Helser Thomas E, Kastelle Craig R, McKay Jennifer L, Orland Ian J, Kozdon Reinhard, Valley John W
Resource Ecology and Fisheries Management Division, Alaska Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 7600 Sand Point Way, Seattle, WA, 98115, USA.
College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, 97331, USA.
Rapid Commun Mass Spectrom. 2018 Oct 30;32(20):1781-1790. doi: 10.1002/rcm.8231.
Stable oxygen isotope ratios (δ O values) measured in fish otoliths can provide valuable detailed information on fish life history, fish age determination, and ocean thermography. Traditionally, otoliths are sampled by micromilling followed by isotope ratio mass spectrometry (IRMS), but direct analysis by secondary ion mass spectrometry (SIMS) is becoming more common. However, these two methods have not been compared to determine which, if either, is better for fish age validation studies. Hence, the goals were to: (1) determine if the δ O signatures from the two different methods are similar, (2) determine which method is better for fish age validation studies, and (3) examine biogeographic and migration history.
Both analytical techniques, micromilling/IRMS and SIMS, were used to measure δ O values in six Pacific cod (Gadus macrocephalus) otoliths. A series of measurements was made from the center of each otolith to its edge to develop a life-history δ O signature for each fish.
The sampling resolution of SIMS analyses was 2-3 times greater than that obtained by micromilling/IRMS. We found an offset between SIMS and micromilling/IRMS δ O values, about 0.5‰ on average, with SIMS yielding lower values. However, the δ O patterns from both methods (i.e., the number of δ O maxima) correspond to the estimated age determined by otolith growth-zone counts, validating fish age determination methods.
Both techniques resolved δ O life-history signatures and showed patterns consistent with seasonal variation in temperatures and changes due to fish migration. When otoliths are large, micromilling/IRMS can provide adequate resolution for fish age validation. However, SIMS is the better option if greater sampling resolution is required, such as when otoliths are small or specimens are longer lived and have compact growth zones.
在鱼类耳石中测量的稳定氧同位素比率（δO值）可为鱼类生活史、鱼类年龄测定和海洋温度记录提供有价值的详细信息。传统上，耳石通过微铣削采样，然后进行同位素比率质谱分析（IRMS），但二次离子质谱分析（SIMS）直接分析正变得越来越普遍。然而，尚未对这两种方法进行比较以确定哪一种（如果有的话）更适合鱼类年龄验证研究。因此，目标是：（1）确定两种不同方法的δO特征是否相似，（2）确定哪种方法更适合鱼类年龄验证研究，以及（3）研究生物地理学和洄游历史。
微铣削/IRMS和SIMS这两种分析技术均用于测量6个太平洋鳕鱼（大头鳕）耳石中的δO值。从每个耳石的中心到边缘进行一系列测量，以得出每条鱼的生活史δO特征。
SIMS分析的采样分辨率比微铣削/IRMS获得的分辨率高2至3倍。我们发现SIMS和微铣削/IRMS的δO值之间存在偏差，平均约为0.5‰，SIMS得出的值较低。然而，两种方法的δO模式（即δO最大值的数量）与通过耳石生长带计数确定的估计年龄相对应，验证了鱼类年龄测定方法。
两种技术都解析了δO生活史特征，并显示出与温度季节性变化和鱼类洄游引起的变化一致的模式。当耳石较大时，微铣削/IRMS可为鱼类年龄验证提供足够的分辨率。然而，如果需要更高的采样分辨率，例如当耳石较小时或标本寿命较长且生长带紧凑时，SIMS是更好的选择。
