Analysis of otolith ⁸⁷Sr/⁸⁶Sr to elucidate salinity histories of Nurseryfish Kurtus gulliveri (Perciformes:Kurtidae) in a tropical lowland river in northern Australia

The Nurseryfish Kurtus gulliveri is found in estuarine and freshwater reaches of tropical rivers in northern Australia and southern New Guinea. Nurseryfish have been recorded in salinities ranging from freshwater to seawater (salinity <0.5-36%), but almost nothing is known of their movements. We conducted coreto- edge transect analyses of 87Sr/86Sr in the otoliths of 16 male and 14 female Nurseryfish collected from a tropical river in northern Australia to examine their movements across a freshwater-estuarine salinity gradient. Comparisons between water and otolith ⁸⁷Sr/⁸⁶Sr showed that most fish had ⁸⁷Sr/⁸⁶Sr values indicative of periods of saline (salinity > 4%) residence during their early life history, usually followed by a distinct transition into brackish (0.5-4%) or fresh water (<0.5%) later in life. No differences were discernible between the salinity histories of male and female Nurseryfish. Analysis of the relationship between the otolith core-to-edge distance and fish mass indicated that most growth occurred in brackish water. We conclude that Nurseryfish are euryhaline rather than diadromous and that analysis of water and otolith ⁸⁷Sr/⁸⁶Sr Sr is an effective method for assessing the movements of tropical riverine fishes and the potential importance of fish migration in driving foodweb subsidies.