Hsieh Yu, Shiao Jen-Chieh, Lin Saul-Wood, Iizuka Yoshiyuki
Institute of Oceanography, College of Science, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei, 10617, Taiwan.
Institute of Earth Sciences, Academia Sinica, No. 128, Section 2, Academia Road, Nankang, Taipei, 11529, Taiwan.
Rapid Commun Mass Spectrom. 2019 Aug 30;33(16):1344-1354. doi: 10.1002/rcm.8476.
Otolith strontium:calcium (Sr:Ca) ratios have been extensively used to study fish migration across a river, estuary and ocean at each life stage. However, otolith Sr:Ca ratios only explain partial variations in salinity and quantitative reconstruction of salinity history of fishes remains a challenge. Using a euryhaline fish Lateolabrax japonicus as an example, this study demonstrated an isotopic method of quantitatively reconstructing the salinity histories and habitat uses of the fish.
Otolith oxygen stable isotopic ratios (δ O values) were measured using a mass spectrometer for subsamples sequentially milled from the otolith core to the edge, and otolith Sr:Ca ratios were measured by an electron probe microanalyzer for the comparison. The mean water temperature within the studied area in the time period of each milled subsample was estimated from the Copernicus database. Based on an isotopic fractionation equation, each δ O value and water temperature pair determined the water δ O value, which was then converted into salinity using published linear equations or an equation that was newly generated for this study.
Individual fish clearly revealed different preferences in habitat use. The retrospectively reconstructed salinity history indicated that most L. japonicus inhabit and can spawn in seawater as well as in brackish estuaries. Few fish lived in freshwater during the young stage based on the analysis of δ O profiles; however, otolith Sr:Ca ratios can only reveal freshwater residence and cannot distinguish residence in brackish water or seawater.
This study demonstrated that otolith temporal microstructure and δ O values are well-suited approaches for quantitative reconstruction of salinity histories of the fish. This method can improve the understanding of the habitat uses of other fishes inhabiting diverse habitats among the rivers, estuaries and oceans.
耳石锶钙（Sr:Ca）比值已被广泛用于研究鱼类在每个生命阶段跨越河流、河口和海洋的洄游情况。然而，耳石Sr:Ca比值只能解释盐度变化的部分情况，对鱼类盐度历史进行定量重建仍然是一个挑战。本研究以广盐性鱼类日本鲈为例，展示了一种定量重建鱼类盐度历史和栖息地利用情况的同位素方法。
使用质谱仪测量从耳石核心到边缘依次研磨的子样本的耳石氧稳定同位素比值（δO值），并通过电子探针微分析仪测量耳石Sr:Ca比值以作比较。根据哥白尼数据库估算每个研磨子样本时间段内研究区域的平均水温。基于同位素分馏方程，每对δO值和水温确定水体δO值，然后使用已发表的线性方程或本研究新生成的方程将其转换为盐度。
个体鱼类在栖息地利用方面明显表现出不同偏好。回顾性重建的盐度历史表明，大多数日本鲈栖息于海水和咸淡水河口，并且能够在这些地方产卵。根据δO剖面分析，很少有鱼在幼鱼阶段生活在淡水中；然而，耳石Sr:Ca比值只能揭示在淡水中的栖息情况，无法区分在咸淡水或海水中的栖息情况。
本研究表明，耳石的时间微结构和δO值是定量重建鱼类盐度历史的合适方法。该方法有助于增进对栖息于河流、河口和海洋等不同栖息地的其他鱼类栖息地利用情况的了解。
