The aquarium trade has been recognized as an important pathway for the introduction of invasive species around the world. This study investigates the availability of ornamental fish species in ten large-size, centrally positioned aquarium stores that control a large share of imports and the Hellenic market chain, and aims to provide a provisional checklist on the aquarium fish trade in Greece. For each recorded species, additional data concerning various aspects (e.g. natural environment, native range, established as aliens, conservation status and threats to humans) were collected from Fishbase, IUCN red list and the scientific literature. Overall, 326 fish species belonging to 64 families were reported according to the store labels. The majority of the species recorded were freshwater (66%), originating mainly from South America and Asia, while most of the marine species (26%) had primarily an Indo-Pacific native distribution. Among the freshwater fishes, Cichlidae and Cyprinidae were the dominant families with 64 and 27 species, respectively, while the family Acanthuridae dominated within the marine fishes with ten species. The vast majority of both freshwater and marine species (>90%) were tropical. Concerning the presence of alien species, 62 ornamental species have been established outside their natural range, with 22 of them positively confirmed as aliens in the European waters. Moreover, 25 species were listed in the critically endangered (CR), endangered (EN) and vulnerable (VU) categories of the IUCN red list. Even more surprisingly, for 192 species recorded, data were missing to assign their conservation status or had not been assessed at all. Finally, the majority of the species (84%) were harmless to humans. However, 35 species (11%) were recognised as potentially harmful (i.e. venomous, ciguatera poisoning, traumatogenic) and two were found to be poisonous if consumed. In conclusion, the aquarium fish sector in Greece is practically uncontrolled given the presence of: a) threatened species, b) species potentially harmful to humans and c) species capable of establishing non-indigenous populations, if released into the wild.

Indigenous freshwater crayfish species (ICS) are important biodiversity components and desirable fishery targets. However, ICS populations are increasingly threatened by various anthropogenic stressors. Moreover, established populations of non-indigenous freshwater crayfish species (NICS) and new ‘waves’ of NICS introductions exert significant pressure on ICS populations at a pan-European level. These effects include direct competition for space and resources as well as crayfish ‘plague’ transmission from introduced North American species. Given low public knowledge of this problematic, considerable risk of future introductions exist as a result of conventional and internet-based aquarium trade, which often lead to deliberate and/or accidental releases of NICS into the wild. In 2011, we conducted a survey of freshwater crayfish species in eleven large-size pet shops located in three major cities and in three large internet-based aquarium companies in Greece. Overall, eight species belonging to three genera (Procambarus, Cherax and Cambarellus) were recorded, originating from the USA, Australia, New Guinea and Mexico. The invasion potential of the three most popular species was assessed using the Freshwater Invertebrate Invasiveness Scoring Kit (FI-ISK). Two species were determined to constitute a ‘very high risk’ of invasion. As such, regulatory measures need to be implemented to monitor the ornamental trade of NICS in Greece and a national framework developed for protecting ICS.

The invasion of non‐indigenous freshwater fish species is one of the most important threats to aquatic biodiversity. Similar to other Mediterranean countries, Greece is considered a hot spot for freshwater biodiversity, with many range‐restricted endemics of high conservation concern. The aim of this study was to undertake a risk screening assessment to evaluate the invasive potential of non‐native, translocated and traded aquarium fishes in Greece by applying the Fish Invasiveness Screening Kit (FISK). In total, 73 freshwater fish species were evaluated by two assessors. FISK was able to discriminate reliably between invasive and non‐invasive species with a threshold of 15.25. Based on mean scores, 30 species were classified as ‘high risk’, of which 17 as ‘moderately high risk’, six as ‘high risk’ and seven as ‘very high risk’. There was a high coincidence rate for the species categorisation between the two assessors, but significant differences in certainty. The results suggest that FISK is a useful tool for assessing risks posed by non‐native, translocated and traded aquarium fish species in Greece.

Ichthyofauna in the Hellenic freshwaters is quite diverse consisted of 161 species (including 28 non-natives), out of the 546 native species in PanEuropean level. However, the status of many aquatic ecosystems is currently degraded as a result of a variety of anthropogenic impacts. This has direct reflection on the conservation status of the indigenous and native fish species whose populations are in many cases declining in favor of the invasive ones. Prussian carp (Carassius gibelio) is probably the most dominant invasive species and currently is considered as naturalized in about 26 lake and river basins. Its successful adaptation and proliferation is mainly attributed to its dual reproductive mode (allogynogenetic and gonochoristic), the opportunistic-omnivorous feeding habits and the ability to withstand and flourish in adverse ecological conditions. Accordingly, it exerts competitive, as well as destructive effects upon the indigenous fish species (sperm parasitism, antagonism for food, spawning grounds and spawning substrates) and therefore, efforts are needed to sustain a relative balance of the populations. This review aims to critically discuss the status of C. gibelio in Greece, the intrinsic causes of establishment and the observed impacts on ecosystems. Finally, reasonable actions are proposed related to exploitation of C. gibelio stocks and a path model for the protection of native fish species and their ecosystems.

The diversity of ichthyofauna in Hellenic freshwaters is highlighted by the presence of 161 species. This actually represents 27.8% of the total fish species recorded in European freshwater, including both native and introduced species, suggesting that Greece represents a ‘hot spot’ for biodiversity. The introduction of alien species for restocking, the aquarium trade and aquaculture was responsible for the deliberate or accidental release of alien specimens into open waters and in some cases for establishing populations. In contrast, commercially valuable alien species (e.g. salmonids, cyprinids and more recently sturgeons) generated significant income to individuals, companies and to the national economy. This paper aims to review the situation regarding the presence of alien fish and crayfish species in Hellenic freshwaters and in particular in aquaculture and to flesh out issues concerning their aquaculture potential.