Discoveries of new species in Maltese waters often turn out to be records of non-native (or ‘alien’) species, that is, species that do not occur naturally in local waters but were directly or indirectly introduced here through human activities.

Colonisation by non-native species very often represents a threat to indigenous marine life as alien species can compete directly or indirectly with native biota, may modify habitats, or lead to the introduction of new parasites and pathogens.

Alien species can also impact the economy or even human health: little wonder therefore that reports of new pest seaweeds, poisonous fish or venomous jellyfish attract public attention.

Documenting the arrival of such newcomers is an important scientific endeavour, and the Marine Ecology Research Group (MERG) at the University’s Department of Bio­logy has been at the forefront of this work for more than a decade.

However, recording new species is only part of the story. For instance, some alien species have only been spotted once or twice and never sighted again (these are referred to as ‘casual’ species), and therefore do not represent a major ecological threat. Other species now occur as reproducing and self-perpetuating populations in the wild (‘established’ species), and some of these have undergone a very rapid population growth to the extent that they affect the diversity or abundance of native species and the ecological stability of the ecosystem (‘invasive’ species).

The latter are clearly of greater concern. To monitor changes in the status of non-native species in Maltese waters, MERG research­ers have been undertaking regular biological surveys.

For management purposes, it is necessary to go beyond records and studies of individual species and to look at the broader picture. How many non-native species occur in Maltese waters? Which plant or animal groups do they belong to? Have they managed to establish themselves? How are they getting here in the first place?

To answer these questions, Julian Evans, Jacqueline Barbara and Patrick Schembri have undertaken an extensive survey of the scientific and other literature recording the presence of new marine species in Maltese waters. The data was then combined with other authenticated but unpublished reports of such species, including sightings made during the ongoing biological surveys by the MERG team.

The blue-spotted cornetfish (Fistularia commersonii) is an Indo-Pacific species that entered the Mediterranean through the Suez Canal in 2000 and reached Malta in 2005. This specimen is some 80cm long and was photographed in Gozo. Photo: Julian Evans

The rate of new records is at an all-time high

From this review it resulted that by the end of last year, 61 authenticated alien species and another five unconfirmed ones had been recorded in Maltese waters. Analysis of the known or probable way these species arrived indicated that most common means of introduction is through boating and shipping. Other species were first introduced elsewhere in the Mediterranean and then managed to spread to Maltese islands under their own steam.

Thirty of these records were made since the turn of the century, clearly indicating that the rate of new records is at an all-time high. This is likely due to the present day warming trend of Mediterranean surface water, which favours the occurrence, establishment and range extension of warm-water species in the central Mediterranean.

In fact, the MERG researchers have also documented another phenomenon – the spread of Atlantic warm-water species to the central Mediterranean – which is almost certainly related to this warming trend. To date, seven such species have been recorded in Maltese waters, so the total number of new species (aliens and Atlantic range extenders) now stands at 73 species.

Overall, the most represented groups were molluscs (21 species), fish (15 species), crustaceans (eight species) and red algae (seven species). More than half of the newcomers (38 species) have established breeding populations, while a further eight species are considered to be invasive.

These species are the seaweeds Lophocladia lallemandii, Womersleyella setacea and Caulerpa cylindracea, the bivalve Brachidontes pharaonis, the crab Percnon gibbesi, and the fish Fistularia commersonii, Siganus luridus and Sphoeroides pachygaster. The latter species, a pufferfish, is particularly interesting because it is one of the Atlantic species that have extended their range to reach the central Mediterranean independent of any human involvement, and is therefore not considered to be an alien species.

Although recognition of the threats posed by invasive species has resulted in the inclusion of management of such species in a number of recent policy actions, including local and EU legislation, these legal documents refer exclusively to ‘alien’ species.

The researchers have argued that all newcomer species have the propensity to disrupt native ecosystems, irrespective of whether they are considered to be ‘alien’ or ‘naturally range-expanding’ species. So although humans were not responsible for the introduction of range-expanding species, for management considerations, assessment and monitoring of such species is as important as for invasive alien ones.

This article was compiled by the Marine Ecology Research Group at the University’s Department of Biology.