The effect of salinity on the requirement for potassium by barramundi (Lates calcarifer) in saline groundwater

Significant international interest exists in utilising inland saline groundwater sources for mariculture; however potassium deficiency is a factor that may limit their use. In this study we investigated the effects of potassium supplementation between 25% and 100% of that found in equivalent salinity seawater on the growth, survival and physiological response of barramundi (Lates calcarifer) at hyperosmotic (45 ppt), near-isosmotic (15 ppt) and hyposmotic (5 ppt) salinities. A K-equivalence of 25% was not tested at 45 ppt because it caused mortality of barramundi in a previous study. Fish reared in 50% K-equivalence water at this salinity survived for four weeks but lost weight; whereas at 75% and 100% K-equivalences fish both survived and gained weight. Homeostasis of blood plasma potassium in these fish was maintained by buffering from skeletal muscle. That these fish exhibited muscle dehydration, increased branchial, renal and intestinal (Na+–K+)ATPase activity and elevated blood sodium and chloride suggests they were experiencing osmotic stress. At 15 ppt, equal rates of growth were obtained between all K-equivalence treatments. Buffering of plasma potassium by muscle also occurred at the two lowest levels of supplementation but appeared to be in a state of equilibrium. Barramundi at 5 ppt displayed equal growth among treatments. At this salinity, buffering of plasma potassium from muscle did not occur and at 25% K-equivalence blood potassium was significantly lower than at all other K-equivalence treatments but with no apparent effect on growth, survival or (Na+–K+)ATPase activities. These data show that proportionally more potassium is required at hyperosmotic salinities compared to iso- and hypo-osmotic salinities and also demonstrate that barramundi have a lower requirement for potassium than other species investigated for culture in inland saline groundwater.