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Mediterranean Sea Warming Causes Marine Species Go Deeper into Cooler Waters to Survive

A recent Tel Aviv University research has discovered that because of the temperature variance across the Mediterranean Sea, species of marine animals, such as fish, mollusks (such as squid) and crustaceans are switching their habitats to reside tens of meters deeper in cooler waters.

Mediterranean Sea Warming Causes Marine Species Go Deeper into Cooler Waters to Survive.
Marine Species. (Image Credit: Shahar Chaikin).

The revolutionary study was led by PhD student Shahar Chaikin under the direction of Prof. Jonathan Belmaker, and together with researchers Shahar Dubiner, all from the School of Zoology in the George S. Wise Faculty of Life Sciences and the Steinhardt Museum of Natural History at Tel Aviv University.

The study findings have been published in the journal Global Ecology and Biogeography, and have far-reaching effects for fishing as well as for future ocean nature reserves.

The team explains that while in the last few decades the whole planet has been warming, this process has been most prevalent in the Mediterranean Sea. The average water temperature in the Mediterranean increases by one degree every three decades, and the rate is only increasing.

One of the crucial questions that has been asked is how, if at all, the numerous species living in the Mediterranean will cope with this unexpected warming. In the last few years, evidence has been collected that certain species have deepened their habitats so as to cope with global warming, while other studies have discovered that species are restricted in their ability to go deeper into cooler water.

It should be remembered that the Mediterranean was hot in the first place, and now we are reaching the limit of many species’ capacity. 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.

Jonathan Belmaker, Professor, School of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University

In the study’s framework, the Tel Aviv University team performed a meta-analysis of data on the depth distribution of 236 ocean species gathered in earlier bottom-trawl surveys.

The data gathered showed for the first time that species deepen their minimum depth boundaries in parallel with warming seawater temperatures, from the west to the east Mediterranean, and on average deepen 55 m spanning the Mediterranean (a range of 60C).

However, the deepening pattern is not even between species: cold-water species were discovered to deepen considerably more than warm-water species, species that reside along a narrow depth range deepen less than species that reside along a wide depth gradient, and species that can survive within in a wider temperature range deepen more than those who can survive only within a narrow temperature range.

“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,” says Shahar Chaikin.

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.

Shahar Chaikin, Study Lead and PhD Student, School of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University

“The deepening effect was found to be more significant among cold-water species. In contrast, 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 added.

The results of the research have a number of suggestions for the future, in the Mediterranean and in general, provided that the response of each species to increasing temperatures can be estimated according to its traits, such as temperature preference. This, for the first time, gives scientists the chance to estimate changes in the composition of the marine community, as well as for the public the chance to get ready for these changes accordingly.

Our research clearly shows that species do respond to climate change by changing their depth distribution and when we think about the future, decision-makers will have to prepare in advance for the deepening of species.

Shahar Chaikin, Study Lead and PhD Student, School of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University

“For example, future marine nature reserves will need to be defined so that they can also provide shelter to species that have migrated to greater depths. And on the other hand, fishing in the future will involve fishing the same fish at greater depths, which means sailing further into the sea and burning more fuel,” Chaikin concluded.

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. We are already seeing deep-sea fish like cod whose numbers are declining, probably because they had nowhere deeper to go.

Jonathan Belmaker, Professor, School of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University

Journal Reference:

Chaikin, S., et al. (2021) Cold-water species deepen to escape warm water temperatures. Global Ecology and Biogeography.


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