Shrinking of fishes exacerbates impacts of global ocean changes on marine ecosystems

Changes in temperature, oxygen content and other ocean biogeochemical properties directly affect the ecophysiology of marine water-breathing organisms1–3. Previous studies suggest that the most prominent biological responses are changes in distribution4–6, phenology7,8 and productivity9. Both theory and empirical observations also support the hypothesis that warming and reduced oxygen will reduce body size of marine fishes10–12. However, the extent to which such changes would exacerbate the impacts of climate and ocean changes on global marine ecosystems remains unexplored. Here, we employ a model to examine the integrated biological responses of over 600 species of marine fishes due to changes in distribution, abundance and body size. The model has an explicit representation of ecophysiology, dispersal, distribution, and population dynamics3. We show that assemblage-averaged maximum body weight is expected to shrink by 14–24% globally from 2000 to 2050 under a high-emission scenario. About half of this shrinkage is due to change in distribution and abundance, the remainder to changes in physiology. The tropical and intermediate latitudinal areas will be heavily impacted, with an average reduction of more than 20%. Our results provide a new dimension to understanding the integrated impacts of climate change on marine ecosystems. Global climate and ocean changes resulting from anthropogenic greenhouse-gas emissions are currently affecting and expected to continue to affect marine organisms1–11. These impacts are fundamentally linked to the close relationship between ocean conditions and the ecophysiology of marine organisms, notably water-breathing ectotherms1,2,13. However, previous studies focus largely on the implication of thermal tolerance and limitations of other environmental factors for the distribution range of these organisms4–6. Few studies have assessed the integrated responses of changes in ecophysiology, distribution and their effects on key characteristics of marine biota such as body size. The size of aquatic water-breathers is strongly affected by temperature, oxygen level and other factors such as resource availability2,14. Specifically, the maximum body weight (W∞) of marine fishes and invertebrates is fundamentally limited by the balance between energy demand and supply, whereW∞ is reached when energy demand= energy supply (thus net growth= 0). This can be expressed by the function that is commonly used to describe growth of fishes15: