Evaluating the metabolic response to temperature using otolith carbon isotopes as an intrinsic metabolic tracer in juvenile chum salmon Oncorhynchus keta

The carbon isotopic compositions of otolith can be used to retrospectively estimate fish field metabolic rates (FMR) and are advantageous for practical applications, particularly for small-sized fish whose metabolic rates are challenging to measure in the field. Based on the proportional contribution of metabolism-derived carbon to otolith carbon, this study validated an approach for juveniles of the anadromous fish species, chum salmon Oncorhynchus keta , by integrating respirometry experiments and stable isotope ratio mass spectrometry (SIA). The isotopic results showed that the compositions of otolith carbon isotope ( δ 13 C otolith ) values were negatively correlated with body mass, aligning with the mass-specific allometric theory. The ratio of metabolism-derived carbon in otoliths ( C resp ) was calculated based on the carbon isotope compositions of the otolith, dissolved inorganic carbon in water (DIC), and diet. The results indicated that up to nearly 50 % of the carbon in juvenile chum salmon otoliths was metabolism-origin. Further, temperature gradient experiments showed that the C resp values increased until around a temperature of 15℃ and fell significantly at 20℃, suggesting that the factorial FMR was restricted at temperatures exceeding the optimal temperature for metabolism ( T opt ). Thus, the relationship between metabolic rate and C resp was validated within the temperature range of 9–15℃. Nonetheless, as a cool-water species, wild chum salmon rarely experience water masses above 15℃. Therefore, our results were feasible to estimate the FMR of juvenile chum salmon in the wild and could be used for reconstructing their metabolic histories, thereby providing insights into the metabolic strategies associated with migration traits.