Calibrating temperature reconstructions from fish otolith oxygen isotope analysis for California's critically endangered Delta Smelt

Rationale: Oxygen isotope ratios (δ¹⁸O values) of fish otoliths (ear bones) are valuable geochemical tracers of water conditions and thermal life history. Delta Smelt (Hypomesus transpacificus) are osmerid forage fish endemic to the San Francisco Estuary, California, USA, that are on the verge of extinction. These fish exhibit a complex life history that allows them to survive in a dynamic estuarine environment; however, a rapidly warming climate threatens this thermally sensitive species. Here we quantify the accuracy and precision of using δ¹⁸O values in otoliths to reconstruct the thermal life histories of Delta Smelt. Methods: Delta Smelt were reared for 360 days using three different water sources with different ambient δ¹⁸O_water values (−8.75‰, −5.28‰, and −4.06‰) and different water temperatures (16.4°C, 16.7°C, 18.7°C, and 20.5°C). Samples were collected after 170 days (n = 28) and 360 days (n = 14) post-hatch. In situ δ¹⁸O values were measured from the core of the otolith to the dorsal edge using secondary ion mass spectrometry (SIMS) to reconstruct temporally resolved thermal life histories. Results: The δ¹⁸O_otolith values for Delta Smelt varied as a linear inverse function of water temperature: 1000 ln α = 18.39 (±0.43, 1SE)(10³TK⁻¹) − 34.56 (±1.49, 1SE) and δ¹⁸O_{otolith(VPDB)} − δ¹⁸O_{water (VPDB)} = 31.34(±0.09, 1SE) − 0.19(±0.01, 1SE) × T ° C. When the ambient δ¹⁸O_water value is known, this species-specific temperature-dependent oxygen isotope fractionation model facilitated the accurate (0.25°C) and precise (±0.37°C, 2σ) reconstruction of the water temperature experienced by the fish. In contrast, the use of existing general fractionation equations resulted in inaccurate temperature reconstructions. Conclusions: The species-specific δ¹⁸O_otolith fractionation equation allowed for accurate and precise reconstructions of water temperatures experienced by Delta Smelt. Characterization of ambient δ¹⁸O_water values remains a critical next step for reconstructing thermal life histories of wild Delta Smelt. This tool will provide new insights into habitat utilization, potential thermal refugia, and resilience to future warming for this critically endangered fish.