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 organisms^{1, 2, 3}. Previous studies suggest that the most prominent biological responses are changes in distribution^{4, 5, 6}, phenology^{7, 8} and productivity⁹. Both theory and empirical observations also support the hypothesis that warming and reduced oxygen will reduce body size of marine fishes^{10, 11, 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.

Figures at a glance

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1. Figure 1: Projected changes in ocean conditions and the expected biological responses of fish communities in terms of distribution and body size.

   

   a, Projected changes in sea bottom temperature. b, Dissolved oxygen concentration. Anomalies in temperature and oxygen are average projections from GFDL ESM2.1 and IPSL-CM4-LOOP relative to the average 1971–2000 values under the SRES a2 scenario. c, Schematic illustrating the expected changes in body size at individual and assemblage levels in a specific region (area enclosed by dashed red line). It is hypothesized that under warming and reduced oxygen levels, the fish at a particular location will have smaller body weight. Together with the invasion/increased abundance of smaller-bodied species and local extinction/decreased abundance of larger-bodied species, mean maximum body weight is expected to lower at the assemblage level.

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2. Figure 2: Predicted mean assemblage maximum body weight (g) and its changes from 2000 to 2050 (20-year average) under the SRES A2 scenario.

   

   a–c, The mean and variation of projections from simulations driven by GFDL ESM2.1 and IPSL-CM4-LOOP are presented. White areas on the maps represent no data. a, Maximum body weight in 1991–2010 is predicted from the Dynamic Bioclimate Envelope Model (left, see Methods). Latitudinal average of mean assemblage maximum body weight in the global ocean in 1991–2010 and 2041–2060 (right). b, The projected percentage changes in mean assemblage maximum body weight between 2000 and 2050 (left) and latitudinal change in average mean assemblage maximum body weight in the global ocean between 2000 and 2050 (right). c, Level of variation in predictions driven by the two earth system models. Areas of agreement between models (coefficient of variation <20 class="mb" span="span">%

) are indicated in red and orange. The data are filtered with a 5-degree running mean across the latitudinal averages.

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Figure 3: Change in individual-level maximum body size of fishes in different ocean basins from 2000 (averages of 1991–2010) to 2050 (averages of 2041–2060).

The thick black lines represent median values, the upper and lower boundaries of the box represents 75 and 25 percentiles and the vertical dotted lines represent upper and lower limits.

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Figure 4: Comparison of relationship between maximum body size ( ) and habitat temperature predicted from the growth model presented in this study (filled dots, solid line) and observations (open dots, broken line).

a, Maximum body weight for Atlantic cod (Gadus morhua) in the North Atlantic based on growth parameters estimated from body size-at-age data from populations in different locations in ref. 28, and b, maximum body weight for North Sea haddock (Melanogrammus aeglefinus) (based on growth parameters in ref. 11.) The slopes of the best fit lines from linear regression for both datasets are significant (p<0 .05=".05" are="are" body="body" both="both" cases="cases" changes.="changes." changes="changes" conservative="conservative" in="in" log="log" maximum="maximum" more="more" observed="observed" over="over" p="p" predicted="predicted" temperature="temperature" than="than" the="the" weight="weight">

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Main

• Main
• Methods
• References
• Acknowledgements
• Author information
• Supplementary information

Global climate and ocean changes resulting from anthropogenic greenhouse-gas emissions are currently affecting and expected to continue to affect marine organisms^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}. These impacts are fundamentally linked to the close relationship between ocean conditions and the ecophysiology of marine organisms, notably water-breathing ectotherms^{1, 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 organisms^{4, 5, 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 availability^{2, 14}. Specifically, the maximum body weight ( ) of marine fishes and invertebrates is fundamentally limited by the balance between energy demand and supply, where 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 fishes <sup>15

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