Fish Metabolism Follows Different Rules Than Scientists Thought

We began curating metabolic rate data from fish studies and found evidence for a surprising scaling relationship. 89, not the 0.67, 0.75, or 1.0 that decades of theory had predicted. This number might seem abstract, but it represents a fundamental relationship governing how fish use energy as they grow.

We were trying to resolve one of ecology's longest-standing controversies: how does metabolic rate scale with body mass in fish? This question matters because the answer underpins predictions about population dynamics and climate change impacts. They curated data from 25 studies spanning 55 independent trials across 16 fish species, from common minnows to European eels. Each dataset required measurements of standard metabolic rate, temperature, and body mass—not trivial experiments, as measuring an individual fish's metabolic rate across a 10-fold range of body sizes is technically demanding.

The analysis revealed strong evidence for a metabolic scaling coefficient of 0.89, with a ΔSIC interval spanning 0.82 to 0.99. This finding directly contradicts the mechanistically derived coefficients that have dominated ecological theory. The 0.67 value, based on surface area constraints, and the 0.75 value from fractal transport networks, and the 1.0 value from proportional scaling—none were supported by the data. We also found that metabolic rates vary 2-3 fold across individuals of the same population, and this variation is repeatable.

"We began curating metabolic rate data from fish studies and found evidence for a surprising scaling relationship."

What emerged was how consistent this 0.89 pattern appeared across such diverse fish species and conditions, yet the models clearly showed that other factors matter too. The best-fitting models included random intercepts and random slopes by species, suggesting that taxonomy, ecology, or lifestyle characteristics influence the underlying metabolic processes. The mechanisms that drive this 0.89 relationship or why it differs from theoretical predictions remain unclear.

This matters because metabolic scaling relationships form the cornerstone of metabolic theory of ecology, linking individual physiology to community patterns and energy flows across landscapes. Our findings suggest that current models predicting how fish populations will respond to climate change may need revision. If fish metabolism scales differently than previously thought, projections of population abundance and species distributions under changing conditions could be systematically biased.

Many questions remain unanswered. What biological mechanisms produce this 0.89 scaling? How do ecological factors like habitat depth, activity level, or feeding strategy modify this relationship? More studies designed specifically to capture wide ranges of body sizes within species are needed, as is understanding whether this pattern holds for other vertebrate groups. The evidentialist framework applied here allows for continuous refinement as new data emerge.

Citation

Jerde, Christopher L.; Kraskura, Krista; Eliason, Erika J.; Csik, Samantha R.; Stier, Adrian C.; Taper, Mark L. (2019). Strong Evidence for an Intraspecific Metabolic Scaling Coefficient Near 0.89 in Fish. Frontiers in Physiology.

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This paper is Open Access.