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

Abstract

Rationale: Oxygen isotope ratios (δ 18 O 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 δ 18 O 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 δ 18 O 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 δ 18 O 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 δ 18 O otolith values for Delta Smelt varied as a linear inverse function of water temperature: 1000 ln α = 18.39 (±0.43, 1SE)(10 3 TK −1 ) − 34.56 (±1.49, 1SE) and δ 18 O otolith(VPDB) − δ 18 O water (VPDB) = 31.34(±0.09, 1SE) − 0.19(±0.01, 1SE) × T°C. When the ambient δ 18 O 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 δ 18 O otolith fractionation equation allowed for accurate and precise reconstructions of water temperatures experienced by Delta Smelt. Characterization of ambient δ 18 O 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.

Accuracy of Otolith Oxygen Isotope Records Analyzed by SIMS as an Index of Temperature Exposure of Wild Icelandic Cod (Gadus morhua)

“…Sub-annual otolith δ 18 O values were measured by SIMS at the NordSIMS laboratory, Department of Geosciences at the Swedish Museum of Natural History in Stockholm, Sweden (Supplementary Table 2). SIMS is a probe-based method for determining the elemental and isotopic composition of solid materials (in otoliths: e.g., Darnaude et al, 2014;Helser et al, 2018;Willmes et al, 2019). Polished mounts were gold-coated, and the stable oxygen isotopes were measured using a CAMECA ims1280 ion microprobe, an advanced high-sensitivity mass spectrometer that uses a focused beam of ions to sample in situ selected micrometer-sized areas of a sample for measurement of isotopic composition.…”

Temperature dependency equation for chub mackerel (Scomber japonicus) identified by a laboratory rearing experiment and microscale analysis

“…Therefore, the application of the equation for inorganic aragonite to chub mackerel would lead to errors in the estimation of their temperature experience, as suggested by previous studies (e.g. Kitagawa et al 2013;Sakamoto et al 2017;Willmes et al 2019). For example, suppose…”

“…The relationship between ambient water temperature and otolith d 18 O has been quantified in various fish species and is often expressed using the following simplified formula: Kim et al (2007;see Kitagawa et al 2013;Sakamoto et al 2017). However, the constants obtained in the d 18 O analysis of several species differed slightly from those of the equation for inorganic aragonite (Kitagawa et al 2013;Sakamoto et al 2017;Willmes et al 2019). Several other studies have also suggested that the constants of temperature dependency equations differ among species (Høie et al 2004;Storm-Suke et al 2007;Godiksen et al 2010;Kitagawa et al 2013;Sakamoto et al 2017).…”

Otolith δ18O Composition as a Tracer of Yellowfin Tuna (Thunnus albacares) Origin in the Indian Ocean

“…Thanks to recent analytical advances, it is now possible to measure stable isotopes with higher spatial/temporal resolution using secondary ion mass spectrometry (SIMS). This technique requires more preparation of the samples to be analysed [42], but in return offers a promising tool to unravel migration patterns and life history characteristics at much shorter timescales [43][44][45][46]. When water temperature and oxygen isotopic composition are known, it is possible to delineate isoscapes (i.e., spatial maps of predicted isotopic variation) that can be used to track potential movements across water masses with distinct isotopic signatures [47].…”