Consistency of elemental and isotope-ratio patterns across multiple scales from individual fish

Assays of elemental and stable-isotope ratios across growth increments of scales have the potential to provide a non-lethal alternative to otolith chemistry for identifying migration and ontogenetic trophic shifts. A central assumption when employing scales as otolith analogues is that any scale from an individual will provide equivalent information about the chemical history of that fish. This assumption was investigated with multiple scales from wild and captive euryhaline Atlantic tarpon Megalops atlanticus from the north-west Gulf of Mexico. Elemental (Sr:Ca) and isotope-ratio (δ¹³ C and δ¹⁵ N) life-history profiles were compared among multiple scales from each fish. All three chemical proxies showed highly consistent patterns among non-regenerated scales, while patterns in regenerated scales diverged, indicating rapid regrowth of interior scale material at the onset of regeneration. Patterns of Sr:Ca and δ¹³ C covaried, supporting their use as salinity proxies, while δ¹⁵ N patterns were consistent with ontogenetic diet shifts. Water samples taken from aquaria holding captive fish were used to calculate partition coefficients for a suite of elements in M. atlanticus scales for future quantification of migratory movements in the region. Together, these results support the assumption that non-regenerated scales from individual M. atlanticus provide equivalent chemical histories, further validating their use as a viable non-lethal alternative to otoliths.