A Method for Improving the Estimation of Natal Chemical Signatures in Otoliths

• Published in: Transactions of the American Fisheries Society (1900) 2008-11, Vol.137 (6), p.1674-1682
• Main Authors: Macdonald, Jed I., Shelley, J. Michael G., Crook, David A.
• Format: Article
• Language: English
• Subjects: Freshwater; Retropinna semoni
• ISSN: 0002-8487; 1548-8659
• DOI: 10.1577/T07-249.1

Laser ablation‐inductively coupled plasma mass spectrometry has been widely used in studies aimed at identifying the natal sites of fish through analysis of the otolith core region. It is critical, then, that measurements of chemistry at the core accurately represent the natal chemical environment, rather than reflecting potential physiological and ontogenetic influences on elemental incorporation during the embryonic and early larval phases. We analyzed sagittal otoliths from 840 age‐0 and age‐1 Australian smelt Retropinna semoni using a 193‐nm Ar‐F excimer laser system with a fast response two‐volume sample cell to demonstrate a rapid depth‐profiling method that enables more accurate estimations of the natal chemistry by sampling a target region of the otolith that excludes the primordium. The method involves (1) examination of validated daily growth increments to identify the target region; (2) measurement of the amount of otolith material removed by each laser pulse; and (3) use of elevated 55Mn concentrations at the primordium to provide a marker to facilitate data selection. The target region in this study was material accreted onto the otolith 2‐10 d posthatch. We sampled this region by excluding data between the maximum value of the Mn peak and the otolith material representing the first 2 d of life. The laser pulses in the acquisition sequence that represented the target were then retained and all other data excluded. This method illustrates the advantages of the uniform energy distribution produced by 193‐nm excimer laser systems for depth profile analysis of otoliths and may provide the basis for a standardized approach to sampling at the otolith core that more accurately reflects the natal chemical environment.