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Rationale: For years, archaeologists, climatologists, and ecologists have used stable oxygen isotope values (δ 13 C, δ 18 O) in fish otoliths from archaeological sites to reconstruct the habitats, paleo-temperature, and seasonality of the fish captures. Otoliths from archaeological sites might have been heated when ancient people cooked the fish for food. Therefore, there are debates as to whether the cooking behaviors would cause further isotopic fractionations of the carbonate in the otoliths., Methods: In this study, we have evaluated the effects of the cooking methods on the otolith δ 13 C and δ 18 O values by comparing the otoliths of the javelin grunter (Pomadasys kaakan) from the same individuals, with the left otoliths taken out before the different cooking processes. Otolith sections of the fish were then made and several subsamples were milled along the microstructures visible in the otolith pairs, mostly annual check rings, followed by the stable isotope analyses., Results: There were no morphological changes between the cooked and uncooked otoliths. The δ 13 C and δ 18 O values were highly consistent for the otolith subsamples between the cooked and uncooked pairs, suggesting none or trivial effects of the cooking processes on the isotopic values of the otoliths. In addition, some javelin grunters showed lower δ 13 C (-5‰ to -6‰) and δ 18 O (-4‰ to -5‰) values deposited in the wide translucent zone of the otoliths, suggesting seasonal migration of the fish to estuaries during the summer., Conclusions: This study suggests that cooking processes do not change otolith stable isotopic compositions and will therefore allow for future research to use the otolith δ 13 C and δ 18 O values to better understand the life history and used habitats of the preserved fish remains in the middens., (© 2021 John Wiley & Sons Ltd.)

Rationale: 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., Methods: Otolith oxygen stable isotopic ratios (δ 18 O oto 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 18 O oto value and water temperature pair determined the water δ 18 O value, which was then converted into salinity using published linear equations or an equation that was newly generated for this study., Results: 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 18 O oto profiles; however, otolith Sr:Ca ratios can only reveal freshwater residence and cannot distinguish residence in brackish water or seawater., Conclusions: This study demonstrated that otolith temporal microstructure and δ 18 O oto 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., (© 2019 John Wiley & Sons, Ltd.)

Rationale: Otoliths are usually used to estimate the age of fish and the chemical composition such as nitrogen stable isotope ratios (δ 15 N values) may record environmental information and ecological role of the fish. However, the isotopic fractionation of δ 15 N values between diets and otoliths has rarely been investigated and remains unclear., Methods: Nitrogen isotopic fractionation between five different diets (δ 15 N diet values) and otoliths (δ 15 N oto values) were elucidated in tilapia Oreochromis mossambica reared in controlled feeding experiments. The otoliths were dissolved with hydrogen chloride and peroxodisulfate was used to oxidize the total organic materials to nitrate, which was further converted into N 2 O gas by denitrification bacteria before the measurement of δ 15 N oto values by isotope ratio mass spectrometry. The δ 15 N values of muscles, gills, scales and livers of the tilapias were also measured by isotope ratio mass spectrometry., Results: The peroxodisulfate oxidation-bacterial conversion method reduced the minimum mass of the otoliths required for analysis to as low as 2 mg, unlike past methods, which have required masses of 8-155 mg. The δ 15 N oto values were not significantly different from the δ 15 N diet values of the five diets. Furthermore, the somatic growth rate had no effect on the δ 15 N oto values. Nevertheless, the δ 15 N values of metabolically active tissues were significantly different from each other and higher than the δ 15 N diet values, due to the deamination of these tissues., Conclusions: These results suggest that diet was the main source of amino acids for the otolith organic matrix and there was no biochemical transamination during the assimilation of dietary amino acids to otoliths. The δ 15 N oto value can be used as a proxy of nitrogen sources of fishes and may have potential application in ecological studies such as the detection of diet shift, migration, trophic levels and environmental changes experienced by the fish population., (© 2018 John Wiley & Sons, Ltd.)

Rationale: Nitrogen and carbon stable isotope ratios (δ 15 N and δ 13 C values) of carbonate-bound organic materials in otoliths can provide information to address the biological and ecological functions of fish. Correct interpretation of otolith δ 15 N and δ 13 C profiles requires knowledge of the metabolic routes of nitrogen and carbon isotopes. However, the isotopic assimilation of δ 15 N and δ 13 C compositions from diets to otoliths has rarely been investigated., Methods: This study traced the daily nitrogen and carbon isotopic assimilation between diets and otoliths using nanoscale secondary ion mass spectrometry (NanoSIMS). Isotopically labeled algae (Tetraselmis chui) were fed to tilapia (Oreochromis niloticus) for 14-17 days. NanoSIMS and conventional isotope ratio mass spectrometry were used to measure δ 15 N and δ 13 C variations in the otoliths and fish muscle, respectively., Results: Otolith δ 15 N values abruptly surged from natural abundance levels by 1000-2300‰ after the fish ate 15 N-spiked algae with δ 15 N values of approximately 2200‰. However, the δ 15 N values of fish muscle increased to only approximately 500‰ at the end of the feeding experiment. Much higher δ 15 N values (3700-14 000‰) and moderate δ 13 C values (60-200‰) were detected in the otoliths after the tilapia ate 15 N- and 13 C-spiked algae with a δ 15 N value of 36667‰ and a δ 13 C value of 272‰. Mapping analysis showed sub-micrometer-scale distribution of 15 N embedded in the otolith growth increments with a low-to-high δ 15 N signal after the tilapia shifted diets from non-spiked to 15 N-labeled algae., Conclusions: These results suggest that otolith nitrogen and carbon isotopes from food were directly assimilated on the same day. Food is the major and in some cases only source of otolith nitrogen isotopes but makes only a partial contribution to otolith carbon isotopes. Therefore, the δ 15 N values recorded in the sclerochronological layers of the otoliths can be used to determine the trophic levels, food sources and diet changes of fish., (Copyright © 2018 John Wiley & Sons, Ltd.)