This increased stratification causes deeper waters to receive less and less oxygen. Furthermore, the metabolism of marine organisms tends to increase with temperature. Consequently, as the oceans warm up, the organisms that inhabit them consume more oxygen. This effect can be huge along coasts, where runoff from fertilizers feeds algae blooms, which in turn feed oxygen-absorbing bacteria. Eutrophication creates more and more ‘dead zones’, including the notorious one in the northern Adriatic.
Some studies have speculated that microplastic pollution also has the potential to exacerbate the oxygen starvation problem. According to this theory, if zooplankton become filled with microplastics, instead of phytoplankton, the phytoplankton will proliferate, again feeding all those oxygen-devouring bacteria on their way to the seabed.
The result is a patchwork of areas that are either too hot or too low in oxygen for various fish to thrive, leading to different escape routes. A much cited example is the case of the south-eastern coast of China, recently colonized by a species called in the area “the Bombay duck” (commercial name in Italian: bumalo), a long and thin fish with a gelatinous consistency.
The boom of this species, which has very few vertebrates, depends on the lack of oxygen in these waters, which has made all the other fish flee, except these which, like jellyfish, need little oxygen to survive. In general, large fish have a higher metabolism and need more oxygen.
The seas of the future – warmer and hypoxic – will not only contain fewer types of fish, but also smaller, stunted fish and more bacteria, which unfortunately are producers of greenhouse gases. The tropics will empty as fish move to more oxygenated waters and many species face extinction.