Otolith microchemistry of two amphidromous galaxiids across an experimental salinity gradient: A multi-element approach for tracking diadromous migrations

An increasing number of studies are uncovering considerable flexibility in migration patterns of diadromous fishes. The development of otolith microchemical techniques has largely driven this research and led to an appreciation of the significance of facultative diadromy in the life history of numerous species. However, validation experiments need to be undertaken for each species and life stage of interest before diadromous migrations can be confidently reconstructed. These validation experiments are required to establish a salinity calibration series against which the otolith microchemistry of unknown individuals can be compared. To facilitate research on facultative amphidromy in galaxiids, we reared the larvae of two species, Galaxias maculatus and G. argenteus, in five different salinities (2, 5, 10, 20, 34). We tested whether trace element signatures of fish reflected their salinity treatment, and hence whether otolith microchemistry could reconstruct diadromous migrations. Distinguishing low salinity (2 and 5) from high salinity (20 and 34) treatments was straightforward using otolith Sr:Ca alone. The five salinity treatments resulted in five distinct multi-trace element signatures for both species (DFA classification success of 85% and 92% for G. maculatus and G. argenteus, respectively). Otolith lithium showed a similar trend to otolith Sr:Ca (ie. higher in saltwater), and otolith Rb:Ca showed a surprising negative trend with salinity despite higher ambient Rb concentrations in saltwater. Our results suggest otolith Li:Ca and Rb:Ca should be considered as part of a multi-trace element approach when investigating diadromous migrations, particularly when non-marine Sr levels may be high.