Project Manaia

Invasive Species and the Mediterranean Sea

The damage caused by invasive species

Invasive species are organisms that are not native to a particular area and habitat. They can be very harmful on both an economic and environmental level. In order to become invasive these organisms must adapt to their new environment quickly and reproduce in large numbers. 

If an invasive species is successful it can feed on native species until there are little or none left and outcompete native species for their prey food. They can destroy habitats by making changes to their new environment like eating a species or plant that helps create that habitat. Also, as they are in a new location, there may be no natural predators to hunt them, so their numbers become uncontrolled.

Puffer Fish - Torquigener Flavimaculosus

Invasive species can develop rapid adaptations to help them cope with the different environmental challenges in their new habitat. This could be more prolific breeding techniques or a greater tolerance to changes in water temperature. It could be a faster metabolism, which can lead to faster rates of development to maturity and consequently better protection against predators and more opportunity to breed. 

These aggressive breeding and feeding habits can force native species to be displaced and seek new habitat or to die out due to lack of food. This can lead to catastrophic changes to food webs and an overall reduction in community biodiversity.

How invasive species enter the Mediterranean

There are many factors that give invasive species the opportunity to enter the Mediterranean Sea and become established, but nearly all of them are a direct result of human intervention.

Boats travelling large distances across seas, or being trailered cross-country, can carry invasive species. Organisms can survive in ballast water and then be ejected into a new sea or cling to the hulls of boats and become dislodged during cleaning. Equipment like fishing gear, bait products, propellers, anchors, scuba-diving gear, and other types of fishing tools has also been involved in the transport of non-native fauna and flora.

The Suez Canal is an artificial sea level canal that took ten years to construct and was completed in 1869. It was designed as a short cut between the North Atlantic and the Indian Ocean, avoiding a much longer journey via the South Atlantic and around the Cape of Good Hope. Overtime it has been widened on a number of occasions, most recently in 2010 increasing its depth in order to allow vessels to pass with drafts of up to 66ft. There are no locks on the canal and sea water flows freely. This unnatural opening and connection between two unrelated seas as well as the huge amount of traffic that passes along the canal has created a huge rise in invasive species entering the Mediterranean.

Boats on the Suez Canal
Boats on the Suez Canal

A recent market-driven surge in the marine aquaculture of non-native species of exotic fish and shellfish has also been highlighted as an entry point for invasive species. Marine aquaculture is the breeding, rearing and harvesting of aquatic species. It is usually used for things like oysters, clams, mussels, shrimp and salmon and grew out of a decline in wild, free-ranging, fisheries. Due to the large numbers of sites it is adding to the creep of invasive species, but also to the unintentional introduction of associated pathogens and parasites that come with them.

Major shipping routes and sites of aquaculture in the Mediterranean Sea
Major shipping routes and sites of aquaculture in the Mediterranean Sea

Human intervention can also come in the form of pollution and the dumping of drainage water from breeding farm tanks and public aquariums or the direct release of unwanted pets into our water systems. 

And of course, climate change is the overriding factor. Our sea levels are rising, which means the habitats in coastal areas are changing as is the types of species that can survive there. These changes favour generalist species that thrive in a wide variety of environmental conditions, rather than specialist species with more specific environmental needs and survival tactics. This in turn leads to a decrease in bio-diversity. 

Also our oceans are warming and these conditions will favour the introduction of more species from the Red Sea into the Mediterranean and help them to spread north and west into Europe. Species that are native to the Mediterranean will be forced to move further towards the poles in the search of cooler water and less competition from these invasive species. 

We are now at a point where stopping the spread of invasive species is largely economically unviable and in some cases impossible. However good cleaning and management of equipment and following good sustainable sailing practices will help. Eating invasive species that have been caught in the local area can help to reduce their numbers and thinking carefully about how you dispose of waste water can also make a difference. You can also help us to monitor the creep of invasive species through the Mediterranean by recording any sitings on our report form.

Submit your sightings of invasive species of the Mediterranean

Writer – Caroline Anderson for Project Manaia, November 2022

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