the fish

In May 1994, an outbreak of a relatively new and serious shrimp disease killed more than 95% of the Penaeus vannamei at an aquaculture farm in Kahuku, Hawaii. The malady that ravaged this farm and decimated its shrimp stocks was Taura Syndrome, an illness that previously caused more than $100 million worth of damage to shrimp farms in South America but had not previously attacked shrimp farms in Hawaii.

While the severity of this bout of Taura Syndrome is somewhat atypical, the shrimp farmer's experiences in coping with the disease underscore the devastating impact that illness can have on the larval and growout production of marine shrimp, and the need for practical, cost-effective tools for disease management. Scientists working at the University of Hawaii, The Oceanic Institute, and the State of Hawaii’s Aquaculture Development Program are seeking to learn more about Taura Syndrome and the Infectious Hypodermal and Hematopoietic Necrosis (IHHN), two major diseases that endanger the growout and broodstock production of Penaeus vannamei and P. stylirostris in Hawaii. While some important strides have been made in developing new preventative methods to combat these illnesses, additional research needs to be done, especially on methods to control the disease.

Taura Syndrome Virus

In 1994 the cause for Taura Syndrome was uncertain. Work conducted in Hawaii and Arizona led to the discovery of a previously unknown virus now called the Taura Syndrome Virus (TSV). Subsequent to the discovery of TSV, researchers at the University of Arizona have shown TSV as the direct cause of Taura Syndrome. While Taura Syndrome Virus can infect both Penaeus vannamei and P. stylirostris, the expression of the disease and the severity of its symptoms differ in each animal. In general, P. stylirostris is much more resistant to TSV than P. vannamei, and P. stylirostris stocks afflicted with TSV can still be successfully harvested and sold by a farmer with good management practices. In contrast, TSV may cause mortality rates as high as 75-80% in P. vannamei, significantly impacting the crop's economic viability.

TS and Penaeus vannamei

Taura Syndrome generally attacks juvenile P. vannamei (0.1 to 5.0 g) within two to four weeks after stocking in growout ponds or tanks. TSV is primarily an illness of the cuticle epidermis (outer exoskeleton) in shrimp. Shrimp in the chronic phase of TSV have scattered, black-spot lesions along their outer skin or shell. During TSV outbreaks, dead and dying shrimp will often be seen in seines or cast nets used for routine population sampling or found lying along the bottom of the grow-out tanks or raceways. Shrimp afflicted with the acute phase of TSV appear weak and disoriented, have soft-shells, and have expanded chromataphores (pigment spots) that may alter their color slightly. Infected shrimp also have empty digestive tracts.

Solutions

More research must be conducted in order to learn more about the way Taura Syndrome Virus is spread and to develop new methods to control the disease. One approach that minimizes the harmful impacts of TSV is to grow P. stylirostris instead of P. vannamei in farms that have experienced high mortalities due to TSV infection episodes.

IHHNV and Runt Deformity Syndrome

The pattern of susceptibility seen with the Taura Syndrome Virus is reversed with another important marine shrimp virus, the Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV). While IHHNV is fatal to Penaeus stylirostris, P. vannamei are relatively resilient to the disease with certain modifications in management practices.

In P. vannamei, IHHNV infection can cause Runt Deformity Syndrome (RDS), an illness that often has a significant impact on a farm’s productivity by reducing a shrimp’s potential for growth. While healthy shrimp crops generally grow at uniform rates (simplifying harvesting, sorting, and marketing), IHHNVinfected shrimp crops typically have a large disparity in growth rates. RDS is a complex interaction between virus and host.

Some scientists believe the impact of IHHNV is most severe when a shrimp is infected early in its life —either as an embryo or as a larvae. In farms stocked with IHHNV-infected shrimp that were exposed to the virus at an early life-stage (i.e. spawning, hatchery), fewer pounds of shrimp per unit area are produced using the same amount of feed. Crop losses from poor feed conversion, reductions in weight and increased sorting costs need to be closely managed by a farmer in order to turn a profit.

Financial support for this work was provided by the Center for Tropical and Subtropical Aquaculture through a grant from the United States Department of Agriculture (USDA grant #94-38500-0065). Partial support was provided by the Aquaculture Development Program, Hawaii Department of Land and Natural Resources (contract #34096). Production support was provided by the University of Hawaii Sea Grant College Program (SOEST), under Institutional Grant No. NA36RG0507. The views expressed herein are those of the author and do not necessarily reflect the views of USDA, CTSA, NOAA or any of its subagencies. UNIHI-SEAGRANT-FS-96-02.

Specific Pathogen-Free Broodstock

Penaeus vannamei broodstock that have not been infected with IHHNV will produce offspring that grow normally even in an IHHNVinfected environment, as long as these animals are stocked as juveniles. While specific pathogenfree stocked P. vannamei may eventually develop an IHHNV infection due to the virus they will continue to grow within a normal size range and fetch the same prices as non-infected shrimp. The offspring of the specific pathogen-free broodstock are still succeptable to IHHN; consequently IHHN-free shrimp should be used for broodstock rotation and consistent production.

New research

In the past, aquaculture researchers sought ways to remove IHHNV that reside in pond environments after an outbreak of disease. However, these efforts have had little success. Attempts to kill IHHNV by disinfecting a pond using lime were problematic with variable results. Current research is investigating methods of using high temperatures (i.e., solar radiation) rather than high pH to kill these shrimp viruses. Scientists are also seeking to better understand the ways in which TSV and IHHNV are transmitted. To this end, studies are examining the possible role that bird feces may play in spreading IHHNV and TSV infections among shrimp facilities and populations.

Where to turn for help

If your shrimp crop is experiencing any of the symptoms mentioned above, or if you would like more information on these marine shrimp diseases, contact one of the following:

Aquaculture Development Program
at Anuenue Fisheries Research Center
1039 Sand Island Parkway
Honolulu, Hawaii 96819-4347
(808) 845-9561 Phone
(808) 845-4334 Fax

The University of Hawaii Sea Grant Extension Service
1000 Pope Road, MSB 226
Honolulu, Hawaii 96822
(808) 956-2873 Phone
(808) 956-8191 Fax

Information on the culture of the milkfish (Chanos chanos) from the FAO Cultured Aquatic Species Information Programme.

biological features

Body fusiform, elongated, moderately compressed, smooth and streamlined. Body colour silvery on belly and sides grading to olive-green or blue on back. Dorsal, anal and caudal fins pale or yellowish with dark margins. Single dorsal fin with two spines and 13-17 soft rays. Short anal fin with two spines and 8-10 soft rays, close to caudal fin. Caudal fin large and deeply forked with large scale flaps at base in adults. Pectoral fins low on body with axillary (inner basal) scales. Pelvic fins abdominal with axillary scales and 11 or 12 rays. Scales cycloid, small and smooth, 75-91 on lateral line. No scutes (modified pointed scales) along belly. Transparent 'adipose' tissue covers eye. Mouth small and terminal without teeth. Lower jaw with small tubercle at tip, fitting into notch in upper jaw. No bony gular plate between arms of lower jaw. Four branchiostegal rays supporting underside of gill covers. Gill rakers fine and numerous. Attains typical length of one metre but may reach maximum length of 1.8 m (male).

Profile

Historical background

Milkfish farming in Indonesia, Taiwan Province of China and the Philippines started about four to six centuries ago. Culture methods in a variety of enclosures are constantly being improved upon. Since the 1970s, large investments have been made in the Philippines (as well as in Taiwan Province of China, Indonesia and Hawaii) in terms of infrastructure, research, credit and training in support to the milkfish industry. For example, the Southeast Asian Fisheries Development Center (SEAFDEC) Aquaculture Department (AQD) was established in Iloilo, Philippines in 1973 with a special remit to find solutions for milkfish aquaculture problems. Government agencies and fisheries institutions were also involved in a national effort to intensify milkfish farming from the mid 1970s until now. In this work, research and development on farming systems, breeding and fry production technologies was carried out. There was no attempt at genetic improvement but fry translocation and trade occurred between Indonesia, Taiwan Province of China and the Philippines and geographic variations and heterogeneity were documented. More recently, unconfirmed reports indicate that milkfish are now being cultured to fingerling or juvenile size in the South Pacific Islands and in Singapore as tuna bait.

Milkfish farming was previously a traditional industry, with little emphasis on producing sexually mature, reproductively active fish in captivity. The traditional milkfish industry depended totally on an annual restocking of farm ponds with fingerlings reared from wild-caught fry. As a result, the industry suffered from regional, seasonal and annual variations in fry availability. These variations are generally unpredictable, and may be quite large over short periods of time.

Thus, the central problem faced by the international milkfish industry was to find a way to produce a reliable, adequate, high quality supply of milkfish fry that was not subject to large unpredictable variations in time and space. During the past decade, much progress has been made, particularly in regard to milkfish propagation and the mass production of fry by private hatcheries, research institutions and government agencies. Instead of relying on wild-caught fry, milkfish farms in the Philippines, Taiwan Province of China and Indonesia now obtain the majority of their fry from hatcheries, mainly due to the significant shortage of wild-caught fry.

Habitat and biology

Milkfish (Chanos chanos) is the only species in the Family Chanidae. Its distribution is restricted to either low latitude tropics or the subtropical northern hemisphere along continental shelves and around islands, where temperatures are greater than 20 °C (Red Sea and South Africa to Hawaii and the Marquesas, north to Japan and south to Victoria, Australia; and in the Eastern Pacific from San Pedro, California to the Galapagos).

Adults occur in small to large schools near the coasts or around islands. They are well developed, migratory, large (up to 1.5 m and 20 kg), and mature sexually in five years. Milkfish only spawn in fully saline waters. The activity is most often correlated with the new or full moon phases, takes place mostly in the night and, in most regions, has one or two seasonal peaks. In the natural environment, spawning takes place near coral reefs during the warm months of the year, and populations near the equator spawn year-round. Juveniles and adults eat a wide variety of relatively soft and small food items, from microbial mats to detritus, epiphytes and zooplankton.

Milkfish is a heterosexual fish; hermaphrodism has not been reported. In natural spawning stocks the sex ratio is almost equal, with a slightly higher amount of females. The determination of sex is very difficult, because there are no easily identifiable morphological differences between males and females; however, the pheromone PGF2a (prostaglandin) has been found to be an effective way to identify mature male milkfish.

Milkfish eggs (1.1-1.2 mm in diameter) and larvae (3.5 mm at hatching) are pelagic and stay in the plankton for up to 2-3 weeks. Egg division begins an hour after and hatching occurs 35-36 hours after spawning. In the wild, eggs are probably released in deeper oceanic waters and in the outer reef region. Older larvae migrate onshore and settle in coastal wetlands (mangroves, estuaries) during the juvenile stage, or occasionally enter freshwater lakes. The larvae eat zooplankton and can thrive and grow in water as warm as 32 °C. They then migrate onshore and where they can be caught by fine-mesh nets operated along sandy beaches and mangrove areas; these 'fry' are 10-17 mm long and are used as seedstock in grow-out ponds, pens and cages. In the wild, juveniles are found in mangrove areas and coastal lagoons, and even travel upriver into lakes; they go back to sea when they get too large for the nursery habitat, or when they are about to mature sexually.

Milkfish can reach a maximum size of 180 cm SL (male/unsexed) and 124 cm SL (female). The maximum recorded weight and age is 14.0 kg and 15 years respectively. Resilience is low, with a minimum population doubling time of 4.5 - 14 years. Its fisheries importance is highly commercial, especially in aquaculture, and it is also used in game fish as bait. It is especially valued as a food fish in Southeast Asia.

Production

Production systems

Seed supply

Milkfish fry can either be obtained through collection from coastal areas or littoral waters or can be produced in captivity. The supply of wild fry is often unpredictable; catches in recent years have diminished and cannot satisfy the demand from ongrowing farms.

Fry from captive broodstock and spawners

To develop broodstock under captive conditions, large juvenile milkfish may be stocked, fed and maintained in floating sea cages in protected coves or in large, deep, fully saline ponds (as practiced in the Philippines), or in large deep concrete tanks on land (as practiced in Indonesia and Taiwan Province of China), until they reach sexual maturity with an average body weight of at least 1.5 kg. Land-based broodstock facilities are entirely dependent on fresh pumped seawater supplies and are often integrated with a hatchery.

Broodstocks reach maturity in five years in large floating cages, but may take 8-10 years in ponds and concrete tanks. On average, first-spawning broodstocks tend to be smaller than adults caught from the wild. As a result, first-time spawners produce fewer eggs than wild adults, but larger and older broodstocks produce as many eggs as wild adults of similar size. Broodstocks of about 8 years old and averaging 6 kg produce 3-4 million eggs.

Breeding milkfish in captive conditions and the mass production of fry, as practised in Taiwan Province of China, Indonesia and the Philippines, is mostly dependent on natural spawning, which assures high survival rates. Artificial induction is not normally used. On days when natural spawning occurs, the fish may feed less than usual but show increased swimming activity and exhibit chasing, occasional leaping, and water-slapping activities from late noon to early evening. Spawning usually takes place around midnight but daytime spawning sometimes occurs.

Wild-caught fry

Wild-caught fry are collected with fine-mesh seines and bag nets of various indigenous designs in the Philippines, Taiwan Province of China and Indonesia. The most commonly used gear are push net 'sweepers' and dragged seines.

Hatchery production

Milkfish hatcheries consist of larval rearing tanks, culture tanks for rotifers (Brachionus) and green algae (e.g. Chlorella) and hatching tanks for brine shrimp (Artemia). Larval rearing may be either operated in outdoor or indoor systems, depending on the specific conditions in the countries where fry are being produced.

Hatchery operations utilise either intensive (high stocking density, high volume tanks, daily feeding and water exchange) or semi-intensive (low stocking density, high volume tanks, minimal water exchange, feeding with mixed diet) systems, with an average survival rate of 30 percent (from stocked newly-hatched larvae). After hatching, the larvae are ideally kept at 50/litre in hatchery tanks (either concrete, fibreglass, canvas or polypropylene-covered earthen tanks) maintained with Chlorella and fed with rotifers during the early stages and later with copepods or brine shrimp for a total of 3-4 weeks. Following this, their size ranges between 2-3 cm and they are ready for transport to nurseries.

The fry may change hands two or more times before being used for grow-out; each time this happens, they are sorted and counted, transported, and stored for different periods of time. Fry are a highly perishable commodity and some of them die during gathering, storage, transport, nursery rearing and grow-out. The technologies for fry storage and transport are generally effective, although perhaps not yet optimised. Fry are stored in a cool place in plastic basins or clay pots at 100-500/litre, in water of 10-25 per cent, which is renewed daily. Dealers may store fry for 1-7 days, depending on the demand. Fry can be maintained on wheat flour or cooked chicken egg yolk for 1-2 weeks but soon begin to die, despite continued feeding. Recently, micro-encapsulated feeds have become commercially available for finfish but the cost compared to conventional live feeds is higher.

Nursery

Nursery operations in milkfish producing countries vary according to established cultural practices.

In Taiwan Province of China, where commercial hatchery and nursery productions are integrated enterprises, milkfish fry are generally grown in either earthen ponds or elevated canvas or concrete tanks at intensive stocking densities of >2 000/litre.

In Indonesia, a well established backyard-type nursery is used. This consists of a series of elevated canvas or concrete 1-2 tonnes tanks and similar stocking densities to those used in Taiwan Province of China are employed.

In the Philippines, milkfish nurseries are integrated with grow-out facilities, where wild-caught or hatchery-reared fry are first acclimated into nursery compartments which comprise one third to one quarter of the total area of the Brackish water pond. Fry are stocked at a density of up to 1 000/litre and are fed with a naturally-grown micro-benthic food known as 'lab-lab' which grows on the fertilised pond bottom. Nursery rearing has also been carried out in hapa type suspended nylon nets installed in Brackish water ponds or lagoons and in freshwater lakes within the grow-out compartments, a traditionally practice in the Philippines. When natural food is becoming depleted, artificial feeds such as rice bran, corn bran, and stale bread or formulated feeds are provided. In about 4-6 weeks, the fry grow to 5-8 cm juveniles, which is the ideal size for releasing into grow-out ponds or pens. Depending on the desired grow-out period, juveniles or fingerling size milkfish are kept in nurseries or transition holding tanks up to the required stocking size of 30-40 g. Nursery rearing from fry to fingerling size normally achieves 70 percent survival.

Ongrowing techniques

Milkfish may be ongrown in ponds, pens or cages. Pond culture

Culture of milkfish in ponds may be in shallow or deep water systems.

Shallow water culture is practiced mainly in Indonesia and the Philippines. Milkfish are traditionally cultured in shallow Brackish water ponds in which the growth of benthic algae is encouraged through inorganic or organic fertilisation. Milkfish will survive on benthic algae alone only if the productivity of the algae exceeds the grazing rate of the fish; otherwise, supplemental commercial feeds are applied. The 'lab-lab' culture system in the Philippines is equivalent to shallow water culture in Taiwan Province of China. 'Lab-lab' is the term used in this country for the algal mat (and all micro-organisms associated with it) in the ongrowing ponds.

Brackish water ponds in the Philippines were mostly excavated from 'nipa' and mangrove areas. Shallow water pond design generally consists of several nursery and production ponds with a typical area of 2 000 m² for nursery ponds and 4 ha for production (ongrowing) ponds. Typically, ponds have a depth of 30-40 cm and are provided with independent water supplies.

The average yield of a typical integrated nursery, transition and shallow grow-out system that produces 3 crops a year is 800 kg/ha. Modified modular pond designs consisting of a series of grow-out compartments with a maximum of eight crops a year have been shown to increase yield to a high as 2 000 kg/ha.

Deep water culture was developed in the mid 1970s in response to the decline of profitability of shallow water culture, and the limited and increasing value of land and manpower resources. Deep-water ponds provide a more stable environment and extend the grow-out period into the winter season. Most deep-water milkfish ponds have been created by converting either shallow water ponds or freshwater ponds, with a depth of 2-3 m. Production from these systems has sharply increased in Taiwan Province of China, having expanded from 23 percent of the total production in 1981 to 75 per cent in 1990.

Most milkfish ponds in the Philippines and Indonesia are of the extensive and semi-intensive type, with large shallow pond units, tidal water exchange, natural food, minimal use of fertiliser alternating with commercial feeds and other inputs, and low to medium stocking rates (50 000-100 000/ha). The Taiwanese method of production, on the other hand, employs intensive stocking densities (150 000-200 000/ha). Few diseases or infestations have been recorded so far in milkfish grow-out farming in these Asian countries.

Pen culture

This system was introduced in the Philippines in 1979 in the Laguna Lake. At that time, the lake had a very high primary productivity, which met the nutritional needs of milkfish. Because of the low rate of input and the high rate of return, the pen culture area increased sharply from 1973 to 1983, and exceeded more than 50 percent of the total lake surface, which is 90 000 ha. As the primary production of the lake could not meet this sudden expansion of aquaculture, and feeding became necessary to meet the nutritional requirements of the cultured fish, the pen culture practices developed in lakes were later introduced into inter-tidal areas in the Philippines along coves and river estuaries as well. Pen operators stock fingerlings at 30 000-35 000/ha and provide supplemental commercial diets. However, disease spreads among culture pens and causes mass mortality. Government regulations are now being considered to maintain sustainable yields from this type of farming.

Cage culture

Fish cages are smaller and more restricted enclosures that can be staked in shallow waters or set-up in deep water with appropriate floats and anchors. Cage farming of milkfish is commonly carried out in marine waters along coastal bays. Stocking rates (in the Philippines) are quite high, from 5 up to 30/m^³.

Feed supply

In the past, traditional feeding practices for milkfish grow-out production have consisted of natural food ('lab-lab') or a combination of phytoplankton and macroalgae (Enteromorpha intestinales, Cladophora spp. or Chaetomorpha linnum) encouraged by fertilisation. In the 1980s however, special commercial feeds for milkfish were developed and became almost exclusively used. As cage and pen culture technology proliferated in the 1990s, both in marine and inland waters, extruded milkfish feeds were further developed into floating and semi-floating forms, while sinking forms were used for pond and tank-based grow-out. Feed supplies are now manufactured commercially in the form of starters, growers and finishers, which are administered according to the production stage of the milkfish.

Harvesting techniques

Milkfish are normally harvested at sizes of 20-40 cm (about 250-500 g). There are three known methods used for harvesting milkfish:

• Partial harvest. Selective harvest of uniformly grown milkfish from grow-out facilities (i.e. cages, pens, ponds, tanks) using seine or gillnets, retaining the undersize fish and harvesting only the commercial sized stocks, with an average body weight of 250 g or larger.
  
  
• Total harvest. Complete harvest in one crop period from grow-out facilities (i.e. total draining of ponds by gravity or pump, hauling of the entire net cage structure, seining or the use of gillnets in pens). The harvest size at this stage may vary from 250-500 g.
  
  
• Forced harvest. Emergency harvesting, regardless of fish size or grow-out stage, which is carried out during 'fish kills' due to oxygen depletions that are attributed to algal blooms, red tide occurrence, pollution or other environmental causes.

Handling and processing

200-400 g milkfish are harvested and marketed mostly fresh or chilled, whole or deboned, frozen, or processed (e.g. fresh frozen deboned, fresh frozen deboned descaled, and smoked fish deboned). In general, all marketed milkfish are produced in farms, only a few being caught from natural waters. In some countries (e.g. the Philippines) fishing for adult milkfish is officially banned in order to protect the natural broodstocks.

There are two known post-harvest processing techniques for milkfish, which are the traditional (i.e. drying, fermentation and smoking) or non-traditional methods (i.e. bottling, canning and freezing) and value-added products such as 'surimi' and deboned products as practiced in Taiwan Province of China and in the Philippines.

Regulations and standard protocols for manufacturing milkfish products exist for both domestic consumption and export, as follows:

• Good Manufacturing Practices (GMPs). Plant construction.
• Personnel hygiene and sanitation.
• Standard Sanitary Operating Procedures (SSOPs).
• HACCP compliance.

Production costs

Milkfish farming is a centuries-old industry in Indonesia, Taiwan Province of China and the Philippines. It has been slow to modernise and now faces challenges from competing aquaculture species and current economic realities. The domestic market is large and the export market has globally expanded. Milkfish price and personal income affect the amount of milkfish consumed in the countries of origin. Studies conducted in Taiwan Province of China and the Philippines concluded that price and income had a negative and positive elasticity coefficient, respectively.

The following are the major determining factors affecting the cost of production in milkfish:

• Type of culture system: costs are lowest in systems dependent only on natural food; costs increase as artificial feed is introduced; costs are highest in systems dependent totally on commercial feeds.
• Increasing production: with milkfish production steadily increasing and culture practices becoming more intense, a big surplus of this commodity is foreseen in the near future.
• Cost of feed: feeds account for 60 to 80 percent of the total production cost.
• Low farm-gate prices: on average, the farm-gate price for milkfish is only about USD 2.00/kg in the Philippines. As the supply of milkfish is expected to increase way above demand, fish farmers cannot demand a higher farm-gate price even though they may be spending heavily to cover production costs.
• Lack of post-harvest facilities for value-adding and processing

Diseases and control measures

The major diseases affecting milkfish are included in the table below. In some cases antibiotics and other pharmaceuticals have been used in treatment but their inclusion in this table does not imply an FAO recommendation.