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.

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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Summary The length–weight relationships of three endemic species (Rutilus prespensis, Rutilus panosi and Rutilus ylikiensis) and one cosmopolitan (Rutilus rutilus) species are presented for 15 Greek lakes.

Morphological patterns among species of the genus Rutilus from the southern Balkan Peninsula were analysed to identify morphological differentiation and its palaeogeographic history. Seven meristic and 65 morphometric characters, based on 20 landmarks, were studied on 711 specimens from 15 Greek lakes. Multivariate analyses demonstrated a well-supported differentiation of the western and northern from the eastern Rutilus species' populations, separated by the Pindos Mountains. In North-western Greece, there are three isolated endemic species, in different biogeographical areas (R. prespensis in the Southern Adriatic, R. panosi in the Ionian and R. ylikiensis in the Attico-Boeotia), while in the Northern Aegean, multiple non-homogeneous populations of the cosmopolitan R. rutilus can be found. This distribution is clearly in accordance with the biogeographical scenario for the distribution of freshwater fauna of the Balkan Peninsula. This study confirms differentiation of the populations of Lakes Kastoria, Volvi, Vegoritida and Doirani, and emphasizes the likely existence of cryptic species within R. rutilus populations.