Preservation Effects on Stable Isotope Values of Archived Elasmobranch Fin Tissue: Comparisons between Frozen and Ethanol-Stored Samples

Elasmobranch fin tissue has been sampled and archived for decades to support genetics research. However, these collections have the potential to provide additional information on the trophic ecology of and habitat use by elasmobranch species. The use of fin tissue is especially attractive considering the threatened status of many elasmobranchs and the call for limiting mortalities. Yet, the use of fin samples for stable isotope analysis requires either that (1) storage methods do not alter tissue isotope values or (2) any alterations in isotope composition that occur during storage are predictable. In this study, paired fin tissues sampled from Smalltooth Sawfish Pristis pectinata and cownose rays Rhinoptera spp. were stored frozen and in ethanol and were subsequently analyzed for carbon (delta C-13) and nitrogen (delta N-15) isotope ratios. Fin delta C-13 and delta N-15 values were highly correlated between treatments for both taxa (r(2) >= 0.80). For Smalltooth Sawfish, ethanol storage significantly increased fin delta C-13 values by 0.5 +/- 0.1% (mean +/- SE) and decreased fin delta N-15 values by 0.1 +/- 0.1% relative to frozen samples; differences were similar for cownose rays (delta C-13: 0.2 +/- 0.2%; delta N-15: 0.2 +/- 0.1%) but were not significant. A range of approximately 3% for delta C-13 between treatments could have effects on data interpretation, suggesting the use of regressions for ethanol correction of delta C-13 values, although trends were comparable between frozen and ethanol-preserved samples without correction. Given the low variability in delta N-15 values, a correction is not warranted. For endangered species such as the Smalltooth Sawfish, stable isotope analysis of ethanol-archived fin samples can provide important information regarding habitat use and trophic ecology, with potential significance for conservation and management strategies. The general uniformity in isotope ratio shifts observed for archived samples between the two taxa suggests that these findings can be generalized across elasmobranch species.