It is recognised worldwide that invasion of native biological systems by non-indigenous species may represent a threat to the integrity of these systems, the economy and even human health.

The most represented groups were molluscs

The environmental impact of invasive marine species may be so severe that the International Maritime Organisation of the UN (IMO) has identified the introduction of aliens as one of the four greatest threats to the world’s oceans.

Alien species may affect recipient ecosystems through predation, competition, contamination of the native gene pool by exotic genes (for example, through hybridisation), habitat modification, and through the introduction of new parasites and pathogens.

The high number of non-indigenous species in the Mediterranean Sea has been attributed to such human activities as seafaring, commerce and tourism, to modifications that make some habitats such as lagoons, estuaries and marinas particularly susceptible to invasion, to aquaculture, and to the opening of the Suez Canal, which has led to the introduction of hundreds of species from the Red Sea into the eastern Mediterranean, a phenomenon known as ‘Lessepsian migration’.

Over the past half century or so there has been a clear trend towards overall warming of the Mediterranean Sea, which seems to be favouring the introduction, establishment and range extension of warm-water alien species.

In 2008, the Mediterranean Science Commission (CIESM: Commission Internationale pour l’Exploration Scientifique de la Mer Méditerranée) launched a programme called ‘Tropical Signals’ with the aim of detecting, monitoring and understanding the effects of climate warming on Mediterranean marine biodiversity.

CIESM invited the Marine Ecology Research Group at the Department of Biology of the University of Malta to form part of this programme since the location of the Maltese Islands, close to the boundary between the western and eastern Mediterranean bio-regions, makes them an important station for monitoring the entry and spread of alien marine species in this sea.

The ‘Tropical Signals’ group is currently made up of 21 research teams from 15 countries, including Malta.

A first task carried out by the Marine Ecology Research Group was an inventory of non-indigenous species reported from Maltese waters, to act as a baseline for future comparisons.

Up to the start of the ‘Tropical Signals’ programme, 39 authenticated alien species and another nine unconfirmed ones had been recorded from Maltese waters. Of the accepted records, 25 had become established, that is, they are present as reproducing populations in the wild. The rest were either casual (only recorded once or twice and have not established breeding populations) or questionable (species for which insufficient information exists to decide if they are established or casual).

The most represented groups were molluscs (14 species), fish (13 species) and seaweeds (10 species). Six species (three seaweeds, a crab and two fish) were classified as invasive in Maltese waters, that is, species whose population has undergone a very rapid growth and which may affect the diversity or abundance of native species and the ecological stability of the ecosystem.

Since the start of the ‘Tropical Signals’ programme, additional non-indigenous species have been reported from Maltese waters by workers from the Marine Ecology Research Group and by others.

These include a sponge, twojellyfish, seven molluscs, a sea-squirt and five fish, bringing the number of non-indigenous species confirmed to have reached Maltese waters to at least 55; slightly more than half have become established.

Summing up what is known so far, it seems that a few species have been introduced deliberately, and many more have been accidentally introduced by human activities, and that for the latter there appears to be a correlation with increasing marine traffic to Malta and theCentral Mediterranean.

Other species, mainly warm-water Atlantic and Red Sea ones, first established populations in the western and eastern Mediterranean respectively, and then spread to the central Mediterranean, including the Maltese Islands, under their own steam or assisted by shipping.

The rate of occurrence of non-indigenous marine species in Maltese waters seems to be increasing with time, a phenomenon thatmay possibly be facilitated bythe general warming trend of Mediterranean surface water that has been ongoing for the pastfew decades.

The Marine Ecology Research Group welcomes reports from sea-users and others, of unusual or new marine organisms.

One can contact Patrick Schembri at the Department of Biology (tel: 2340 2272) or by e-mailing patrick.j.schembri@um.edu.mt.

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