When people think of shrimp, what usually comes to mind? We’d bet you a bag of prawn crackers that most would picture whiteleg shrimp (Litopenaeus vannamei) or tiger prawns (Penaeus monodon), both of which spurred multi-billion dollar industries, making both aquaculturists and seafood lovers pretty happy. But there’s a new shrimp on the block – and it might soon claw its way from the rivers of Asia to groceries and restaurants worldwide.
The giant river prawn (Macrobrachium rosenbergii), or GRP, is well known to most aquaculturists. The discovery that its larvae needed brine to survive beyond five days is the stuff of aquaculture legend. But most cosmopolitan seafood consumers don’t know much about it. It’s the largest member of Macrobrachium, a genus meaning ‘big arm’ in the order Decapoda – which includes crabs, shrimp, prawns, lobsters, crayfish and other familiar critters seafood consumers crave.
The 240 or so Macrobrachium species have a tropical and subtropical distribution, inhabiting numerous rivers, ponds, lakes and streams across every continent except Europe and Antarctica. Most species are amphidromous and require both fresh and seawater to complete their lifecycles. Eggs wash downstream to the sea, then metamorphose into juveniles which crawl as far as 100km upstream to breed and repeat the cycle.
The GRP grows larger than any other Macrobrachium species. Females reach 25cm while the larger males top out at 32cm, excluding their impressive claws or chelipeds which can breach 60cm.
Males are divided into three morphotypes: small males (SM) have short translucent claws; mid-sized orange claws (OC) have large yellow-orange claws as long as their bodies; while large blue claws (BC) have bright blue claws twice as long as their bodies. Blue claws dominate orange claws, while small males sit at the bottom of the heap. The presence of higher-caste males inhibits the growth and development of both male and female GRPs.
Wild GRPs live in shallow, muddy lakes and rivers with good vegetation. Able to crawl on land and even up relatively vertical surfaces like waterfalls, they are largely nocturnal – spending days half-buried in mud and detritus. As night falls, they forage and hunt for worms, crustaceans, molluscs, fish and carrion. People have caught and eaten GRPs for thousands of years in Asia but modern farming began just a half-century ago.
GRPs in aquaculture
Generations of river and lakeside communities in South East Asia, particularly in Myanmar and Bangladesh, have stocked GRPs in pens and pools, but science-based farming began only in the 1960s. The first major GRP aquaculture breakthrough was in 1961, when Food and Agriculture Organization (FAO) expert Shao-Wen Ling discovered that GRP larvae require brackish water to survive beyond five days.
This finally allowed aquaculturists to produce enough juveniles for grow-out experiments in ponds by 1963. Hawaii-based fisheries biologist Takuji Fujimura followed up this discovery with a system for commercial mass-rearing by 1972, spurring the first GRP business ventures in Hawaii and other locations.
Research and development projects eventually sprouted in Asia, Europe, the Americas and Africa, with the United Nations Development Programme (UNDP) funding GRP farm research in Thailand in the 1980s, helping the country produce 3,000 tonnes by 1984.
By 2012, annual global GRP production had ballooned to 220,254 tonnes, valued at US$1.2 billion. According to the FAO’s 2014 data, the top producers in 2012 were China (57 percent), Bangladesh (19 percent), Thailand (11 percent), Vietnam (4 percent), India (3 percent), Taiwan (3 percent) and Myanmar (2 percent), with Asia supplying over 98 percent of global trade.
Farming GRPs requires a bit of consideration. Commercial hatcheries take from 32 to 35 days to produce post larvae (PL) using 12 percent brackish water, plus a mixture of live brine shrimp and egg custard for food. Hatcheries are either flow-through or use a recirculating aquaculture system (RAS). Pond stocking densities can range from one to four prawns per square metre in extensive systems to up to 20 prawns per metre in intensive systems. A big challenge is that males are aggressive and not only fight, but cannibalise each other, leading scientists to develop novel solutions, discussed later.
Though they can subsist on natural pond biota, five to seven months of supplemental pellet feeding are standard before harvesting. With dry pellets containing 30-35 percent protein, an FCR of 3:1 or even 2:1 can be achieved. Polyculture systems with freshwater fish can further lower this as the prawns can subsist on detritus, leftovers and faeces.
Sorting should be done from the fifth to the 11th month to catch and sell the larger blue-clawed males. When these are removed, the less-dominant orange-clawed males in turn develop to become BC males. Without removing the dominant BC caste, males never fully metamorphose into the latter, larger stages and the batch suffers from cannibalism. After a year, the entire pond should be drained and the remaining harvest brought to market.
Harvesting and processing
A key step after harvesting is proper handling as processed GRPs can become a bit “mushy” when their internal organs are crushed by improper harvesting, transport and storage. GRPs cannot be piled or stacked like other shrimp as their internal organs are prone to damage, greatly lowering meat quality. The FAO recommends icing and washing GRPs in chlorinated water immediately upon harvest, right by the pond’s edge.
Post-processing after harvest is much more important for GRPs than most other types of seafood as their meat – and market value – degrades fast if not well tended.
Why farm Macrobrachium?
Farming GRPs has many advantages. It is highly profitable and applicable for both inland, large-scale operations and artisanal or small-scale aquaculture. The prawns can sell for over $15 per kilogram, so large-scale farming can reap good returns.
GRPs are particularly suited for freshwater polyculture, alongside carp, tilapia, barbs, pangasius and other fish, as they require very little input or cost and can make unused pond substrates productive and profitable. Throughout Asia and Brazil, it has been found that adding GRPs to rice fields is also ecologically sound – reducing the need to produce rice with pesticides. The prawns eat pests and other insects, improve soil fertility and feed on the seeds of common rice-field weeds, while the rice absorbs the nutrients discharged by GRPs, resulting in better water quality for rice fields.
The way forward
One of the biggest challenges is that GRPs are familiar mostly to Asian consumers, rather than being established in a truly global market. Unlike whiteleg shrimp and tiger prawns, GRPs have yet to become mainstays of the global seafood industry.
A good strategy might be to better market and package the species, similar to how pangasius was globally marketed as “cream dory”. Potential names include “blue shrimp” or “blue prawn” – or even “river lobster”, as an easier and more quickly produced alternative to marine lobsters.
“The key is to enticingly ‘dress up’ GRPs displayed in markets and groceries so they look more delectable to consumers,” says Gilbert Pang, co-founder of Asia Aquatixs. “We were quite surprised to find that buyers were willing to pay more for both live and frozen GRPs compared to whiteleg shrimp or tiger prawns. This might be because marketable GRPs are a bit larger and because the public impression of GRPs is that they aren’t farmed in sprawling commercial plots like marine shrimp.”
Khoo Eng Wah, managing director of the Sepang Today Aquaculture Centre (STAC) in Malaysia, agrees. “Demand is currently greater than supply and most of our GRPs are sold live to neighbouring Singapore at very good prices – about US$15 to $20 per kilogram.”
The freshwater prawns are already replacing tiger prawns in some areas. “In Thailand, GRPs are now being used as substitutes for tiger prawns when making tom yam soup,” notes Dr Maria Rowena Eguia of SEAFDEC, an international body pushing for sustainable fisheries and aquaculture.
Increased market visibility and acceptance will dramatically boost export demand, a requirement to move this species beyond subsistence aquaculture and regional consumption and onto a global stage.
Another big hurdle in successfully farming GRPs is that dominant males are especially vicious towards each other and cannot be reared as intensively as marine shrimp. Males fight and have a strict caste system, so only a few large blue claws will lord over more numerous mid-sized orange claws, who in turn will dominate a huge number of small, clear (and frequently unmarketable) prawns in a culture system.
Current stocking levels range from four to 20 per square metre or lower – compared for instance with whiteleg shrimp, which can be stocked to as many as 150 per square metre. The strict caste system also results in highly variable harvests – with GRPs of varying sizes produced over the same period.
Similar to farming tilapia, a good solution is to create monosex GRPs to raise either all-male or all-female batches. A recent report featured in Nature magazine highlighted a scientific breakthrough to produce monosex GRP populations of either gender using androgenic gland cell transplantation.
A 2017 study by Levy et al found that all-female GRP batches showed better performance than mixed-sex batches in terms of survival rate, feed conversion and yield, with harvested animals of the same size. All-female GRPs are injected with suspended hypertrophied androgenic gland cells when young.
Though female GRPs are slightly smaller than the largest BC males, at 25cm, most GRPs in a rearing system will reach maturity and are of uniform size. All-female monosex aquaculture is a good way of navigating the issues around the complex and cannibalistic caste society of GRP males, while eliminating the need for constant culling and other labour-intensive harvest and management practices.
Raising all-male GRP batches also has distinct advantages, says Gilbert Pang. “Gene-silencing is used to induce female shrimp to produce all-male offspring. The advantage of all-male batches is that males spend much less time fighting and trying to dominate each other because there are no females to impress and mate with. Most of the animals spend their time feeding instead of fighting – shortening production time while improving productivity.”
“Size definitely matters – male GRPs grow significantly larger than females and all-male batches produce fairly uniform-sized prawns which sell for more than whiteleg shrimp or tiger prawns. More and more inland farmers are switching from marine- to freshwater-prawn farming because of potentially higher margins.”
With a little brains and a lot of prawn, The Fish Site is excited to see GRPs finally take their place as top contenders for the global seafood industry.
An excellent resource for those wanting to learn more about Macrobrachium is the UN FAO GRP farming guide. Farmers can also contact Singapore-based Asia Aquatixs, which produces monosex PL. Malaysia-based STAC organises courses for farmers interested in improving their GRP farming skills, while SEAFDEC provides sustainable aquaculture support for farmers across South East Asia. To learn about the latest developments in GRP culture while meeting and exchanging ideas with global GRP experts, readers can register for the international conference Giant Prawn 2019, which takes place 15-18 November in Shanghai.