Co-variation and trade-offs in ontogenetic scaling of growth and metabolic rates in teleost fish

Why growing fish matter to everyday life

From goldfish in home aquariums to salmon on our dinner plates, fish turn food into body mass using energy, just like we do. A long-standing idea in biology claims that this energy use follows a strict mathematical rule that applies to nearly all living things. But this new study on seven species of bony fish asks a deceptively simple question: as a single fish grows from small to large, does its metabolism really follow that fixed rule, and how is its growth affected? The answers challenge textbook assumptions about how bodies use energy and reveal hidden trade-offs that could shape survival, reproduction, and responses to a changing world.

Old rules about size and energy

For nearly a century, many biologists have embraced the idea that metabolic rate – the pace at which organisms burn energy – follows a universal pattern. According to this view, energy use rises with body size in a very predictable way, so that bigger animals use more energy overall but less per unit of body mass. This thinking underpins the “metabolic theory of ecology,” which suggests that the same simple rule helps explain growth, reproduction, and even how ecosystems function. Yet critics have long pointed out that real animals show lots of variation around this supposed law, implying that biology may not be governed by a single neat equation.

Following the same fish through life

Most previous work compared different species, or different individuals within a species, at one point in time. This study instead followed 389 individual fish, from seven species including trout, guppies, clownfish, and zebrafish, repeatedly over their lives. For each fish, the researchers measured body mass, standard (maintenance) metabolic rate – the energy cost of simply staying alive at rest – and, for most species, maximum metabolic rate during strenuous activity. The difference between maximum and maintenance rates, called metabolic scope, represents the energy available for everything beyond bare survival, such as swimming, digestion, and reproduction. By tracking these traits an average of 6–7 times per individual, the team could calculate how each fish’s metabolism and growth changed with size over its own lifetime, rather than inferring patterns from one-off measurements.

Metabolism grows faster than growth

Across species, the researchers found that as individual fish grew, their maintenance, maximum, and total aerobic capacity all increased more steeply with size than classic theory predicts. On average, these metabolic traits scaled closer to a simple one-to-one rise with body mass than to the widely cited “three-quarters power” rule. By contrast, growth rate – how quickly fish added body mass – increased much more modestly with size. Importantly, individuals whose growth rate increased more strongly through life also tended to show a steeper rise in maintenance metabolism. In other words, fish that ramped up growth as they got bigger carried higher energy costs for basic upkeep, suggesting that rapid growth and elevated background metabolism go hand in hand.

The hidden cost of growing fast

The story becomes more nuanced when looking at metabolic scope, the energy budget available for activities beyond maintenance. Here, the researchers uncovered a trade-off: individuals and species with more strongly increasing growth tended to have a shallower, or even declining, increase in metabolic scope as they grew. Put simply, fish that accelerate growth through life often end up with less spare aerobic capacity relative to their maintenance needs. This means they may have less energy left over for demanding tasks like escaping predators, coping with heat or low oxygen, or producing offspring, even though they reach larger sizes more quickly.

What this means for fish and for us

These findings show that neither metabolism nor growth follows a fixed, universal scaling rule. Instead, the way each fish’s metabolism scales with size depends closely on its growth pattern, and faster growth comes at the cost of higher running expenses and reduced “wiggle room” in energy use. In rich, predictable environments, that trade-off might pay off: rapid growth can help fish outgrow size-selective predation. But in harsher or changing conditions, a shrinking metabolic safety margin could hurt survival and reproduction. By revealing how growth and metabolism co-vary within individual animals, this study challenges influential theories and highlights that life’s energy budget is more flexible—and more constrained—than simple formulas suggest.

Citation: Rosén, A., Andreassen, A.H., Storm, Z. et al. Co-variation and trade-offs in ontogenetic scaling of growth and metabolic rates in teleost fish. Commun Biol 9, 338 (2026). https://doi.org/10.1038/s42003-026-09588-w

Keywords: metabolic scaling, fish growth, aerobic scope, energy trade-offs, life history