The effect of salinity on the requirement for potassium by barramundi (Lates calcarifer) in saline groundwater G.J. Partridge a,b, ⁎, A.J. Lymbery b a Aquaculture Development Unit, Challenger TAFE, Fremantle, Western Australia, Australia b Fish Health Unit, Centre for Fish and Fisheries Research, Murdoch University, Murdoch, Western Australia, Australia ABSTRACT ARTICLE INFO Article history: Received 6 September 2007 Received in revised form 12 February 2008 Accepted 25 March 2008 Keywords: Inland saline aquaculture Barramundi Lates calcarifer Potassium deficiency (Na+–K+)ATPase 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. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. 1. Introduction Secondary salinity affects over 380 million hectares of land in over 20 countries worldwide (Ghassemi et al., 1995; Lambers, 2003). Countries including Australia, China, India, Israel and the USA have a demonstrated interest in utilising their affected land and water resources for commercial mariculture (Ron et al., 2002; McNevin et al., 2004; Zhu et al., 2004; Barman et al., 2005; Partridge et al., 2008). In Australia, over 60% of saline groundwater sources range from 5 to 45 ppt, a range suitable for the culture of many euryhaline species (Partridge et al., 2008). Although the ionic composition of saline groundwater generally reflects that of seawater, the exact composition varies both locally and regionally. This variability relates to the nature and timing of recharge and the nature of the weathered material between the soil surface and bedrock (George, 1990). One factor, however, that appears consistent worldwide is a deficiency of potassium, relative to equivalent salinity seawater (Fielder et al., 2001; Partridge and Furey, 2002; Saoud et al., 2003; Zhu et al., 2004; Shakeeb-Ur-Rahman et al., 2005). This deficiency is primarily caused by the fact that potassium is preferentially taken up by cation exchange sites in clay soils (Stumm and Morgan, 1996). Saline groundwater can contain as little as 5% of the potassium found in equivalent salinity seawater (i.e. K-equiva- lence) (Fielder et al., 2001) to as high as 75% K-equivalence (Partridge and Furey, 2002); however, in a review of saline groundwater sources, Partridge et al. (2008) reported that most of those assessed for mariculture contain approximately 20% K-equivalence. As with all animals, potassium is the most abundant intracellular ion in fish and plays many important physiological roles including the maintenance of cellular volume and membrane potentials and the generation of nerve impulses (Epstein et al., 1980; McDonough et al., 2002). In fish, potassium plays additional critical roles in osmo- and iono-regulation and acid/base balance (Marshall and Bryson, 1998; Evans et al., 2005). Barramundi (Lates calcarifer) tolerates salinities from freshwater (Rasmussen, 1991) to at least 55 ppt (Shirgur and Siddiqui, 1998) and has been identified as a suitable species for inland saline aquaculture in both Australia (Partridge et al., 2008) and India (Jain et al., 2006). Aquaculture 278 (2008) 164–170 ⁎ Corresponding author. Aquaculture Development Unit, Challenger TAFE, Fremantle, Western Australia, Australia. Tel.: +61 8 9239 8032; fax: +61 8 9239 8081. E-mail address: gavin.partridge@challengertafe.wa.edu.au (G.J. Partridge). 0044-8486/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2008.03.042 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online