Animals require energy for all life functions. Energy reflects rates of chemical and physiological processes inside their bodies and governs performance, growth, behaviour, social interactions, rates of food consumption and drains on ecosystem resources. We quantify energy demands of animals by measuring metabolic rates. However, traditional methods of measuring metabolic rates are unsuitable in wild fish, leading us to try and create useful alternatives. We tested the idea that there may be an internal chemical marker in fish that acts as a natural proxy for metabolic rates.
Our study species was snapper (Chrysophrys auratus). Snapper is an iconic and valuable fishery species found throughout the Indo-pacific region.
We reared juvenile snapper at 4 different temperature treatments for up to 2 months. Then we used intermittent-flow respirometry to calculate metabolic rates. intermittent-flow respirometry involves placing the fish in enclosed chambers for 24 hours and measuring the rates of oxygen consumption. Using this technique, we could calculate Standard Metabolic Rates, the minimum energy usage needed to sustain life; Maximum Metabolic Rate, the upper limit of metabolic capacity; and Absolute Aerobic Scope, illustrating energy niches and fitness/performance windows.
We then measured carbon (δ13C) and oxygen (δ18O) stable isotopes in otoliths (ear stones) using isotope-ratio mass spectrometry. Otoliths are paired calcified structures in all teleost fish that continuously grow, forming layers like an onion (or an ogre!), but are inert so trap chemical signatures within the layers throughout the fish’s life.
We found that under higher temperatures, standard and maximum metabolic rates significantly increased, while carbon and oxygen isotopes in otoliths significantly decreased. We then investigated the relationships between isotopes and metabolic rates. We discovered negative logarithmic relationships between the carbon isotopes and standard and maximum metabolic rates. Additionally, exponential decay curves were observed between proportions of metabolically sourced carbon in otoliths and both measured and theoretical metabolic rates.
Our results show that carbon isotopes display strong potential as a quantitative metabolic proxy in wild fish.
Otolith archives are stored in universities and research institutes across the globe. Our study shows that these collections possess untapped potential that could create lifetime metabolic profiles for a wide range of species, locations and years. These chemical approaches have particular value for populations where direct monitoring is unfeasible, particularly historical or extinct species and inaccessible deep-sea species. This powerful chemical tool can be used to reconstruct lifetime physiological histories of wild fish, providing valuable insights for wide use in conservation and management of fish populations.
For more information:
Martino, Doubleday, Chung, Gillanders (2020) Experimental support towards a metabolic proxy in fish using otolith carbon isotopes. Journal of Experimental Biology doi:10.1242/jeb.217091