater, they have turned this area of intensive aquaculture into a hotspot for fish diseases.

NNV is a rather special virus. Chi Shua-chi, a professor of life sciences at National Taiwan University and an expert in fish virology, explains that NNV can go through neurofibrils on the outer surface of fry to enter the spinal cord, brain and retina, where the virus mass replicates, destroying the tissue in the brain and retina. "Because infected fry decompose quickly after dying, the NNV is released into the water. At ample levels, a large number of fry will come down with the virus." Chi explains that experiments have proven that when infected fry are mixed with uninfected fry, or when infected fry are removed en masse but the pool or pond isn't disinfected thoroughly enough, then healthy fry can catch the virus via "horizontal transmission."

NNV can also move from the tissue of the digestive tract's vagus nerve to the brain or retina, where it mass replicates. Groupers are carnivorous fish, and healthy, fast-growing fish often eat weaker young fish. The gorging fish can then acquire the virus through their digestive tracts.

What's more, both iridovirus and NNV can be transmitted to fry via their feed. Grouper fry require large numbers of live copepods to eat, Chi explains, and cur-rently fish farmers raise copepods, which are fed with dead fish and bycatch, in the same -earthen ponds that they use to hold mature fish. Under these circumstances, the copepods easily become virus carriers and thus agents of horizontal transmission.

The thorniest problem is water pollution in the wider environment. When this reporter toured areas of intensive aquaculture development along the Pingtung coast, she saw an alarming assortment of pipes climbing the levees. Because the intake and effluent pipes for the ponds and tanks of each farm were individually installed, it was hard to distinguish between them. Unavoidably, pipes drawing water into ponds were located not far from pipes expelling effluent with a high density of virus into the sea. "This makes it very easy for an epidemic to spread throughout the whole area of intensive aquacultural development along the coast," says Chi.

A farmer holds up cute and pretty grouper fry. Grouper farmers are eagerly awaiting vaccines that are expected to be approved for sale later this year.

Fry can also catch these viruses from fish carriers. According to current understanding, iridovirus and NNV cannot infect fish eggs, but can grow in the reproductive glands of carrier fish and may be found in the fluid expelled with the eggs. This can potentially result in "vertical transmission." And if breeding fish are weak and their immune systems impaired, the viral load within their bodies will rise.

"Many people mistakenly believe that a vaccine is all that is needed to solve the problem," say Chou Hsin-you and Chi Shao-chi, who spoke at a conference on grouper breeding techniques held in early December 2010. If you want to prevent the spread of these grouper viruses, they say, then you've got to examine breeding methods and health controls (such as those covering fish immunity and feed, including live feed) and select fish for breeding with an eye toward blocking every entry point for a virus. What's more, you need to mobilize all breeders and fish farmers to unite in working toward the same goal.

Preventing disease is like going to war: Imposing tight preventative measures that block all avenues of transmission-whether through water, feed, breeding stock, or tools-poses great challenges.

Scholars have learned from experiences with NNV in Japan, where the virus wiped out sole stocks before being successfully stamped out.

"Back then the steps taken to prevent vertical transmission involved screening the breeding fish and rejecting all carriers," says Chou. "Eventually, all of Hokkaido had only 10 breeding fish." Vectors in vertical transmission include the eggs of carriers. Consequently, it was necessary to use ozone and electrolysis to clean the water and reduce the quantity of virus on the surface of the eggs.

Preventing horizontal transmission requires working on several fronts at the same time: inspecting fry to determine if they are carriers, cleaning live feed, sanitizing breeding tools and the water both going in and coming out of tanks and ponds, and making sure that workers often wash their hands, work boots, fish tanks, buckets and so forth. Anything that can serve to carry a virus must be effectively sanitized.

"Some fish farmers who are already sanitizing seawater with ozone have asked me why their fish are still sick." Chi says that several factors have to be precisely calculated in order to bring about "effective disinfection." These involve the ozone machine's design and specifications, the sanitization process, the amounts of ozone used, and the proper length of time.

Chi points to three main factors that play a role in viral outbreaks: the viruses, the environment, and the afflicted organisms themselves. NNV and the iridovirus are potent viral strains, but if proper controls can be exercised over the breeding environment, and further steps can be taken to bolster the fish's own resistance-such as through vaccinations and improved nutrition-the situation can improve.

"Although vaccines bolster the fish's resistance, there are many limiting factors," says Chi. First of all, they can only be used after the immune organs are mature (about 18-19 days after hatching). When fry reach 3 cm (at 40 days) their immune system will be more mature, and the results will be even better. The second restriction is that orally administered or water-immersed vaccines require at least two weeks to create a sufficient number of antibodies, and that period of immunity lasts for three months. Consequently, if the young fish encounter stressors (such as changes in water quality, temperature or oxygenation) during the first two weeks, those experiences can cause a loss of immunity, meaning that the fish can potentially still fall ill.

(right) The academic community recommends moving one-to-four-month-old fry inside and strictly controlling water quality in order to prevent the spread of bacteria and the transmission of viruses. The photo shows a lab at National Kaohsiung Marine University engaging in "drawer rearing." They have already succeeded in raising green groupers to maturity in these drawers.

In reality, NNV and the iridovirus were already a problem more than a decade ago when fish farmers in Taiwan began to raise groupers. Back then breeders practiced "carpet breeding"-using huge numbers of eggs to provide sufficient numbers of surviving fry. But with growing numbers of groupers dying from viruses, what new measures can breeders take to adapt? It's a cause for concern.

Pan Jianzhang, who raises 6-cm grouper fingerlings in Jiadong, says that all fry can be carriers. The key point is whether they will actually come down with the virus. So how do you go about selecting strong-bodied fry with high levels of resistance? That depends upon the individual experience of those raising the fish. "Every-one views things a little differently." He typically looks at pond controls, water color, and the presence or absence of bubbles to determine whether the situation is good or bad.

Freshness of feed is essential, and when fry have to change tanks or ponds for cleaning, great care must be taken to ensure they are not long out of water. Because the seawater in fishponds is running 24 hours a day, so much is being used that it's hard to completely disinfect it. Except for those raising high-value giant groupers, most farmers cannot afford to invest in water circulation systems. Green groupers are of relatively low value. Pan can't justify the expense of a circulation system, and it's possible that sanitizing the water wouldn't even resolve the problem.

Pan says that most farmers raising 3-cm grouper fingerlings in Pingtung don't disinfect their seawater. Facing lower and lower survival rates, the farmers are at their wits' end.

ISO grouper standards

With the menace posed by viruses, the Fisheries Agency of the Council of Agriculture has hired researchers from National Taiwan University, National Taiwan Ocean University, and the Animal Health Research Institute to carry out research on vaccines, and has teamed up with the COA's Bureau of Animal and Plant Health Inspection and Quarantine (BAPHIQ) to establish a quality control system for aquacultural products. It aims to implement first-rate production and disease prevention standards at one stroke.

BAPHIQ's director-general Hsu Tien-lai explains that the Ma administration's push for high-value specialty agricultural products has largely been aimed at export, so that producers must adapt to the demands of the international market. And international markets in recent years have been requiring that exporters show concern for human health issues and environmental responsibility in addition to animal disease control. Various nations have developed different quality control systems. Agricultural exports to the United States, for instance, must meet Hazard Analysis Critical Control Point standards, and those to Europe must meet the EU's Good Agricultural Practices standards. There is another set of standards to meet for export to Japan. These various regulations cover the whole process of agricultural production, from plant and animal diseases, to environmental impacts, to food safety.

Because different nations make different demands, the BAPHIQ, in order to help producers planning to export to meet the market conditions of the importing countries, has created new export inspection guidelines, and revised the Fisheries Agency's quality standards at the production end, creating a unique quality-control system for Taiwan that is similar to ISO standards. These standards will be implemented throughout the production process via permits and inspections.

With regard to grouper exports, overseeing agencies have developed a quality control system for breeding stock and their offspring, which is known as the "high bio-security isolation production system." It starts with inspection of fry, so as to ensure that none of them have any viruses. And it also includes standards for water supply, feed, and sanitation of equipment. The Fisheries Research Institute has already established a model breeding farm and hatchery in Tainan in the hope of getting the aquaculture industry to copy its methods of isolated production, which offers a high level of safety. So far, a few fish farmers in Tainan have adopted the model. Since the Tainan coast doesn't suffer from the same pollution problems found along the Pingtung coast and enjoys relatively clean water, survival rates under the new regimen have climbed as high as 80%.

Hsu Tien-lai believes that if fish farmers adopt this set of quality controls and vaccinate young fish before they are placed in ponds, then, based on the 70-80% survival rates for vaccinated fish in NTU labs, it should be possible to attain survival rates of 30-40% for vaccinated fish raised outside, despite the numerous complications.

Huang Youyi, who leads the Fisheries Agency's aquaculture group, explains that these quality controls are expected to be authorized in the middle of 2011. They will then be promoted in the hope of their being widely adopted.

Fish farmers and academics have called for the government to take drastic measures to resolve the problem of polluted seawater being drawn into fish ponds by overhauling the system of intake and effluent pipes along the coast. Although the government is currently building a seawater supply station to provide fish farmers with water in Pingtung's area of highest production, most farmers believe that the supply will be insufficient to meet their needs. "It used to be that a few hours was all that was needed to completely change the water in the ponds. If farmers end up needing to wait their turn for a day or two, that additional time will make it easier for the fish to suffer health problems, and in the end we're going to end up using our own pipes to intake water."

The general plan to solve the damage wrought by grouper viruses is ready to be put into place, and various government agencies are introducing measures of their own. But will these be enough to meet the needs of fish farmers on the front lines and resolve various implementation problems? The answer remains to be seen.

Disease control measures in grouper aquaculture

Important supplementary measures:Veterinary management and process control in production (P); diagnosis and treatment (D); good aquacultural practices (G); disease monitoring (M); laboratory support (L); management of aquacultural facilities and water supply in production zones (A); vaccine development and use (V).
source: BAPHIQ