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Atlantic fish species (1) a recent spread through the Strait of Sicily from the
eastern to the western basin after 2004/2005;
Atlantic fish species have shown two patterns of spread
(Table 1): a) continuous dispersal throughout the whole (2) a constant spread into the western basin through the
western basin (for example the opah Lampris guttatus, Gibraltar Strait since their first reported entry into the
the ringneck blenny Parablennius pilicornis and the Mediterranean, involving the establishment of
bastard grunt Pomadasys incisus), and b) patchy Mediterranean populations;
dispersal, as in the case of the Bermuda sea chub
(3) a sequential spread into the western basin through
Kyphosus sectatrix and the snake eel Pisodonophis
the Gibraltar Strait, with alternating spreading phases
semicinctus. All of these fish species have reached the
spaced several decades apart. These pathways are
Adriatic Sea (at least the southern part) or the eastern
considered natural, i.e. they are not human induced.
basin. Two species, P. pilicornis and P. incisus, have
displayed a stable distribution in the western basin since The changes in the main current circulation in the
they first arrived in the Mediterranean and established Mediterranean during the last few decades coupled with the
permanent populations. These two species spread both present warming of the western basin may well explain the
clockwise and anticlockwise along the western recent spread of Red Sea species towards the western
Mediterranean coasts, eventually meeting on the basin. The reversal of the North Ionian Gyre in 1997 from
south-east coast of France (Fig. 10; Parablennius anticyclone to cyclone, driven by the Bimodal Oscillating
pilicornis, dates in black). The number of individuals of all System mechanism, had a major impact on exchanges
Atlantic species has increased during the last decade, between the eastern basin and the Ionian Sea (Civitarese et
whether their spread has been continuous or patchy, and al., 2010). Moreover, recent studies (including Soto-Navarro
these species are now relatively frequent where they were et al., 2012) have detected a rising salinity trend in Atlantic
rare before. Their increasing numbers are having an waters in the period 2003–2007, implying a higher salinity
impact on native populations and may lead to a reduction input into the Mediterranean. The sharp rise in the
in the abundance of endemic species. abundance of several Atlantic species in the western basin
since 2006 maybe related to this trend. However, the
A review of the spread and/or establishment of these geographical distribution of these species is sometimes
species suggests that their successful invasion may have highly complex because the natural pathways may be
occurred along three main pathways: disturbed by human transportation.

Fig. 11. Map of the western Mediterranean with the dates of first observations of Parablennius pilicornis (in black; triangles mark the
meeting point along the south-eastern French coast) and its potential pathway of expansion (arrows) (modified from Pastor & Francour,
2010). Kyphosus sectatrix (in red), another Atlantic species, shows a patchy distribution and two phases of spreading,
at the beginning and at the end of the 20th century (modified from Francour & Mouine, 2009).
Parablennius pilicornis. Photo: M. Otero - Kyphosus sectatrix. Photo: K. Bryant

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