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

Area of Science:

• Physiological Ecology
• Isotope Geochemistry
• Fisheries Science

Background:

• Metabolic rate is fundamental to fish survival, reproduction, and ecological interactions, yet challenging to measure in wild populations.
• Stable isotopes, specifically carbon isotopes (δ¹³C) in otoliths, show potential as indicators of lifetime metabolic signatures.
• Experimental validation is essential to confirm the utility of otolith δ¹³C as a metabolic proxy.

Purpose of the Study:

• To experimentally validate the use of stable carbon isotopes (δ¹³C) in fish otoliths as a proxy for metabolic rate.
• To investigate the relationship between environmental temperature, metabolic rates (SMR, MMR, AAS), and otolith isotopic signatures (δ¹³C, δ¹⁸O).
• To establish quantitative relationships between otolith δ¹³C and metabolic rate components for application in wild fish.

Main Methods:

• Juvenile Australasian snapper (Chrysophrys auratus) were reared under varying temperatures.
• Standard metabolic rate (SMR) and maximum metabolic rate (MMR) were determined using intermittent-flow respirometry.
• Otoliths were analyzed for stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes using isotope-ratio mass spectrometry.

Main Results:

• Increasing temperatures led to significant decreases in otolith δ¹³C and δ¹⁸O, alongside increases in SMR and MMR.
• Negative logarithmic relationships were observed between otolith δ¹³C and both SMR and MMR.
• Exponential decay curves linked the proportion of metabolically sourced carbon in otoliths (M_oto) to SMR, indicating physiological thresholds regulate these relationships.

Conclusions:

• Otolith δ¹³C serves as a reliable proxy for metabolic rates, reflecting both basal energy for subsistence and activity metabolism.
• The quantitative relationships established support the use of otolith δ¹³C for reconstructing lifetime metabolic trends in wild fish.
• This validated proxy offers a powerful tool for ecological and fisheries research, overcoming limitations of direct metabolic measurements in natural environments.