Two trials were conducted to determine the effect of sudden decrease in salinity of raw and potassium-fortified inland saline water on western
king prawn Penaeus latisulcatus osmoregulation, ionoregulation and condition. Prawns were subjected to salinity decrease over 1 h from 32 to
25 ppt in the first trial and from 27 to 20 ppt in the second trial in three water types: inland saline water with potassium fortified to 100% and 80%
of the marine water concentration (IS100, IS80), and raw inland saline water (ISW). In the first trial condition and ingestion rate were monitored
over 19 days following salinity change. In the second trial condition, haemolymph osmo- and iono-regulation were recorded over 48 h following
salinity change. In the first trial, 100% mortality was observed in ISW by day 13, with final survival 94% in IS80 and 100% in IS100. Tail muscle
moisture content increased significantly (Pb0.05) over time in both trials and in all water types, suggesting loss of energy reserves. In the second
trial, serum osmolality, sodium concentration and osmoregulatory capacity decreased following salinity change, stabilising by 24 h in IS100 and
IS80 but continuing to decrease till 48 h in ISW, suggesting partial breakdown of osmoregulatory function in the potassium-deficient medium.
Prawns were stronger regulators of divalent than monovalent cations. These trials demonstrate that potassium-deficient inland saline water
requires fortification with potassium to allow prawn survival and efficient osmoregulation.