Salt balances of inland shrimp ponds in Thailand: implications for land and water salinization

Abstract

Brackish water ponds for farming black tiger shrimp (Penaeus monodon) have recently proliferated in inland areas of Thailand's central plain, raising concerns about land and water salinization. The environmental impacts of inland shrimp farming were assessed by analysing the salt balance for an inland shrimp farm. Field data on water fluxes and pond salinity, collected from nine ponds in Chachoengsao Province from May–July 1999, were used to model the salt balance for a typical shrimp pond over one growout cycle. During growout, seepage represented 38% (11.5 tonnes crop−1) of salt losses, pond discharge 33% (9.7 tonnes crop−1), and accumulation of salt in pond sediment 6% (1.8 tonnes crop−1). About 23% of the initial salt content remained at harvest and could have been recycled. However, the majority (84% on average) was discharged to the irrigation canals. Much of the salt in pond sediment was also exported to the canal system through tidal flushing of the ponds. Field measurements of salinity were taken in adjacent canals and rice paddies to explore the impacts of salt exports from shrimp ponds. Pond discharge caused increases in canal salinity above levels that would impact on yields of irrigated rice and orchard crops, the main land uses in the region. Elevated soil and water salinity in adjacent rice fields was probably related to lateral seepage from the ponds. The salt budget was also modelled for a zero discharge farming system, promoted by proponents of inland shrimp farming as having few impacts. However, the results suggest that, even in zero discharge ponds, almost half of the initial pond salt content is exported through seepage (45%, 12.4 tonnes crop−1), with another 6% (1.8 tonnes crop−1) deposited in sediments. While techniques exist for mitigating much of the salt leakage, the likelihood of their uptake in Thailand is low. Further expansion of inland shrimp farming in the central plain is therefore likely to contribute substantial salt load to the area's land and water resources.