William Cheung, University of British Colombia, Canada, and colleagues employ a model to examine the integrated biological responses to climatic changes of over 600 species of marine fish due to changes in distribution, abundance, and body size. The climate scenario used in the study would mean an increase in world temperatures of between 2 and 5.4 °C (3.6 to 9.7 F) by 2100, the second biggest gain of six scenarios used by the U.N. panel of climate experts.
As water gets warmer, it also gets lighter, limiting the mixing of oxygen from the surface layers towards the colder, denser layers where many fish live. Oxygen is one of the key ingredients for body growth of fish. Their assemblage-averaged maximum body weight is expected to shrink by 14–24 % globally from 2000 to 2050. Average maximum sizes of fish in the Indian Ocean, which has the most tropical waters, are likely to shrink most, by 24 %. This is followed by a decline of 20 % in the Atlantic and 14 % in the Pacific.
Fish stocks are likely to shift from the tropics towards cooler seas to the north and south. Ranges for most fish populations are said to shift towards the poles at a median rate of 27.5 to 36.4 km (17.1–22.6 miles) a decade from 2000 to 2050.
The results provide a new dimension to understanding the integrated impacts of climate change on marine ecosystems. The effects of the fish shrinking on marine food chains is still unclear, but believed to be large with, e.g., cod only being able to eat fish that fit into their mouth.
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