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

Abstract

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 (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios. Fin δ¹³C and δ¹⁵N values were highly correlated between treatments for both taxa (r² ≥ 0.80). For Smalltooth Sawfish, ethanol storage significantly increased fin δ¹³C values by 0.5 ± 0.1‰ (mean ± SE) and decreased fin δ¹⁵N values by 0.1 ± 0.1‰ relative to frozen samples; differences were similar for cownose rays (δ¹³C: 0.2 ± 0.2‰; δ¹⁵N: 0.2 ± 0.1‰) but were not significant. A range of approximately 3‰ for δ¹³C between treatments could have effects on data interpretation, suggesting the use of regressions for ethanol correction of δ¹³C values, although trends were comparable between frozen and ethanol-preserved samples without correction. Given the low variability in δ¹⁵N 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.

Received March 27, 2014; accepted July 30, 2014

ACKNOWLEDGMENTS

This work was supported primarily by funding from the U.S. Department of Commerce (DOC), National Oceanic and Atmospheric Administration's (NOAA) National Marine Fisheries Service (NMFS) through Section 6 (Cooperation with the States) of the U.S. Endangered Species Act under Grant Award NA10NMF4720032 to the Florida Fish and Wildlife Conservation Commission (FFWCC). The statements, findings, conclusions, and recommendations presented here are those of the authors and do not necessarily reflect the views or policies of the DOC or NOAA. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Government. We thank the Charlotte Harbor Field Laboratory staff and volunteers for dedicated fieldwork, A. Hussey for stable isotope analysis, and N. Hussey and G. Paterson for helpful comments. We also thank C. Sweeting and one anonymous reviewer for insightful reviews. This research was conducted under Endangered Species Permit numbers 1475 and 15802 issued to FFWCC by NMFS.