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Polychlorinated biphenyls (PCBs), hexachlorocyclohexane (HCH) isomers, dichlorodiphenyltrichloroethane (DDT) and its metabolites, other organochlorine pesticides such as hexachlorobenzene (HCB), chlordane compounds (CHLs, including trans-chlordane and cis- and trans-nonachlor) and the heavy metal mercury were quantified in muscle and liver of the European chub (Leuciscus cephalus, Linnaeus, 1758) and in the muscle of the barbel (Barbus cyclolepis, Heckel, 1837) at two sampling sites of the River Nestos, Greece. PCBs in muscle and DDTs in the liver tissues were the predominant organochlorinated contaminants. Among the PCBs, congeners 47 (up to 9.60 ng g(-1) wet wt.), 180 (up to 1.15 ng g(-1) wet wt.) and 190 (up to 1.50 ng g(-1) wet wt.) were the most frequent and abundant. The contamination degree by the sum of PCBs on the fish tissue samples from the River Nestos is lower or similar to PCB levels found in other ecosystems. Among the organochlorine pesticides, essentially only p,p'-DDD, p,p'-DDE and alpha-, beta- and gamma-HCH were found, with the former appearing at mean levels up to 30.71 ng g(-1) wet wt. From a public health standpoint, residue organochlorine pesticide levels from our work are considerably lower than the recommended tolerance limits. Finally, mean values of Hg in chub were significant lower (up to 31.04 ng g(-1) wet wt.) compared to those detected on barbel (up to 169.27 ng g(-1) wet wt.). The concentrations of Hg in fresh water fish from the River Nestos did not exceed WHO and US EPA health guidelines, and were suitable for human consumption.

The freshwater fish fauna of Greece includes 106 species of which 79 are native primary species. Of these 38 species are endemic to Greece and southern parts of neighbouring countries. Endangered species are divided into three groups: wholly endangered, locally endangered and rare and threatened. In the first group are eight taxa, usually found in a very restricted area (Eudontomyzon hellenicus, Alosa caspia vistonica, Pseudophoxinus beoticus, Ladigesocypris ghigii ghigii, Barbus euboicus, Barbus cyclolepis cholorematicus, Pungitius hellenicus and Knipowitschia goerneri). The second group includes five species with a wide area of distribution but some local populations which are extinct or in obvious decline (Salmo trutta, Pseudophoxinus stymphalicus, Vimba melanops, Valencia letourneuxi, Proterorhinus marmoratus). The third group comprises nine endemic taxa, mainly with a wide distribution in Greece, or widely distributed in Europe but represented in Greece by local populations, sometimes recognized as subspecies (Phoxinellus pleurobipunctatus, Barbus prespensis, Gobio kessleri banarescui, Gobio uranoscopus elimeius, Carassius carassius, Orthrias barbatulus vardarensis, Pungitius platygaster, Knipowitschia thessala, Economidichthys pygmaeus). These taxa often have rare or threatened local populations. The main reasons for the decline of fish species are: agricultural and industrial activity, overfishing, partial or total destruction of many karstic springs, public and private works abstracting water, drainage, irrigation, tourist and industrial installations, dams, mixing of fish faunas of different origin and introduction of exotic species.