Small temperature increases will benefit the growth of many cold-blooded animals, but laboratory studies indicate that warming can eventually exceed physiological limits, resulting in reduced growth. Evidence shows that this may have already happened for a fish species—the banded morwong—in the Tasman Sea. Climate change can affect organisms both directly, by affecting their physiology, growth, and behaviour1, and indirectly, for example through effects on ecosystem structure and function1,2. For ectotherms, or ‘cold-blooded’ animals, warming will directly affect their metabolism, with growth rates in temperate species predicted to increase initially as temperatures rise, but then decline as individuals struggle to maintain cardiac function and respiration in the face of increased metabolic demands3,4. We provide evidence consistent with this prediction for a marine fish (Cheilodactylus spectabilis) in the Tasman Sea; one of the most rapidly warming regions of the Southern Hemisphere ocean5. We estimated changes in the species’ growth rate over a 90-year period using otoliths—bony structures that fish use for orientation and detection of movement—and compared these changes to temperature trends across the species’ distribution. Increasing temperatures coincide with increased growth for populations in the middle of the species range, but with reduced growth for those at the warm northern edge of the species’ distribution, indicating that temperatures may have already reached levels associated with increased metabolic costs. If warming continues, the direct metabolic effects of increasing temperatures on this species may lead to declining productivity and range contraction.