The length-weight relationships of the cyprinid fish Carassius gibelio (Bloch, 1782) are described for the 12 most important lakes of Greece. Carassius gibelio is an allogynogenetic fish species, which was introduced into Greek lakes during the late 1970s. The values of the exponent b of the length-weight relationships ranged from 2.33 to 3.38, and varied with the trophic state of the lake. In eutrophic lakes these b values were significantly (P < 0.001) lower than in oligotrophic or mesotrophic lakes. Relationships between the b values and phosphorus concentrations were logarithmic: b ¼ 1.37-0.13 log (PO 4 -P); total length, fork length and standard length were linear (in all cases: r 2 > 0.95). In most of the lakes, populations consisted of females, with the exception of Pamvotis, Doirani and Koronia lakes where a small proportion were males.

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Lake Pamvotis is a tertiary natural shallow lake located in the NW of Greece. For biogeographical reasons, it historically contained only four fish species: two endemics (Phoxinellus epiroticus, Squalius pamvoticus), one native to the West Greece (Barbus albanicus) and the ubiquitous Anguilla anguilla. These species were almost exclusively present in the lake before 1920, forming remarkable commercially exploited populations. From the 1930s through the 1990s, 20 species were introduced into the lake for purposes of eutrophication control or to enhance the fishery. These introductions, some of them accidental have led to the apparent decline, even loss, of the native species. During the last three decades the fish fauna of the lake has shifted from the native, clearwater species to a predominance of introduced species, mainly those adapted for turbid eutrophic water (Rutilus panosi, Cyprinus carpio and Carassius gibelio, and several Asian cyprinids). The current fish assemblage in the lake is dominated by introduced species particularly the Mosquitofish (Gambusia affinis) and the Lourogobios (Economidichthys pygmaeus) in the littoral zone, the benthopelagic allogynogenetic Prussian carp (C. gibelio), and the opportunistic Trichonis roach (R. panosi), in the pelagic zone.

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Toxin-producing cyanobacteria in lakes and reservoirs form a threat to humans as well as various forms of aquatic life. This study is an investigation into the occurrence and distribution of Microcystins (MCYST) in 13 Greek Lakes. The distribution of MCYST in water and surface scum and toxin bioaccumulations in the omnivorous fish species Carassius gibelio were surveyed in all lakes. Considerable amounts of MCYST were found in water and scum of all lakes, irrespective of the trophic state, the type of the lake, and the reported dominant cyanobacterial species. Toxin accumulation in six tissues (liver, brain, intestine, kidney, ovary, and muscle) of C. gibelio was also analyzed. Even though the target organ for MCYST is the liver, in our study, MCYST were found also in the rest of C. gibelio tissues in the following order: liver > intestine > kidney > brain > ovaries > muscle. Risk assessments were carried out, taking into account the WHO guidelines and the tolerable daily intake (TDI) for MCYST. Our findings suggest that the amounts of MCYST found in water of Lakes Kastoria, Koronia, Pamvotis, Doirani, Mikri Prespa, Petron, and Zazari, pose adverse health risks. Also, it is likely to be unsafe to consume C. gibelio in Lakes Koronia, Kastoria, Pamvotis, and Mikri Prespa due to the high concentrations of accumulated MCYST.