Experimental support towards a metabolic proxy in fish using otolith carbon isotopes

Metabolic rate underpins our understanding of how species survive, reproduce and interact with their environment, but can be difficult to measure in wild fish. Stable carbon isotopes (δ¹³C) in ear stones (otoliths) of fish may reflect lifetime metabolic signatures but experimental validation is required to advance our understanding of the relationship. To this end, we reared juvenile Australasian snapper (Chrysophrys auratus), an iconic fishery species, at different temperatures and used intermittent-flow respirometry to calculate standard metabolic rate (SMR), maximum metabolic rate (MMR) and absolute aerobic scope (AAS). Subsequently, we analysed δ¹³C and oxygen isotopes (δ¹⁸O) in otoliths using isotope-ratio mass spectrometry. We found that under increasing temperatures, δ¹³C and δ¹⁸O significantly decreased, while SMR and MMR significantly increased. Negative logarithmic relationships were found between δ¹³C in otoliths and both SMR and MMR, while exponential decay curves were observed between proportions of metabolically sourced carbon in otoliths (M_oto) and both measured and theoretical SMR. We show that basal energy for subsistence living and activity metabolism, both core components of field metabolic rates, contribute towards incorporation of δ¹³C into otoliths and support the use of δ¹³C as a metabolic proxy in field settings. The functional shapes of the logarithmic and exponential decay curves indicated that physiological thresholds regulate relationships between δ¹³C and metabolic rates due to upper thresholds of M_oto Here, we present quantitative experimental evidence to support the development of an otolith-based metabolic proxy, which could be a powerful tool in reconstructing lifetime biological trends in wild fish.