Lake Egirdir is the second largest lake in Turkey and its fish fauna in 1950's was consisted of Barbatula mediterraneus, Cobitis turcica, Capoeta pestai, Cyprinus carpio, Hemigrammocapoeta kemali, Pseudophoxinus egridiri, Pseudophoxinus handlirschi, Vimba vimba and Aphanius anatoliae. Sander lucioperca were introduced to the lake and a considerable amount of pikeperch fishery become possible during the 1970s. However, during the 1980's, most of the native fish species in the lake disappeared due to predatory effects of the pikeperch. Cannibalism rate of pikeperch increased (96%) and their size in the catch decreased because of unsufficient prey fish and overfishing. Moreover, with the collapse of crayfish stock because of a fungal infection, the lake fishery became deteriorated. In the followed years, Gambusia affinis, Knipowitschia caucasica, Tinca tinca, Carassius gibelio, Alburnus chalcoides, Seminemacheilus ispartensis, Atherina boyeri and Pseudorasbora parva were introduced to the lake and cannibalism rate in pikeperch population decreased to 96%. Annual catch in the lake decreased to 38-450 tons in the last five years from 2000 tons in the years of 1970s. At the time of this research 13 fish species was present in the lake and 7 of these were native while 6 were alien species. Consequently, the alien fish species introduced to Lake Egirdir have caused a change in the lake's native fish fauna, destroyed its endemic species as well as giving damage to both the food-chain structure of the lake's ecosystem and the socioeconomic structure of the region.