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Invasive species: Are they really threatening the Mediterranean Sea and local fisheries?

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The lionfish is the fastest invasive species to spread in the Mediterranean Sea.
The lionfish is the fastest invasive species to spread in the Mediterranean Sea.
By Marie Lecoq

Invasive species are becoming a global concern. In the last 20 years, the number of non-native species in European waters has increased to almost 1,300.

The issue is most acute in the Mediterranean Sea, which is home to 69 per cent of them and although only 10 per cent are categorised as invasive.

Species such as the Atlantic blue crab in Catalonia or the lionfish in Cyprus can pose a major threat to local marine life.

What are invasive species?

A species becomes invasive if it starts to spread in large numbers and cause significant ecological problems - such as outcompeting the local species - or cause problems for people in terms of jobs and livelihoods.

How do they get to European waters

There are a number of ways these species get introduced into new environments, but the main reason is human activity.

Researchers believe the main global culprits are trade and commercial vessels.

Small organisms can be brought to new seas either by attaching themselves to ships or being carried inside their ballast water - the water they use to balance the vessel when it is not full of cargo. When the boats release this water to load their cargo, all the small organisms that were sucked in with the water at the beginning get released into the new environment.

The second main factor of species invasion is aquaculture and imports. In Northern European seas, for example, researchers believe that around 46 per cent of non-native species arrived via oyster culture imports through propagules - what an organism creates to reproduce itself - in the shells.

“Because the shells of the oysters are so intricate, you get all of the propagules of lots of other species in there, things like tunicates and seaweeds. And so they've started to spread and invade countries like France, Spain and the UK,” said Jason Hall-Spencer, a professor of marine biology in Plymouth.

Why has the Mediterranean become such a hotspot for non-native species?

“Hundreds and hundreds of invasive species have come through the Suez Canal”, explained Hall-Spencer, who has also worked in Cyprus to combat the invasion of lionfish in the Mediterranean.

The man-made waterway connects the Mediterranean Sea to the Red Sea in Egypt. It is an essential trade route between Europe and Asia because it reduces a ship’s journey by 9,000 km.

For 80 years, the Mediterranean Sea was protected by a natural barrier created by the difference in salinity between the Bitter Lakes and Lake Timsah, and the freshwater from the Nile.

But then in 2015, the canal was widened and deepened which meant that what was used to block the spread of these invasive species, which was a high salinity area that would kill most organisms, was removed, meaning “they [the invasive species] can easily pass through the canal,” explained Hall-Spencer.

Now that this natural barrier is down, more and more species are crossing over, despite the Mediterranean Sea and the Red Sea having very different ecosystems.

How do these newcomers survive in Mediterranean waters?

The Mediterranean region is warming 20 per cent faster than the global average with sea temperature expected to rise by 3.5 degrees celsius in the next 70 years, according to a United Nations’ United Nations Environment Programme Mediterranean Action Plan (UNEP/MAP) report produced by Plan Bleu, a UNEP/MAP regional activity centre.

The Eastern Mediterranean is one of the places in the world that is warming fastest due to climate change.
Jason Hall-Spencer
Professor of Marine Biology, Plymouth

“The Eastern Mediterranean is one of the places in the world that is warming fastest due to climate change. And so the conditions that are in the Eastern Mediterranean Sea, for example, around Cyprus or Lebanon, are perfect for many Red Sea species. And so we're seeing them invade and do really well,” Hall-Spencer said.

“So it's a combined effect. The canal is the source of the problem, but secondarily is the fact that the temperature of the water is getting more akin to the Red Sea,” he added.

But climate change and trade are not the only reasons non-native species thrive in the Mediterranean. Overfishing is also a big issue as it is killing off their potential predators.

“There's an acute biodiversity problem [in the Mediterranean Sea] in all of the big fish, pretty much, being hunted and killed by humans. And it skewed the whole system towards small organisms,” said Hall-Spencer.

Are all invasive species dangerous?

Not all invasive species are a threat but some “are voracious predators”

Some species can be pretty harmless to the environment, such as the Pacific oyster. However, others can have disastrous effects - not only on biodiversity but also for local economies.

“Fish are a particular problem in the Mediterranean because obviously fish eat other fish and invertebrates and some of them are voracious predators,” Hall-Spencer said.

The Red Sea pufferfish, for instance, is highly poisonous for humans and cannot be eaten. They are aggressive predators that have even been known to destroy fishing nets to get at the catch inside.

And if they are caught in a net with other fishes, they will attack and poison them, spoiling the entire catch.

Copyright: Guillaume Comparat
The lionfish arrived from the Red Sea through the Suez canalCopyright: Guillaume Comparat

The lionfish is also a great predator but it has one economic advantage over the pufferfish: it makes for a tasty meal.

Hall-Spencer worked on a project funded by the European Union called RELIONMED to find ways to reduce its population size in Cyprus.

As part of the project, the group organised teams of divers to hunt the fish.

“We were able to catch hundreds,” Jason said. “We were able to get rid of most of the lionfish in Marine Protected Areas [in Cyprus], and then over a period of months, they started to arrive back. So we monitored the situation and then we would remove them again.”

As part of the project, they have also created incentives to grow a market for the fish in Cyprus.

“The lionfish actually tastes absolutely delicious,” explains Jason. “And so there is a hope that by fishing it or even overfishing it, people around the Mediterranean could reduce its population size.”

But getting people to see the benefits of eating lionfish is a delicate balancing act as you run the risk of making invasive species attractive.

“If these species become so commercially successful, there will be incentives not to overfish and to protect the stocks and help them build,” Hall-Spencer said.

What is being done to address the issue?

In 2015, the European Commission launched the Invasive Alien Species Regulation.

The IAS Regulation provides a list of all non-native species that are restricted in the EU - these restrictions include keeping, importing, selling, breeding and growing. It also provides a set of measures to prevent and tackle species invasions.

It has also joined international efforts to protect 30 per cent of the world’s ocean

to help rebuild the stocks of large fish and predators.

At a global level, in 2017 the International Maritime Organisation implemented a convention that regulates how and where cargo boats release the water they carry in their tanks.

However local and international efforts might not be enough if the root of the problem is not addressed.

As long as the Suez Canal’s natural barrier is broken, invasive species will continue to make their way into the Mediterranean.

“What's needed is the reinstatement of a high salinity area and that can be done using desalination plants,” Hall-Spencer said.

“Egypt has got areas that are quite arid, so it needs to make fresh water using the ocean and the by-product of that is very salty brine and that could be pumped into and into areas of the Suez Canal to raise the salinity to such a high level that would prevent the spread of the invasive,” he added.

“Unless you turn off that tap, the Suez Canal tap of the invasion, then all of these measures won't work,” he said.

Video editor • Marie Lecoq