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Otolith laser assays for otolith ageing

Learn about beam-based elemental assays advantages and disadvantages, instruments, reconstruction ability and migration analysis.

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Advantages and disadvantages

Beam-based elemental assays target a particular age or date range in the sectioned otolith. This takes advantage of the chronological growth sequence recorded in the otolith.

These types of targeted assays are popular for:

It’s advantageous to take an age-structured approach, particularly since the beam sizes of the current generation of instruments approach the width of a typical daily increment. As a result, the assay can be limited to the time scale of interest, whether it’s:

Disadvantages of the approach include:

However, the most significant disadvantage is the reduced sensitivity and precision of beam-based assays compared to their solution-based counterparts.

Assay instruments

There are a wide variety of sophisticated instruments available for probed assays of the otolith, but the most frequently used include:

No single instrument type is sensitive to each element, nor is any one instrument preferred for use in all assays. However, the minor elements such as sodium (Na) and potassium (K) can generally only be measured accurately with an electron microprobe, while the trace elements require PIXE or LA-ICPMS.

Reconstructing migration histories

Reconstructing migration histories using elemental composition along an otolith growth sequence is similar to stock discrimination based on analysis of the otolith core. These techniques share many of the same assumptions.

However, migration analysis expands the scope of the interpretation. It links a series of elemental assays along an otolith transect to the growth chronology recorded in the otolith. This allows the reconstruction of migration pathways structured by age or date.

Migration analysis is one of the most powerful applications of otolith microchemistry, although its successes to date have been largely limited to the detection of anadromy. Subtler migration patterns are now being detected with LA-ICPMS, which is more sensitive to non-physiologically regulated trace elements.

Previously reported difficulties in interpreting migration histories may have been due to the use of the electron microprobe. This is more suited to assays of physiologically regulated elements.

Migration analysis

Several forms of migration analysis have been reported. One of the most successful has been the comparison of elemental trajectories among fish to determine a common history or divergence age/date.

This is a particularly robust application, since it:

In many respects, this comparative approach is similar to the use of elemental fingerprints as biological tracers.

A potentially more powerful approach is that in which the otolith elemental trajectory is linked to shifts in temperature and water chemistry along possible migratory routes. Such an approach is the basis for the detection of anadromy using Sr:Ca ratios. In these ratios, the shift between freshwater and saltwater environments is so marked in the otolith it’s nearly unmistakable.

Similar interpretations in more homogeneous environments appear possible. However, they’re complicated by the assumption that a given elemental concentration at any point along an otolith transect reflects the environment in the same way.

In other words, ontogenetic changes in otolith elemental composition are assumed not to exist. Yet, such changes have been clearly documented, even in fish held under constant environmental conditions.

As a result, observed trends in concentration across an otolith could reflect a shift in their environment or an age-related change in incorporation rate independent of the environment. It could also reflect both.

With further experimentation, we should be able to factor out ontogenetic shifts in elemental composition, thus simplifying the interpretation of possible migration pathways. However, it’s not always clear from published reports whether reconstructed distributions have been adjusted for ontogenetic effects.

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