Metabolic rate, defined as the rate of energy expenditure, is influenced by a variety of biotic and abiotic factors, including food availability, life history strategies, and environmental temperature. Oxygen consumption is a commonly used method to quantify the metabolic rate of marine organisms. By measuring different components of metabolism—such as resting metabolic rate (RMR), standard metabolic rate (SMR), and maximum metabolic rate (MMR)—we can estimate critical thermal limits (upper and lower lethal temperatures), thermal sensitivity (the rate of metabolic change per °C), and optimal temperature (the most favorable temperature for physiological performance) in laboratory-controlled experiments. However, these metabolic measurements reflect only a snapshot of an organism’s condition under controlled settings and do not capture the full spectrum of energy expenditure associated with behavioural and physiological responses in wild, free-ranging individuals. Field metabolic rate (FMR), which represents the actual energy use in natural environments, is notoriously difficult to measure in wild animals.
To overcome these challenges, we have begun applying a novel isotopic proxy to reconstruct the metabolic history of marine organisms. This proxy is based on the stable carbon isotope values (δ13C) recorded in biogenic carbonates such as otoliths (bony fish), shells (bivalves), and cuttlebones (cuttlefish). Because these structures grow continuously throughout an organism’s life, the δ¹³C values along their growth increments offer a time-resolved record of metabolic level.
The aim of this study is to expand the application of this novel isotopic proxy to marine vertebrates by establishing the relationship between δ¹³C values in bone carbonate and oxygen consumption rates. Our results reveal a consistent relationship between the bone-derived isotope proxy and measured oxygen consumption, allowing us to develop a transformation equation linking the two. These findings demonstrate the potential of this method for estimating field metabolic rates in other marine vertebrates, including sharks and elasmobranchs.
Understanding the physiological and behavioural responses of marine organisms to environmental change is critical for informing effective conservation and management strategies. This isotope-based approach offers a promising tool for advancing such efforts by providing retrospective, individual-level insights into metabolic performance in the wild. This study was published in Limnology and Oceanography Letters.
Hsieh, C. Y., Liu, T. Y., Tseng, Y. C., Shirai, K., Wang, P. L., Wu, G. C., & Chung, M. T. (2025). Estimation of lifelong metabolic rates in marine fish: A combination of oxygen consumption measurements and δ13C metabolic proxy derived from vertebral structural carbonates. Limnology and Oceanography Letters. https://doi.org/10.1002/lol2.70009
