In an interglacial period some 800,000 to 700,000 years ago, temperatures rose by about four degrees Celsius, causing fish in the dimly lit twilight zones of the oceans to reduce their body size by as much as 35 percent. The oceans have probably absorbed less CO2 as a result.
Several studies have already predicted that the current global warming will cause fish living in the light-flooded top 200 meters of the sea to become smaller. By the year 2050, a decrease in size of 14 to 24 percent compared to the early 2000s was predicted. The reason for this is, among other things, the lower oxygen content of warmer waters, which at the same time reduces the oxygen demand of the fish at a depth of 200 to 1000 meters increases.
Fossils analyzed for study
“Until now, however, there have been hardly any studies that have addressed the effects of global warming on the deeper ocean layers, the twilight or the mesopelagic zone,” explains Konstantina Agiadi from the Institute of Paleontology at the University of Vienna in a press. release. Using fossils from the middle Pleistocene, when cold and warm periods alternated relatively quickly, she studied how the body size of fish evolved during that time.
To do this, she analyzed so-called otoliths from two ice ages and one interglacial period. These hearing stones from the inner ear of bony fish enable the animals to perceive sound and balance and are usually preserved in sedimentary rock. Otoliths are a few millimeters to a few centimeters in size. Their size tells you the size of the fish they’re from, and their shape tells you the species of fish. This allows the researchers to use the ear stones to reconstruct past fish faunas.
Samples taken from Rhodes
For their study, which the Viennese researchers published in the journal “Proceedings B” of the Royal Society, the research team, led by Agiadi, analyzed fish otoliths from sedimentary formations on the island of Rhodes, Greece, that date from 800,000 to 700,000 years ago. . It found that during the interglacial period, when global temperatures rose by four degrees Celsius, the average size of fish stocks in the twilight zone of the eastern Mediterranean declined by 35 percent. The individual species generally did not follow this trend, but the species composition shifted towards a higher frequency of smaller species.
Fish from this part of the ocean play a central role – as the scientists illustrate with the help of so-called lantern fish: this group of small fish gets its name from luminescent organs along the body and on the head that emit blue, green or yellow light. Although small, this family of fish makes up more than half of the fish biomass in the deep sea. The biomass of twilight zone fish is estimated to be between two and about 20 billion tons, which is 100 times the mass of fish caught annually worldwide.
Current global warming may also have an effect
Not only with this enormous mass, but also because of their way of life, the lanternfish in particular “make an important contribution to the so-called biological carbon pump – a natural mechanism for reducing carbon dioxide in the atmosphere,” according to Agiadi. Because the fish migrate vertically – controlled by daylight: At night – protected from the darkness – they swim hundreds of meters to the sea surface, where they eat plant plankton that has absorbed CO2 from the atmosphere through photosynthesis during the day. When the fish then migrate back down to avoid daylight, they transport the carbon absorbed with the plankton from the surface to the deep sea.
For this reason, co-author Martin Zuschin, head of the Institute of Paleontology, expects that the shrinking of twilight zone fish due to current global warming will negatively affect the ability of the oceans to absorb carbon dioxide from the ocean in the foreseeable future. absorb atmosphere. future.
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