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Age & Growth Program

Recent Advances in Otolith Geochemistry as a Tool for Understanding Fish Biogeography and Age Validation

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Figure 1. The University of Wisconsin-Madison Secondary Ion Mass Spectrometer (WiscSIMS) IMS-1280 ion microprobe.  Dr. Thomas Helser, Age and Growth Program Manager, is at the helm. 

The Age and Growth (A&G) Program is collaborating with colleagues at the University of Wisconsin, Department of Geosciences Secondary Ion Mass Spectrometer (WiscSIMS laboratory) to analyze otoliths for stable oxygen isotopes. The WiscSIMS IMS-1280 ion microprobe, funded by the National Science Foundation, is being used to explore new applications of in situ analysis of stable isotope geochemistry (Fig. 1). The advantage offered by the IMS-1280 ion microprobe over conventional isotope mass spectrometry is the dramatic reduction in analysis spot sizes. 

Recent developments in the analytical capabilities of the IMS-1280 have allowed in situ analysis of otoliths on sub-annual, even daily, timescales. The ion microprobe can analyze discrete samples (~ 2 ng) that are thousands of times smaller than those required by conventional acid digestion/gas-source mass spectrometry (10-100 μg). The increased spatial resolution (sample diameter = 10 μm with depth of ~1 μm) allows for finer temporal resolution of measurements while maintaining high accuracy and precision.

Researchers in the A&G laboratory are currently using the IMS-1280 to study the detailed signatures of otolith d18O measurements from Bering Sea yellowfin sole and Pacific cod as a tool to understanding environmental history and age validation. 

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  Figure 2.  Composite photographs of polished surfaces of matching (a) roasted and (b) unroasted yellowfin sole otolith cross sections labeled with δ18O spot analysis locations (white numbering) and calendar year assignments (black numbering) for each transect. Inset: Roasted (darker) and unroasted otolith halves embedded in epoxy mount.

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  Figure 3. Measurements of δ18O (‰ VSMOW, ±2 S.D.) made at WiscSIMS from the otolith center to the edge of Transect A (upper panel) and Transect B (middle panel) of the roasted otolith half. Solid vertical lines indicate estimated transition between juvenile and adult phases (age-7). Dashed vertical lines indicate locations of translucent growth zones, with calendar years of formation labeled at top of each panel in italic type (1981-2007)

In one study, we conducted high-precision otolith d18O analysis using the IMS-1280 ion microprobe to investigate the life history of a yellowfin sole (Limanda aspera), a Bering Sea species known to exhibit age related migratory behavior.. The oxygen isotope ratio (d18O) of aragonite fish otoliths is dependent on the temperature and d18O of ambient water and can thus reflect a fish’s environmental history.  The IMS-1280 offers a spatial-resolution advantage over conventional acid-digestion techniques for stable isotope analysis of otoliths, which is especially given their compact nature (Fig. 2).

The yellowfin sole otolith was cut transversely through its core and one half was roasted to eliminate organic contaminants. Values of δ18O were measured in 10-μm spots along three transects (two in roasted half, one in unroasted half) from the core toward the edge (Fig. 2).  Otolith annual growth zones were dated using the dendrochronology technique of crossdating to accurately assign fish age and calendar year for each growth zone.

The results of this analysis yielded a total of 73 spot analyses of δ18O over Transect A, which spanned the years 1981 through 2007 (Fig. 3). No analyses were attempted in the growth band for 2008 as it was too narrow and close to the edge of the otolith.

The number of 10-μm-diameter spot samples per annulus within Transect A ranged from 1 to 7. Transect B spanned the years 1981 through 1989, and the number of spot samples per annulus ranged from 3 to 23 (total n = 78; Fig. 3). The linear sampling density of both Transects A and B was approximately 50 spot analyses per mm. Measured values of δ18O ranged from 29.0-34.1‰ (relative to Vienna Standard Mean Ocean Water).

Ontogenetic migration from shallow to deeper waters was reflected in generally increasing
δ18O values from age-0 to approximately age-7 and subsequent stabilization after the expected onset of maturity at age-7. Cyclical variations of δ18O within juvenile otolith growth zones, up to 3.9‰ in magnitude, were caused by a combination of seasonal changes in the temperature and d18O of ambient water.  The IMS-1280 ion microprobe at WiscSIMS obtained a high-precision and high-resolution record of relative environmental conditions experienced by a yellowfin sole that is consistent with population-level studies of ontogeny. (Continued)

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