Warming Mediterranean Causes Marine Species’ Cooler Water Migration

A recently-released Tel Aviv University (TAU) report has found that several marine species, including fish, mollusks and crustaceans have changed their habits to seek cooler waters – up to 55 meters across the climactic gradient of the Mediterranean Sea.

The study was led by PhD student Shahar Chaikin under the supervision of Prof. Jonathan Belmaker, and along 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 results of the study were published in the journal Global Ecology and Biogeography, and have far-reaching implications for both fishing and future marine nature reserves.

The researchers claim that while the entire planet has been warming over the last few decades, the process is particularly marked in the Mediterranean Sea. The average water temperature in the Mediterranean rises by one degree every thirty years, and the rate is only accelerating. Recent evidence has shown that some species in the sea have migrated to cooler waters as a way of attempting to adapt to global warming. Other studies have noted that this option is to all marine species that would like to deepen 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,” explains Belmaker. “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.”

The TAU researchers conducted a meta-analysis of data on the depth distribution of 236 marine species collected in previous bottom-trawl surveys. The data collected revealed for the first time that species deepen their minimum depth limits in parallel with warming seawater temperatures, from the west to the east Mediterranean, and on average deepen 55 meters across the Mediterranean (a range of 6⁰ C). The research showed that there was not uniform distribution of the deepening pattern; cold-water species were found to deepen significantly more than warm-water species, species that live along a narrow depth range deepen less than species that live along a wide depth gradient, and species that can function within in a wider temperature range deepen more than those who can function only within a narrow temperature range.

“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,” said Chaikin.

The study is important because it now allows researchers of the Mediterranean – and more generally – to feel increasingly confident in predicting each species’ likely response to rising temperatures, including their temperature preference.

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

A 2017 TAU study, revealed that foreign species crossing the Suez Canal were harming indigenous species and habitats in the Mediterranean Sea, damaging vital marine resources and raising concerns about human health. This latest research highlights the struggle that marine species continue to face in the Mediterranean.