Macroecological study of otolith-derived field metabolic rates of marine fishes

Abstract

Metabolic rate - the energy expenditure of an organism over time - is considered by some ecologists to be the driver of all other ecological processes. Several theories seek to describe how metabolic rates vary at a macroecological scale, particularly with respect to individual body mass and temperature. Body size and temperature scaling of metabolic rate (and linked biological and ecological processes) underpin many ecological models, particularly models predicting regional to global scale responses of organisms to climate change. The relationship between body size, temperature and metabolic rate is therefore of primary concern to ecologists. Some theories propose universal scaling exponents to describe the relationships between metabolic rate and body mass or temperature, while other theories suggest that scaling exponents are context-dependent. Field metabolic rate - the full, time-averaged costs of an organism living in the wild - is relatively understudied compared to metabolic rates measured in laboratory settings. Despite their great importance to humans, the field metabolic rates of teleost fishes are especially understudied, due to a lack of methods appropriate for determining metabolic rate in aquatic animals. Here, I used a proxy derived from stable isotope analysis of otolith aragonite (Cresp values) to estimate the field metabolic rates of 114 species of marine teleost fishes. I investigated the effects of two ecological traits - species’ thermal realm and depth of occurrence - on field metabolic rates, after accounting for body mass and temperature. In the full dataset, field metabolic rate scaled with body mass with an exponent of 0.90 and with temperature with an Arrhenius activation energy of 0.26. Importantly, the scaling of field metabolic rate with temperature varied with thermal tolerance range; scaling was steeper in stenothermic groups compared to eurythermic groups. Both thermal realm and depth of occurrence had significant effects on field metabolic rates. The mean field metabolic rate of polar species was elevated compared to temperate species, suggesting partial metabolic cold adaptation. Deeper-dwelling species, operating at similarly cold temperatures, did not show high field metabolic rates, likely due to increasing food and light limitations with increasing depths. My results emphasise the importance of studying metabolic rates across a range of taxa. Current marine ecosystem models, which use scaling exponents derived from terrestrial animals, are likely overestimating the effects of climate change on body mass, while underestimating the ability of marine animals to mitigate the effects of rising temperatures on their metabolic rates, and the costs of that mitigation. Studying field metabolic rates in fish specifically enables more appropriate model parameterisation, and the C_resp method is a useful tool to aid in this endeavour.

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ORCID for Clive Trueman: orcid.org/0000-0002-4995-736X

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