KERKENNAH ISLANDS (Tunisia): A decade ago, Tunisian fisherman Ahmed Chelli’s nets bulged with fish and octopus that he sold at the local market in the Kerkennah islands. Today, he pulls up only “ISIS” the name locals have given to the blue crabs that have invaded their fishing grounds in the fast-warming waters of the Mediterranean.

“The fisherman, instead of finding fish to provide an income, he finds something that cuts his nets,” Chelli complained.

For more than 70 days this summer, a marine heatwave cooked the waters of the western Mediterranean.

It was the worst swelter for the western part of the basin in the last four decades, said marine ecologist Joaquim Garrabou at Spain’s Institute of Marine Sciences, who monitors temperature gauges in the sea’s near-coastal waters.

Temperatures climbed higher, and the heatwave lasted longer, than any other to hit the waters west of Sicily since record-keeping began in 1982, Garrabou said, based on preliminary findings from his analysis.

“We’ve been witnessing marine heatwaves during the last 20 years,” said Garrabou, who’s also coordinator of the T-MEDNet marine monitoring network. He and his colleagues have found that about half of the worst heatwaves on record in the entire basin have hit since 2015.

“Almost every year, some area of the Mediterranean suffers,” he said.

Measurements taken by European Space Agency satellites show that, from June through September, the waters off north Africa and southwest Europe were 2 to 5 degrees Celsius above the 1985-2005 daily averages. Temperatures peaked at nearly 31C in some parts.

By September, populations of sponges, sea-stars, fish, and mollusks were dying en masse in the waters off France and Spain. Corals bleached to bone white.

Around Tunisia, the underwater warmth encouraged reproduction among invasive species such as the blue crab, said Hamdi Hached, an environmental consultant in Tunis at the Friedrich Naumann Foundation for Freedom.

The crabs likely first arrived from the Indo-Pacific via ship ballast water, and were first documented in the Mediterranean in 1898. But, with the last decade of warming, the population has exploded - eating and outcompeting valuable native species.

With blue crab larvae thriving at water temperatures around 30C, there’s no end in sight.

Hached said the pincered crustacean’s “ferocity and the destructive ability” has inspired the caliphate-themed nickname “ISIS” by the fishermen of the Kerkennah islands — which lie about 20 km off the northern coast of Tunisia.

“It has a very large appetite to devour all the creatures around it, becoming a curse on the fishermen in the region.”

While tourism drives most of the sea’s economic activity, worth $450 billion in 2017 according to the World Wildlife Fund, there are millions who rely on the sea’s bounty for their livelihoods.

But as climate change makes the Mediterranean among the world’s fastest warming seas with temperatures rising about 20pc faster than the global ocean average that bounty is under threat.

The rapid warming is due partly to the fact that the Mediterranean is a relatively shallow and contained basin. With an area of about 2.5 million square kilometers (970,000 square miles), it is a “climate change hotspot because it’s a small sea,” Garrabou said.

There are few connections between the sea and the Atlantic Ocean to the west, so there are “not a lot of ways out for warm water,” he said. The overall water temperature is now 0.4C higher on average than it was 30 years ago, data show.

Acute marine heatwaves can form when warm air temperatures coincide with stable ocean conditions - when there is less mixing between the colder, deeper layers of water and the warmer surface layer.

This summer, southern Europe suffered blistering temperatures on land, which scientists said provided the perfect set-up for an ocean heatwave to unfold in the waters, as the ocean absorbs excess heat in the atmosphere.

Published in Dawn, November 14th, 2022