Deep ocean fishes will get smaller as the water warms

A new study led by the University of Vienna has found that fishes living in the deeper parts of the oceans (below 200 meters depth in the water column) would most likely decrease in size as the climate continues to warm. According to the experts, this change may have major ecological effects, impacting a variety of marine ecosystems.

The scientists retrieved fish otoliths – small stones located in the inner ear of bony fishes which facilitates their sound and balance perception – from sedimentary formations dating between 800 and 700 thousand years ago from the island of Rhodes in the Aegean Sea. The morphology of these body structures is particular to each fish species, and their size directly reflects the size of the individual fish they come from, thus allowing researchers to get an idea of fish sizes in past fish faunas.

“Thanks to the otolith analysis we have found that fishes during the interglacial period were smaller in size by 35 percent, when the global temperature had increased by 4°C, which could happen again nowadays due to the ocean warming,” explained study lead author Konstantina Agiadi, a geoscientist and paleontologist at the University of Vienna.

This study is one of the first to address the consequences of climate change on the deeper parts of the oceans – the so-called “mesopelagic zone” (200 to 1000 meters deep). The researchers focused on changes in lanternfishes, a group of tiny mesopelagic fishes named for their capacity to produce their own light. 

Currently, lanternfishes are major contributors to the biological carbon pump, a natural mechanism reducing atmospheric carbon dioxide concentrations. After phytoplanktonic organisms absorb CO2 from the atmosphere through photosynthesis, each night lanternfishes travel hundreds of meters upward to the ocean surface and return to the mesopelagic zone, bringing massive amounts of carbon from the surface to the depths of the ocean. Moreover, these fishes make up more than half the fish biomass in the deep sea, and over 100 times more than the total global annual fishery catches.

“Knowing the response of these organisms to ocean warming is key, since they contribute to ecosystem stability, reduce atmospheric carbon dioxide, and are a huge food resource for other organisms in the marine food web,” concluded study co-author Marta Coll, a researcher at the Institut de Ciències del Mar (ICM-CSIC) in Barcelona.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.

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By Andrei Ionescu, Earth.com Staff Writer

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