Tolerance limit for fish growth exceeded by warming waters

Climate change can affect organisms both directly, by affecting their physiology, growth, and behaviour(1), and indirectly, for example through effects on ecosystem structure and function(1,2). 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 demands(3,4). 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 ocean(5). 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.