Invasive species: do they really threaten the Mediterranean Sea and local fisheries?
Invasive species are becoming a global concern. Over the past 20 years, the number of non-native species in European waters has increased to nearly 1,300.
The problem is most acute in the Mediterranean Sea, which is home to 69% and although only 10% are classified 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 begins to spread in large numbers and cause significant ecological problems – such as competition with local species – or cause problems for people in terms of jobs and livelihoods.
How do they get to European waters
These species are introduced to new environments in several ways, 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 by being carried inside their ballast water – the water they use to balance the ship when it is not full of cargo. When ships release this water to load their cargo, any small organisms that were sucked in with the water at the start are released back into the new environment.
The second main driver of species invasion is aquaculture and imports. In the seas of Northern Europe, for example, researchers believe that around 46% of non-native species arrived via oyster imports through propagules – what an organism creates to reproduce – in the shells.
“Because oyster shells are so complex, you get all the propagules of a lot of other species in there, things like tunicates and seaweed. And so they started spreading and taking over countries like the France, Spain and the UK,” said Jason Hall-Spencer, professor of marine biology at Plymouth.
Why has the Mediterranean become a hotspot for non-native species?
“Hundreds and hundreds of invasive species have passed through the Suez Canal,” said Hall-Spencer, who has also worked in Cyprus to combat lionfish invasion in the Mediterranean.
The man-made waterway connects the Mediterranean Sea with the Red Sea in Egypt. It is an essential trade route between Europe and Asia as it cuts a ship’s journey by 9,000 km.
For 80 years, the Mediterranean Sea has been protected by a natural barrier created by the difference in salinity between the bitter lakes and Lake Timsah, and the fresh water of the Nile.
But then, in 2015, the channel 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, which which means “that they [the invasive species] can easily cross the canal,” Hall-Spencer explained.
Now that this natural barrier has fallen, more and more species are crossing, although the Mediterranean Sea and the Red Sea have very different ecosystems.
How do these newcomers survive in Mediterranean waters?
The Mediterranean region is warming 20% faster than the global average and sea temperatures are expected to rise by 3.5 degrees Celsius over the next 70 years, according to a report by the United Nations’ Mediterranean Action Program for the Environment (UNEP/MAP) produced by Plan Bleu, UNEP/MAP Regional Activity Centre.
“The Eastern Mediterranean is one of the fastest warming places in the world due to climate change. Thus, the conditions in the eastern Mediterranean, for example around Cyprus or Lebanon, are perfect for many Red Sea species. And so we see them invading and doing very well,” Hall-Spencer said.
“So it’s a combined effect. The channel is the source of the problem, but secondarily the fact that the water temperature is approaching the Red Sea,” he added.
But climate change and trade are not the only reasons why non-native species thrive in the Mediterranean. Overfishing is also a big problem because it kills their potential predators.
“There is an acute problem of biodiversity [in the Mediterranean Sea] in pretty much all the big fish hunted and killed by humans. And that skewed the whole system toward small organisms,” Hall-Spencer said.
Are all invasive species dangerous?
Not all invasive species are a threat, but some ‘are voracious predators’
Some species can be quite harmless to the environment, such as the Pacific oyster. However, others can have disastrous effects – not only on biodiversity but also on 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 example, is highly toxic to humans and cannot be eaten. They are aggressive predators that have even been known to destroy fishing nets to catch the catch inside.
And if they are caught in a net with other fish, they will attack and poison them, spoiling the whole catch.
The lionfish is also a great predator but it has an economic advantage over the pufferfish: it constitutes a tasty meal.
Hall-Spencer worked on a European Union-funded project called RELIONMED to find ways to reduce the size of its population in Cyprus.
As part of the project, the group organized teams of divers to hunt fish.
“We were able to catch hundreds of them,” Jason said. “We were able to get rid of most lionfish in marine protected areas [in Cyprus], then over a period of months they started to come back. So we monitored the situation and then we deleted them again.
As part of the project, they also created incentives to develop a market for fish in Cyprus.
“The lionfish actually tastes absolutely delicious,” says Jason. “And so there is 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 tricky balancing act because you run the risk of making invasive species attractive.
“If these species are so commercially successful, there will be incentives not to overfish and to protect stocks and help them build,” Hall-Spencer said.
What are we doing to solve the problem?
In 2015, the European Commission launched the Invasive Alien Species Regulation.
The EEE 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 fight against species invasions.
He has also joined international efforts to protect 30% of the world’s oceans.
to help replenish stocks of large fish and predators.
Globally, in 2017, the International Maritime Organization established a convention that regulates how and where cargo ships discharge the water they carry into their tanks.
However, local and international efforts may not be enough if the root of the problem is not addressed.
As long as the natural barrier of the Suez Canal is broken, invasive species will continue to enter the Mediterranean.
“What’s needed is to re-establish an area of high salinity and that can be done using desalination plants,” Hall-Spencer said.
“Egypt has quite arid areas so it needs to produce fresh water using the ocean and the by-product of that is very salty brine and that could be pumped in and out of the canal areas from Suez 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 invasion, then all of these measures won’t work,” he said.