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As Mediterranean heats up, fish diving deeper to find colder water, study finds

By Alexandra Mae Jones

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    TORONTO (CTV Network) — As the Mediterranean Sea warms, some marine species are responding by diving deeper to find cooler waters, but not all species are able to endlessly go deeper, according to a new study.

Researchers at Tel Aviv University looked at the depth records for different marine species across the past few decades as the ocean has been warming and found that many species appear to be deepening their habitats.

The study, which looked at more than 200 species living in the Mediterranean, was published in the journal Global Ecology and Biogeography in October.

The Mediterranean is one of the places on the planet where the globe’s general warming has been more pronounced, a news release about the study explained, with the average water temperature rising by one degree every 30 years. While this may not sound like a lot, for many marine species, a shift in temperature by a few degrees can make a big difference in survival.

“It should be remembered that the Mediterranean was hot in the first place, and now we are reaching the limit of many species’ capacity,” Jonathan Belmaker from Tel Aviv University said in the release. “Moreover, the temperature range in the Mediterranean is extreme – cold in the northwest and very hot in the southeast. Both of these factors make the Mediterranean an ideal test case for species’ adaptation to global warming.”

It’s a common theory that in response to warming ocean temperatures, species will attempt to relocate to either a completely different location or dive deeper, but according to the new study, not much is known about how common it is for species to relocate deeper.

Researchers looked at depth records for 236 marine species that had been collected in bottom-trawl surveys conducted within the last few decades. Of those species, 162 were fish, 40 were a type of crustacean and 34 were cephalopods.

“Various studies collect fishing data from trawling – that is, a boat that drags a net along the seabed and collects various species – and these studies often also measure the depth at which the species were caught in the net,” Shahar Chaikin, first author of the study, said in the release.

“We cross-referenced these data with water temperature data, and by analyzing 236 different species we came to a broad and compelling conclusion: there has been a deepening of the depth limits of species’ habitats. The minimum depths for species in the Mediterranean are getting deeper, while the maximum depths remain stable.”

They also found that species already considered cold-water species were able to dive much deeper, while those used to more temperate waters were only increasing their depth slightly.

“There are species that function within a narrow temperature range and at a certain depth that deepen much less, probably because they cannot survive in deeper water,” Chaikin explained.

Researchers pointed out that if this trend continues, it has implications for not only fishing industries, but marine reserves and marine biodiversity.

“Even if species deepen to escape the warm waters and this rapid adaptation helps them, there is still a limit – and that limit is the seabed,” Belmaker said. “We are already seeing deep-sea fish like cod whose numbers are declining, probably because they had nowhere deeper to go.”

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Sonja Puzic

Article Topic Follows: CNN - Regional

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