published on 29 April 2013 in ecosystems
Intruders in the Mediterranean
The Mediterranean Sea (Mare Nostrum, Latin for “Our Sea”) has always been considered of fundamental importance for the economy and the cultural exchange between populations, connecting Italy to the Middle East and Africa. The Mediterranean area is considered the cradle of European culture, in fact many of the main ancient civilisations, like the Egyptians, the Greeks, the Phoenicians and Romans, developed here. Still today about a third of all international cargo traffic passes through this region. Fishing, aquaculture and many commercial activities are an important source of income for Mediterranean countries. Even tourism is an important source of wealth, due to the singular naturalistic and cultural value of this area. The Mediterranean sea is a semi-closed basin with an average depth of 1500 m and a maximum depth of 5267 m. The Mediterranean accounts for only 0.7% of the Earth’s ocean surfaces, but it is estimated that 10,000 to 12,000 animal and plant species, equivalent to about 10% of the total species that inhabit the seas of the planet, live here.
Recently there have been profound changes in the Mediterranean fauna and flora due to climate change and the degradation of ecosystems, which are becoming more and more vulnerable. In the last decades, an increasing number of non-native species has been found in the Mediterranean and theories about the tropicalisation of the Mediteranean Sea have started to circulate and been given ample space on the media, at times accompanied by unjustified scaremongering. Tropicalisation is the process by which species from tropical and sub-tropical areas settle down in the Mediterranean though they were not native to these seas. The tropical species that have arrived in the Mediterranean mainly come from the eastern Atlantic, through the Strait of Gibralter and from the Nile Basin and the Red Sea, via the Suez Canal. In the latter case, the migrations are called lessepsian after the French engineer Ferdinand-Marie de Lesseps, who supervised the construction of the canal. There are also many species that have been introduced intentionally or unintentionally, especially through bilge water (the bilge is the lowest part of the hull of a vessel, it collects wastewater and infiltrations) and ballast water which are discharged into the sea without the necessary precautions.
A dam modifies the Mediterranean
In 1869 the Suez Canal was opened. In the first years after the opening, the migration of species was limited. This phenomenon can be explained in different ways: one of these could be that the high salinity of the Bitter Lakes of the Suez Canal prevented marine species from reaching the Mediterranean. However, the salinity then decreased due to increased marine traffic which disturbs the waters. Another reason could be that a sort of natural barrier had formed, created by fresh water carried by the Nile, whose estuary is near to the outlet of the Suez Canal. However, after the construction of the Aswan Dam, which reduced the flow of the Nile and consequently the supply of fresh water, this barrier was interrupted, allowing alien species to colonise our sea.
Aliens among us
About 55 species have entered the Mediterranean through the Suez Canal, they have adapted to the new conditions and have started proliferating. The rapid adaptation of these species is due to two main factors: the first is that in the Red Sea there are about 1500 species with greater competition between each other respect to the 600 species present in the Mediterranean; the second is that intensive fishing in the Mediterranean, pollution caused by heavy metals and pesticides, the rise in average temperature, the reduction of the Posidonia beds have created imbalances within marine flora and fauna and have left an environmental void. About thirty species have come from the West African coast and from the Atlantic, often attracted by food left-overs thrown out from ships.
Some of the fish species that have been found in the Mediterranean are listed below.
Plotosus lineatus: common name, Striped eel catfish, has settled down along the eastern coasts. The spines of the first dorsal and the pectoral fins are poisonous.
Fistularia commersonii: common name Smooth Flutemouth, its long, slender body makes it very similar to the Trumpetfish; present from Sicily to Sardinia, recently it has also been caught along the coast of the Argentario.
Sargocentron rubrum: common name Mediterranean squirrelfish, has spread in the south-eastern part of the Mediterranean between eastern Libya and the southern coast of Turkey. It is not present in Italian waters.
Leiognathus klunzingeri: common name pony fish, has spread in the south-eastern basin and has formed dense local populations; in Italy this species can be found in Sicilian waters.
Upeneus moluccensis and Upeneus pori, two mullet species that have spread in such an aggressive way that they have displaced the indigenous species.
Lessepsian species have colonised the eastern Mediterranean very rapidly due to the faunistic poverty of the area caused by bio-geographic changes, which have left a number of ecological niches available. Only a small percentage of these species has spread in the western part of our sea because it is more populated by Atlantic and Mediterranean species.
Giant clams and killer algae
Among the tropical species that have started populating the Mediterranean, there are also invertebrates that were introduced intentionally, such as the giant clam of the Philippines (Ruditapes philippinarum), which is native to the Pacific. In 1983, it was introduced for commercial reasons into the Venice Lagoon, where due to the high trophic level, the particular sediment and the movement of the waters as a result of the tides, it found a suitable habitat and proliferated naturally, giving rise to vast populations.
Even among plants there are some cases of tropicalisation, such as the marine seaweed, Caulerpa taxifolia, also known with the rather striking name of “killer alga” because it infests and is harmful to the Posidonia prairies, the green lung of the Mediterranean. It is hypothesised that the seaweed was accidentally released into the Mediterranean from the tanks of the Oceanographic Museum of Monaco in 1984. The speed of propagation of this seaweed is amazing and in just ten years, from the coast of Monte Carlo it has spread all along the Italian coast.
Species on the move
The term “tropicalisation” of the Mediterranean must not be confused with the tendency of some organisms that live in the warmer waters of the southern coasts of the Mediterranean to expand their territory towards more temperate areas where they were absent.
The latter phenomenon has been often attributed to global warming and indeed in the last twenty years there has been an increase in the migration of warm-water organisms to the cooler, northern areas of the Italian seas. However, scientists are still studying whether there is a direct connection, because many other species that live in warm waters, whose environmental requirements are similar to those that have expanded their territory, have not behaved in the same way.
An example of a species that has moved north in these last years is Thalassoma pavo, a phenomenon visible to any snorkeler in Italian seas. In fact, this well-known fish, whose common name is ornate wrasse, is one of the most common fish found at a depth of a few metres and one of the most colourful inhabitants our seas. Until about fifteen years ago, it could only be found in the extreme southern waters of Italy and was common only in Lampedusa and in the Pelagie islands; nowadays it can frequently be seen right up to the Ligurian Sea (but not in the northern part of the Adriatic).
Another example is Sparisoma cretense, or parrotfish, the only Mediterranean species of parrotfish, rather flashy with its bright red colour with green patches and teeth fused together to form a sort of beak, that until recently could only be found in Sicilian and Calabrian waters but now is rather common in the Tuscan Archipelago.
What we can do
For many years now, some random Italian ports have been monitored, such as those of Genoa, Naples and Palermo, in order to study how the new alien species affect our Mediterranean ecosystem, but nothing tangible has been done yet to prevent this phenomenon. A possible solution could be the sterilisation of ballast water and a reclamation policy for areas dominated by very intense maritime traffic.
Edited by Tiziana Bosco