**3. Results**

In total, 800 records were utilized from 169 lakes and 631 lotic sites from 51 river basins. By using solely lotic data from field surveys, our results indicated low NIFS richness (0–1.40 NIFS per 1 km2) throughout the country, with the exception of specific hotspots in central and northern Greece (Figure 3a). On the contrary, by utilizing bibliographic data from lentic ecosystems the patterns of predictive NIFS richness was predominately high (2.7–4.3 NIFS per 1 km2) in most areas of the country, followed by moderate NIFS predicted richness (1.5–2.6 NIFS per 1 km2) in the remaining parts (Figure 3b).

**Figure 3.** Patterns of the predicted richness of non-indigenous fish species derived from (**a**) lotic data and (**b**) lentic data.

Overall, we recorded 52 NIFS in 154 lentic and 51 lotic ecosystems (265 sites) of which 17 were categorized as alien and 35 as translocated. The five most widespread NIFS within Greece included four aliens, namely *Gambusia holbrooki* occurring in 223 locations (53.1%), *Carassius gibelio* occurring in 187 locations (44.52%), *Lepomis gibbosus* occurring in 113 locations (26.9%), *Pseudorasbora parva* occurring in 93 locations (22.14%), and one translocated species *Cyprinus carpio* occurring in 109 locations (25.95%). NIFS richness ranged from single species (176 sites) up to 12 fish species (one site—Lake Pamvotis). Our results indicate that freshwater ecosystems under high NIFS richness are located mainly in lowland areas of western, central, and northern Greece (Figure 3a), usually near large cities and the presence of lentic ecosystems with well-developed commercial and recreational fisheries. On the contrary, areas with low NIFS richness were observed in mountainous regions and within small river basin areas in southern Greece and the Aegean islands (Figure 4a).

**Figure 4.** (**a**) Patterns of predicted richness of non-indigenous fish species (indicating high invasion pressure) and (**b**) geographical distribution range of critically endangered freshwater fish species and protected areas (Natura 2000 network).

The distribution of the CR freshwater fish species in Greece was scattered throughout the aquatic ecosystems of the country (Figure 2), covering approximately 28% (36,708 km2) of the entire area of Greece (Table 2). In most areas, only one CR fish species occurs or is known to have occurred (69.9%); however, in some ecosystems two or more species co-occur (Figure 4b; Table 2). The highest species richness of CR fish species was observed in the north and northeastern, western, and central parts of Greece, while no CR fish species were recorded in the mountainous areas of northern and central Greece, in the majority of the Aegean or Ionian Islands, or the island of Crete (Figure 4b).


**Table 2.** Distributional coverage of critically endangered (CR) freshwater fish species in Greece.

Overlapping areas of CR species with moderate to high NIFS richness (1.5–4.3 NIFS per 1 km2) were relatively high (~50%) (Table 3). These areas were located in the western, central-east, and northern part of Greece (Figure 4). Shared absences indicating both low CR species and NIFS and thus low overlapping areas were observed in the mountainous regions of central Greece, the Aegean Islands, and the Island of Crete (Table 3; Figure 5). The above is to be expected since most Aegean Islands (with the exception of the Island of Lesbos) as well as the Island of Crete lack any CR species.

**Table 3.** The proposed class boundaries (low, moderate, high) of the overlapping areas of nonindigenous and critically endangered freshwater fish species, the actual areas, and the total percentage for each category.


Only, five CR species were recorded exclusively in areas with low NIFS richness (*Barbus pergamonensis, Knipowitschia goerneri, Knipowitschia milleri, Oxynoemacheilus theophilii* and *Pungitius hellenicus*). The CR fish species co-occurring in areas with moderate to high NIFS richness were: *Pelasgus epiroticus* (Lake Pamvotis), *Valencia robartae, Salaria economidisi* (Acheloos basin), *Scardinius graecus* (Lakes Yliki and Paralimni), *Cobitis stephanidisi* (Lake Karla basin), *Alburnus macedonicus* (Lake Doirani), *Barbus euboicus, Squalius* sp. *Evia* (streams of Euboea Island) and *Caspiomyzon hellenicus* (Strymon basin) (Figure 5; Table 4).

A quarter of the overlapping areas (24.8%) fall well within the NATURA 2000 network (Figure 6a), where management bodies could implement control or mitigation programs to minimize the negative impacts from NIFS to native biodiversity. However, the majority of the distribution areas of CR fish species in Greece are located outside of protected areas (75.2%; Figure 6a) and any additional anthropogenic stressors have the potential to increase pressure to the populations of these species. Moreover, the vast majority of the overlapped areas within the protected zones encompassed only one or two CR fish species per 1 km2: 78.4% and 17.2%, respectively (Figure 6b), while three or four CR fish species per 1 km<sup>2</sup> covered considerably smaller areas (4.4% in total; Figure 6b). In addition, the area coverage of moderate to high NIFS richness was similar within the unprotected (49.3%) and protected areas (44.5%) (Figure 6c,d), potentially indicating that no effective preventive measures are applied to prevent NIFS spread in protected areas.


**Table 4.** The distributional coverage and the overlapping areas per critically endangered freshwater fish species for each NIFS predicted richness category (low, moderate, high).

**Figure 6.** (**a**) The overlapping areas of NIFS and CR freshwater fish species in unprotected areas (grey) and protected areas (black); (**b**) the area coverage (%) of CR freshwater fish species richness within the protected areas; (**c**) the area coverage (%) of NIFS richness in unprotected areas; and (**d**) within protected areas.
