Tiger shrimp (penaeus monodon)

INTRODUCTION

Penaeus monodon, commonly known as the giant tiger prawn or Asian tiger shrimp and also known by other common names, is a marine crustacean that is widely reared for food.

Females can reach about 33 cm (13 in) long, but are typically 25–30 cm long and weight 200–320 g.

Males are slightly smaller at 20–25 cm (8–10 in) long and weighing 100–170 g.

The carapace and abdomen are transversely banded with alternative red and white.

The antennae are grayish brown. Brown pereiopods and pleopods are present with fringing setae in red.

Shrimp farming has been practiced for more than a century for food and the livelihood of coastal people in some Asian countries, such as Indonesia, the Philippines, Taiwan Province of China, Thailand and Viet Nam.

Pond of shrimp farming

Penaeus monodon was originally harvested together with other shrimp species from traditional trapping-growing ponds or as a significant by-product of extensive milkfish ponds.

HABITAT AND BIOLOGY

Penaeus monodon mature and breed only in tropical marine habitats and spend their larval, juvenile, adolescent and sub-adult stages in coastal estuaries, lagoons or mangrove areas.

In the wild, they show marked nocturnal activity, burrowing into bottom substratum during the day and emerging at night to search for food as benthic feeders.

Under natural conditions, the giant tiger prawn is more of a predator than an omnivorous scavenger or detritus feeder than other penaeid shrimp.

Protozoea, which have feathery appendages and elongated bodies, moult three times and then metamorphose into the mysis stage. Mysis, which have segmented bodies, eyestalk and tails characteristic of adult shrimp, also moult three times before metamorphosing into PL with similar characteristics to adult shrimp.

PRODUCTION SYSTEMS

Seed supply

Due to their larger size and better survival, captured wild seeds were used commonly in south Asia for extensive ponds, which require a minimal amount of seed for stocking. However, the use of wild seeds has been reduced, due to overfishing and the outbreak of white spot disease in shrimp nursery grounds. Therefore most Penaeus monodon grow-out farms now rely solely on hatchery-produced seeds.

Broodstock

Healthy females (25-30 cm body length and 200-320 g weight) and males (20-25 cm; 100-170 g) captured from the wild are preferably used as broodstock in the induced ovarian maturation process. Broodstock from greater depths (60-80 m), or more than 20 miles offshore, are preferable due to the lower prevalence of shrimp diseases, which are higher in coastal shrimp farming areas.

Shrimp are subsequently induced to moult by manipulating the salinity of the water. After mating has occurred, which is easily determined by the presence of a spermatophore in the thelycum and hardening of the shell, the eyestalk of females is unilaterally ablated for endocrine stimulation. Broodstock are fed with squid, mussel or cockle meat, supplemented by polychaete or Artemia biomass to enhance reproductive performance.

The early stage of ovarian development can be first observed within a week after ablation. Later, gravid females with ripe stage eggs, which can be observed by the opaque diamond-shaped ovary under torchlight, are collected and transferred into spawning tanks. After spawning, these females can be re-used in the maturation process a few times, while males can be further used for several months, depending on prawn health and tank conditions.

After spawning, eggs are generally kept in the same tank for fertilisation until hatching. Nauplii are then collected and cleaned (rinsed with flowing seawater to remove the fat and debris released by the spawner) for transfer to larval rearing tanks or for transportation to other remote hatcheries.

ON GROWING TECHNIQUES

There are three on-growing culture practices: extensive, semi-intensive and intensive, which represent low, medium, and high stocking densities respectively. Due to its benthic feeding habit Penaeus monodon is commercially cultured only in earthen ponds, under widely varying salinities from 2 to 30‰.

Extensive

Commonly found in Bangladesh, India, Indonesia, Myanmar, the Philippines and Viet Nam, extensive grow-out of shrimp is conducted in tidal areas where water pumping is unnecessary. Ponds with an irregular shape according to land boundaries are generally larger than five hectares and easily constructed by manual labour for cost reduction. Wild seeds, which either enter the pond through the gate by the tide or are purchased from collectors, are usually stocked at a density not exceeding 2/m². Shrimp feed on natural foods that enter the pond regularly on the tide and are subsequently enhanced by organic or chemical fertilisers. If available, fresh fish or molluscs may be used as supplementary feed. Due to the low stocking densities, larger sized shrimp (>50 g) are commonly harvested within six months or more. The yield is lowest in these extensive systems, at 50-500 kg/ha/yr. Due to the increase in land costs and the shortage of wild seeds, almost no new extensive farms are being constructed today. After gaining experience in shrimp farming, many farmers have upgraded their ponds to semi-intensive systems to provide better incomes.

Semi-intensive

Semi-intensive ponds (1-5 ha) are commonly stocked with hatchery-produced seeds at the rate of 5 to 20 PL/m². Water exchange is regularly carried out by tide and supplemented by pumping. The shrimp feed on natural foods enhanced by pond fertilisation, supplemented by artificial diets. Production yields range from 500 to 4 000 kg/ha/yr.

Intensive

Intensive farms are commonly located in non-tidal areas where ponds can be completely drained and dried before each stocking. This culture system is found in all Penaeus monodon producing countries and is commonly practiced in Thailand, the Philippines, Malaysia and Australia. Ponds are generally small (0.1 to 1.0 ha) with a square or rectangular shape. Stocking density ranges from 20 to 60 PL/m². Heavy aeration, either powered by diesel engines or electric motors, is necessary for internal water circulation and oxygen supply for both animals and phytoplankton.

Feeding with artificial diets is carried out 4-5 times per day followed by feed tray checking. Final FCR is normally between 1.2:1 and 2.0:1. Since the outbreak of white spot disease, reduced water exchange and closed systems have become commonplace, due to their lower risk of introducing viral diseases through intake water. However, feed and phytoplankton blooms need to be carefully monitored and managed to avoid deterioration of the pond bottom and water quality due to wastes. P. monodon has a habit of slowly nibbling feed on the pond bottom; this causes substantial nutrient losses because pellet stability is generally not longer than two hours.

Harvesting techniques

Bamboo traps are traditionally used for the partial harvest of selected large shrimp in extensive culture. Semi-intensive ponds are commonly harvested by draining the pond by tide through a bag net installed at the outlet sluice gate. Intensive ponds are normally harvested similarly to semi-intensive ponds. If the tide does not allow harvesting, the drainage canal can be blocked to allow the water to be pumped out to reduce the water level. It is still necessary to pick the remaining shrimp by hand after the pond has been drained.

In Thailand, artificial sluice gates are temporarily installed inside the pond for the harvesting of many closed system ponds where a sluice gate for water exchange is not necessary. Shrimp are then trapped in this artificial gate during the pumping out of the water. For the live shrimp market, ponds are partially harvested by cast net in the early morning. Due to its burrowing habit, a drag net is not practical unless it is installed with electric shock gear to stimulate the shrimp to jump.

MARKET

In general, Penaeus monodon is the most prominent farmed crustacean product in international trade and has driven a significant expansion in aquaculture in many developing countries in Asia. Market prices during its early development were quite good due to little competition and strong demand from the Japanese market. International markets appear to have become almost saturated since global production reached 600 000 tonnes/yr. The price for P. monodon has since fallen, particularly during the booming of the production of Litopenaeus vannamei in Asia from 2001-2004.

There has been increasing change in consumer preference from shrimp to marine fish, due to the lower cholesterol and higher omega-3 in fish.