Evaluation of micromilling/conventional isotope ratio mass spectrometry and secondary ion mass spectrometry of δ 18 O values in fish otoliths for sclerochronology

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.