An insatiable demand for shrimp
Shrimp culture is increasing faster than any other warm-water aquaculture sector with shrimp farmers in South and Central America and the USA setting new production records last year (15% increase). And yet in Asia, the home of shrimp farming, harvests fell for the third year in succession as disease and environmental concerns continue to cause problems in production for small and large scale producers.
The brackish-water culture of shrimp farming evolved in the
traditional 'tambaks' of Java, the 'bheris' of
Bengal and tidal ponds in Ecuador. But these traditional systems,
characterized by minimal management methods and low yields, are now
being superceded by more modern semi-intensive and intensive culture
methods which produce higher yields for Asia and Latin America.
The
majority of shrimp production is still provided by Asia (70%) but
other subtropical and tropical countries, such as Ecuador, are
becoming increasingly important in overall global production. However,
this dramatic expansion has raised the question of sustainability and
a number of environmental issues should be considered if the world's
insatiable demand for shrimp production is to continue.
Shrimp farming has received rather bad press over the last decade. More recent findings have revealed that, although mistakes have been made and environmental damage has been a problem, much of this has been reduced as the industry has developed and learnt from experience. One of the most serious accusations is that shrimp farmers destroy mangrove forests, displace the rural people dependent on this complex ecosystem and affect the fry of other commercially important finfish and shellfish industries. This was particularly highlighted by problems in India last year which resulted in the Indian Supreme Court trying to ban the future development of shrimp farming in many parts of the country. Concern was also raised in Africa over the proposed 6000 hectare shrimp farming project in the mangroves of the Tanzanian Rufiji delta. Despite assurances from the African Fishing Company that the shrimp farms are to be situated behind the mangroves on land which is not used by local people, Tanzania's National Environment Management Council (NEMC) has advised the government to reject the proposal.
But not every mangrove forest used for shrimp farming has had a detrimental effect. Indeed, it is now generally known that ponds dug from cleared mangroves are too acidic for growing shrimp and have to be limed at considerable cost. It is therefore more practical and cheaper to site farms inland from mangroves. Farmers are learning to send waste water through the forests which act as excellent natural filters (and may even benefit the trees from the organically enriched water) and, where this is not possible, most farmers are endeavouring to clean their effluent water to avoid problems of pollution. One of Mexico's leading shrimp producers (AquaNova) is also one of several major companies involved in actively propagating mangrove seedlings for reforestation.
Away from the mangroves, environmental problems can still occur in shrimp farming although mistakes can often be attributed to the ignorance of small-scale farmers when they are left to 'feel' their way into the business. Extension services for shrimp farming are few and far between, especially when compared to agricultural systems. Dr. Janet Brown, Head of the Tropical Prawn Unit at the Institute of Aquaculture, Stirling, feels very strongly that if consumers are demanding products of the highest quality, then supermarket retailers should provide more money and support in order "to promote sustainable production and to avoid some of the more serious problems that are now associated with shrimp farming."
The Institute of Aquaculture, Stirling, with funding from DFID, has been involved in monitoring the environmental impacts of different types of pond in Thailand, Asia's main producer of shrimps. By comparing ponds stocked at two different stocking densities (50 shrimp/m2 and 90-100 shrimp/m2) and different stages of development (newly constructed and after a few production cycles), scientists have been able to build up a picture of long-term nutrient inputs and outputs from the ponds and this revealed some surprising discoveries. For instance, it had been assumed that the high levels of food and fertiliser going into the pond would result in high nutrient levels due to the amount of uneaten food and animal waste left in the system. However, it was found that the actual nutrient content of the sediment was relatively low and that the bulk of the sediment came from severe erosion of the pond walls due to the action of aerators. This presents disposal problems for the large amounts of nutrient-poor sediment that has to be regularly taken out of the ponds.
It was also assumed that regular changing of the pond water was an effective technique in managing nutrient levels. However, this was often found not to be the case as the plankton are actually quite effective at reducing high ammonia levels and changing the water only resulted in seriously reducing the plankton populations. The Institute of Aquaculture intends to use the data collected over an 18 month period for mathematical modelling so that more effective management strategies, such as the use of pond liners to prevent wall erosion, can be produced.
One of the other major areas of practical concern for the future of shrimp farming is the provision of broodstock. The major species for culture (60% of current production) is Penaeus monodon. This species is highly productive in the wild but attempts to breed the species in captivity have largely failed with the result that the pressure on wild caught mature females is increasing. With other species of fish, decreasing stocks usually leads to a loss of interest and for the most part, species are then left to recover, at least for a while. However, increasing rarity of P. monodon is simply leading to a rapid rise in price of mature female broodstock.
Shrimp larvae are also often harvested using fine-mesh nets. The nets inevitably trap large quantities (99%) of other crustacean and fish larvae which are then discarded. Finding ways to reduce the reliance on wild caught broodstock is therefore essential. In Bangladesh, as marine shrimp farming has declined, small-scale giant freshwater prawn (Macrobrachium rosenbergii) production has increased. To avoid dependence on wild caught larvae, which is nonetheless an important activity for resource poor people, DFID has developed a locally manufactured low cost hatchery which is now operational in Bagerhat and provides employment opportunities for local people.
Public awareness about shrimp farming is increasing as environmental groups continue to highlight the negative sides of the industry. International organizations, such as the UN, are also exerting pressure on governments to legislate for environmentally safe and sustainable shrimp aquaculture. Many of the demands being made of the industry are logical and necessary but for an activity that is currently worth nearly 6 million dollars the amount of guidance and support that is available is inadequate in comparison to the size and value of this fast growing sector. Governments should undoubtedly do more to safeguard their export earnings as well as their environments but retailers, who would be very hard hit by trade embargoes if the situation was to worsen, should also shoulder some of the costs. It is this sector in particular that should invest in a future trade that is productive but not necessarily detrimental to the environment nor to the people and other industries that are also dependant on these valuable aquatic regions of the tropics.
