Global heating is killing the Mediterranean

01 July 2021

Coral and mollusks are disappearing and fishers venture ever further from the shore. Heatwaves and increased temperatures create imbalances in the marine ecosystem, reducing biodiversity.

The increase in temperatures weighs heavily on these latitudes. We’ve all experienced it: on land we suffer the heat, searching for shade in the hottest hours, resorting to fans and air conditioners to find relief from the heat.

In the waters of the seas and oceans the temperature is increasing too: and those who live there suffer just as much as we do, if not more. The consequences of global heating were discussed in the webinar Climate Change in the Mediterranean as part of Slow Fish 2021.

Goodbye, fish

Una cosa è certa, e ce la racconta Federico Betti, istruttore subacqueo, dottore di ricerca in biologia ed ecologia marina, zoologo marino e docente del corso di ittiologia presso Università di Genova: «Se la temperatura sale, come sta accadendo, alcune specie scompaiono. Il loro posto verrà preso da altri organismi più adatti a vivere nelle mutate condizioni». Il mare, in altre parole, può adeguarsi: «Siamo noi, gli esseri umani, a non saperci adattare a questi cambiamenti – prosegue Betti -. Il problema, infatti, è che storicamente l’uomo utilizza l’ambiente marino per una serie di attività come la pesca, l’acquacoltura, lo sport subacqueo, la nautica, e anche semplicemente per tuffarsi e fare un bagno: tutte queste attività sono state possibili in un mare di un certo tipo. Se il mare cambia, cambieranno le attività che sarà possibile fare». 

The effects on fishing, for example, are clear enough. Lorenzo Dasso, architect, fisher and restaurateur of the Slow Food Cooks’ Alliance, shared his experience. He goes out himself with his own boat to fish for his own restaurant’s menu: “I’ve got 40 years of experience fishing, and there’s no doubt that over the last ten, twenty years the ecosystem has changed. Until recently you could still find shellfish near the shore: mussels, limpets, even oysters. Today the coastal cliffs are becoming a desert.”

Global heating is killing the Mediterranean

Effects on fishing

To catch fish now boats travel further from the shore, to reach areas where the temperatures are lower. “The white mullet used to live from 20 to 40 meters from the coast, now we find them at over 70 meters away. The stargazer was found at 20 to 25 meters, and now from 35 to 70. The Rajiformes were once found at 25-30 meters from the coast, nowadays from 40 to 60 meters. The pink shrimp, which we used to go up to 200 meters to find, is now caught from 300 to 400 meters out. And the red shrimp has ventured even further out. Once we found it at 400-500 meters from the shore, now, over 700.”

The quantity caught, Dasso continues, “is around the same as ten years ago, but we have to make twice as much effort: this says a lot about the health of the sea.” Those who fish don’t need thermometers to know how much the temperature has risen. “Ten years ago a block of ice to conserve fish on the boats lasted five hour, today it lasts three.”

Once upon a time there was coral

“In the last twenty or thirty years, the waters around the coast of the Ligurian Sea have seen a temperature increase of one or two degrees,” explains Federico Betti. “Which might not seem much, but in reality marine ecosystems have changed completely, and the effects are very serious. For over twenty years we’ve been seeing mass die-offs, the most serious in 1999 and 2003. The long and powerful heatwaves have affected deep-water life like for example soft corals, which are important because the shelter lots of juvenile fish, as well as sponges and mollusks; the Noah’s Ark shell, and recently even the fan mussel, a mollusk endemic to the Mediterranean, has become an extinction risk because of an protozoic infection which, presumably, has been facilitated by the increased temperatures.”

Global heating is killing the Mediterranean

Phenomena of this type, which happen underwater and are therefore all but unknown to the general public, have serious consequences: “We see the homogenization of the seafloor, and it means that are ever less arborescent and sponge species in our seas: a reduction in biodiversity that causes imbalances in the ecosystem and gives rise to new and unusual interactions between organisms.” In other words, the ecosystem is simplified. “When this happens it’s never a positive sign, because it means the environment is less resilient and less resistant, thus more fragile.”

What happens five-thousand meters under the sea?

The Mediterranean Sea, off the coast of the Peloponnese, is up to five-thousand meters deep. An immense and mysterious space, mostly unknown. “Of the pelagic zone, in the open sea, we know little or nothing,” admits Maurizio Würtz, Professor of the Department of Biology at the University of Genoa. “We know something of the first fifty meters, and we know something of the sea floor around the coast, but there remains a great question mark regarding animal and vegetal biomass and the movement of the waters between 50 and 5000 meters. What we know is that the Mediterranean a thermohaline machine, which changes according to the temperature and salinity levels.”


The waters of the Mediterranean are never less warm than 12°C, a unique case amongst the world’s seas. This is because of the low depth of the Gibraltar Strait – around 300 meters – which separates it from the much colder Atlantic Ocean. “This characteristic means that in the Mediterranean and in particular in the Ligurian Sea, there’s a frequent mixing of waters of the deep and the surface, which are cooled by the wind. This mixing allows for a sort of fertilization of the surface waters and creates favorable conditions for the entire food chain.”

The increase in water temperatures, however, risks breaking the equilibrium. “Our worry,” continues Würtz, “is that the surface waters heat up so much as to block the vertical water flows, and then there’s no more mixing with the deeper waters, thus preventing the regeneration of fish stocks which sustain the large predators like the tuna.”

What can we do? Action is clearly needed to mitigate the effects of the climate crisis, as is political action to “safeguard the ecological functionality and the processes the Mediterranean a sea rich in biodiversity, despite its reduced size compared to great oceans,” Würtz concludes.