Impacts of ocean warming on fish size reductions on the world’s hottest coral reefs

The impact of ocean warming on fish and fisheries is vigorously debated, with major potential consequences for ecosystems and industries. Leading climate change theories project limited adaptive capacity of tropical fishes, with 14-39% size reductions predicted by 2050 due to a mass-scaling limitation of oxygen supply in larger individuals. Using the world’s warmest coral reefs in the Persian /
Arabian Gulf as a natural laboratory for ocean warming, where species have survived >35.0°C summer temperatures for over 6,000 years and are estimated to be 14-40% smaller at max size compared to cooler locations, we found two adaptive pathways across 10 metabolic and swimming performance metrics that
appear to have facilitated survival at elevated temperatures. Comparing Lutjanus ehrenbergii and Scolopsis ghanam from the Persian / Arabian Gulf to individuals from typical present-day coral reef temperatures in the Gulf of Oman (summer max 32.0°C) across 27.0, 31.5°C and 35.5°C, our data reveals that despite limited adaptive capacity, the species and populations surviving in Persian / Arabian Gulf
show a lower-than-expected rise in basal metabolic demands and a right-shifted thermal window facilitating a capacity to maintain oxygen supply and aerobic performance to 35.5°C. Importantly, our findings challenge the leading theoretical oxygen-limitation explanations for warming-induced size reductions in fishes, as the species examined here did not fully comply with mass-scaling predictions. Instead, our data suggest a mismatch in energy acquisition and demand as the primary cause of size reductions. Our data support a modified unifying resource-acquisition theory encompassing aspects of existing oxygen limitation theories as well as resource limitation and ecological metabolic theory, to explain species-specific size reductions due to ocean warming. Our findings help understanding of why diversity is declining as a consequence of ocean warming, and why smaller individuals are evolutionarily favored at elevated temperatures.