Otolith microstructural and microchemical changes associated with settlement in the diadromous fish Galaxias maculatus

The presence of a settlement mark in the otoliths of the common galaxid Galaxias maculatus was validated by examining the relationship between changes in otolith microstructure and otolith microchemistry. Two methods were used to examine the microchemistry of otoliths: (1) laser-ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICPMS) to examine changes in Sr isotope ratios, and (2) laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS) to examine changes in Ba/Ca and Sr/Ca. Both analytical techniques detected changes in otolith microchemistry consistent with movement from oceans into rivers (settlement). There was a strong correlation between the timing of settlement as indicated by otolith microstructure and both Sr-based methods; however, Ba/Ca was a less reliable marker of settlement for this species. These results support the use of this settlement mark for further otolith based studies of age, growth and the reconstruction of settlement histories in G. maculatus and demonstrate the potential utility of otolith microchemistry as a method for validating settlement marks in other diadromous fish.