The utilisation of acid sulphate soils for shrimp (Penaeus monodon) culture on the west coast of Sri Lanka

Abstract

Continued pressure on land resources for shrimp culture has resulted in many shrimp culture developments on acid sulphate soils in South East Asia which are marginal or difficult to manage. The present study included a survey to identify and classify different acid sulphate and potential acid sulphate soils in the areas earmarked for shrimp culture on the West Coast of Sri Lanka. This was supported by on farm investigations into the behavior and kinetics of metals in culture ponds, time series studies on water and soil quality over a culture cycle, plus morphological and histopathological changes in cultured shrimps and monitoring of calcium and magnesium contents in selected tissues. Although the general environment and water quality criteria in the study areas provided promising conditions for culture of Penaeus monodon, survival (35.1%) and production (1240 kg/ha/ crop) wore found to be significantly lower on acid sulphate soils than that on neutral soils. The stability of metals, particularly that of Iron, which is governed by the redox potential-pH relationships of the pond environment, appears to play a significant role in the processes that increase the potential stress to the shrimps cultured in an acid sulphate environment. Under acid sulphate conditions, shrimps showed elevated levels of iron (119.9 pg/g dry wt) and manganese (38.4 pg/g dry wt) in their muscles and unusually high levels of these heavy metals were recorded in gills (1588 and 93.2 pg/g dry wt of iron and manganese respectively) and carapace (778 and 34 pg/g dry wt of iron and manganese respectively) during the latter part of the culture period. Calcium levels in the carapace were relatively low (136 to 260 mg/ g dry wt) throughout the culture period and showed a negative correlation with culture time (r - -0.950; p - .001). Accumulation of hydrated oxides of iron in gills as a result of oxidation of pyrites was confirmed by the Eh-pH relationships monitored in the pond environment and by histochemical, SEM and TEM studies. These insoluble oxides appear to be primarily responsible for gill colour changes and concomitant histological changes in giil, heart and hepatopancreatic tissues. They are clearly detrimental to the normal gill functions of cultured shrimps. Statistically significant correlations were observed between iron in shrimp gills and muscles with iron in the surface sediments (p -.004 and .010 respectively) and the culture period (p >.013 and .010 respectively). Manganese in gills and carapace of cultured shrimps was correlated to the Iron concentration in those tissues (p >.016 and .002 respectively). Traditional management strategies (drying the pond bottom, liming and artificial aeration) although creating promising conditions for shrimp culture under favourable soil conditions, create adverse conditions by favouring the formation of iron (III) oxides in F>onds on acid sulphate soils. Detailed studies on mapping, classification and identification of profile forms in coastal soils provided vital information necessary for land use planning and development of these sediments in shrimp culture. Development of soil classes; sulphidic sand, unripe sulphidic peat, unripe sulphidic muck, acid sulphate muck, raw acid sulphate muck and raw acid sulphate clay have the most serious implications on cultured shrimps and the environment. Ripe clay with sulphidic subsoil, ripe clay with raw acid sulphate sub-soil, half ripe clay with acid sulphate subsoil and sand with acid sulphate sub- soil appear to be the least harmful soil classes for shrimp culture among the sediment types investigated.