Willmes Malte, Lewis Levi S, Davis Brittany E, Loiselle Liane, James Hannah F, Denny Christian, Baxter Randall, Conrad J Louise, Fangue Nann A, Hung Tien-Chieh, Armstrong Richard A, Williams Ian S, Holden Peter, Hobbs James A
Wildlife, Fish and Conservation Biology, UC Davis, One Shields Avenue, Davis, CA, 95616, USA.
California Department of Water Resources, 1416 9th Street, Sacramento, CA, 95814, USA.
Rapid Commun Mass Spectrom. 2019 Jul 30;33(14):1207-1220. doi: 10.1002/rcm.8464.
Oxygen isotope ratios (δ 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 O values in otoliths to reconstruct the thermal life histories of Delta Smelt.
Delta Smelt were reared for 360 days using three different water sources with different ambient δ O 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 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.
The δ O values for Delta Smelt varied as a linear inverse function of water temperature: 1000 ln α = 18.39 (±0.43, 1SE)(10 TK ) - 34.56 (±1.49, 1SE) and δ O - δ O = 31.34(±0.09, 1SE) - 0.19(±0.01, 1SE) × T ° C. When the ambient δ O 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.
The species-specific δ O fractionation equation allowed for accurate and precise reconstructions of water temperatures experienced by Delta Smelt. Characterization of ambient δ O 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.
鱼类耳石（听小骨）的氧同位素比率（δO值）是水体状况和热生活史的重要地球化学示踪剂。三角洲胡瓜鱼（Hypomesus transpacificus）是美国加利福尼亚州旧金山湾特有的胡瓜鱼科饵料鱼，正濒临灭绝。这些鱼具有复杂的生活史，使其能够在动态的河口环境中生存；然而，气候迅速变暖威胁着这种对温度敏感的物种。在这里，我们量化了利用耳石中的δO值重建三角洲胡瓜鱼热生活史的准确性和精确性。
使用三种具有不同环境δO值（-8.75‰、-5.28‰和-4.06‰）和不同水温（16.4°C、16.7°C、18.7°C和20.5°C）的不同水源，将三角洲胡瓜鱼饲养360天。在孵化后170天（n = 28）和360天（n = 14）收集样本。使用二次离子质谱（SIMS）从耳石核心到背缘测量原位δO值，以重建时间分辨的热生活史。
三角洲胡瓜鱼的δO值随水温呈线性反比函数变化：1000 ln α = 18.39 (±0.43, 1SE)(10/TK) - 34.56 (±1.49, 1SE) 且δO - δO = 31.34(±0.09, 1SE) - 0.19(±0.01, 1SE) × T °C。当已知环境δO值时，这种特定物种的温度依赖性氧同位素分馏模型有助于准确（0.25°C）和精确（±0.37°C，2σ）重建鱼所经历的水温。相比之下，使用现有的一般分馏方程会导致水温重建不准确。
特定物种的δO分馏方程能够准确、精确地重建三角洲胡瓜鱼所经历的水温。确定环境δO值仍然是重建野生三角洲胡瓜鱼热生活史的关键下一步。该工具将为这种极度濒危鱼类的栖息地利用、潜在热避难所和对未来变暖的恢复力提供新的见解。
