The Baltic Sea biota consists of four types of natural immigrants of different origin: freshwater, marineboreal, cold-water, and glacial relicts of freshwater and marine origin (Elmgren 1984). Fish species from other regions (like the Mediterranean or North Sea) are non-indigenous immigrants, occurring sporadically, and which should be regarded as merely an enrichment of the Baltic fish community (Grygiel & Trella 2007). Some authors,

I-BET-762 manufacturer like Elmgren and Hill, 1997 and Elmgren, 1984, regard the Baltic Sea, in comparison with other basins, as a unique example of an ecosystem inhabited by few species, functioning at a low level of biodiversity, whereas Grygiel & Trella (2007) consider the Baltic fish community to be of relatively high biodiversity. Be that as it may, there are some 120 marine fish species in the North Sea MG-132 concentration but only 69 in the western Baltic Sea (ICES subdivisions 22–24) (Aro 2000). There are well-documented reports on over 20 non-indigenous marine fish species (NIS), including just one typically invasive species – Neogobius melanostomus (Pallas, 1814) ( Skóra, 1996, Krzykawski et al., 2001, Bacevičius and Karalius, 2005, Grygiel and Trella, 2007, Lampart-Kałużniacka et al., 2007 and Czerniejewski et al., 2008). The occurrence of NIS has been reported not only from the Baltic

Sea, but also from the Mediterranean, considered to be one of the main hotspots Quisqualic acid for marine bioinvasions and is, among European seas, by far the major recipient of NIS, including macrophytes, invertebrates and fish. The most important vectors of NIS in this region are shipping, aquaculture and direct immigration via the Suez Canal. In recent decades, the rate of introductions into the Mediterranean Sea has increased, which has had both ecological and economic

impacts ( Kalogirou et al. 2010). Some species occur unexpectedly in new regions after an expansion of their natural distribution range (Mohr, 1988 and Nehring, 2002); one of these is the thicklip grey mullet, which occurs in the North Atlantic. Its range extends northwards to the Faroes and the British Isles, Iceland and southern Norway. Since the mid-1960s, the species has evidently been spreading from the North Sea into the western Baltic (Mohr 1988). Single specimens were caught in Flensburg Fjord and the Fehmarnsund in the mid-1970s, and in Kiel Fjord and the Trave estuary in the 1980s (Czerniejewski et al. 2008). Ehrich et al. (2006) put Chelon labrosus on the list of fish species occurring in German waters in the North Sea and western Baltic, but the frequency of occurrence in the total number of hauls was extremely low in the former region (0.01%), and zero in the latter one (studies conducted from 1958 to 2005).