A study of the effects of alkalinity and total hardness on postlarvae and juveniles of the giant freshwater prawn Macrobrachium rosenbergii (de Man)

Abstract

The effects of environmental water hardness and alkalinity on the giant freshwater prawn Macrobrachium rosenbergii (de Man) were investigated. Preliminary studies indicated significant interaction effects between alkalinity and hardness and also that the ratio of calcium to magnesium ions in the water was important for growth. At moderate alkalinity a Ca:Mg ratio of 1:1 allowed better growth than ratios of 4:1 and 1:4. In a series of experiments made with postlarval and juvenile prawns, the levels of alkalinity and hardness in the test waters were adjusted separately so that the influence of each factor could be determined independently as well as in combination. Data on aspects of survival, growth and carapace mineralization were recorded.

The results showed that, in contrast to some reports, high water hardness did not necessarily constrain performance provided alkalinity was low (2Smg 1' as CaCOs). On the other hand, high alkalinity (>100mg 1') caused a number of problems. These included increased mortalities (particularly when hardness was low. 20mg 1'), an increased incidence of a pathological condition called "white muscle syndrome” , a reduced size increment achieved at moult while, paradoxically, increasing the moulting frequency, and an enhanced calcium deposition in carapaces during intermoult. In all experiments, postlarvae were more sensitive than juveniles to adverse combinations of alkalinity and hardness. This feature is discussed in relation to Asian prawn farming practises.

The activity of dephosphorylating enzymes in gills from immediate postmoult and from intermoult prawns exposed to low and high alkalinity was measured. The activity was maximal at both low and high alkalinity in postmoult prawns i.e. during the time of expected maximum calcium uptake, and significantly lower in intermoult prawns but only when alkalinity was low. At high alkalinity, activity in intermoult prawns remained at postmoult levels suggesting that this activity was associated with the increased calcium deposition observed in intact animals held in high alkalinity waters.