Egirdir Lake is placed Anatolian Peninsula in 500 km(2), its mean depth takes 16 meters. They are made use of watering and due to oligotrphical characteristic and drinkable water besides fishing (Fish and Crayfish). Since 1950, 9 different species of fishes of Cyprinidae were lived this lake. There were cause irreversible alterations by way of implantation of pike-perch (Sander lucioperca) in here for to ann more economical production. In the first year while the while the level of prosperity of the people rising, and the following years there were obtained catching low recolte neither carp nor pike-perch catching. Local government and fisherman were negative effected in this situation, and they were implantation in the lake in early 1990. In this implantation who nobody knows Carassius auratus gibelio Bloch, 1783 was implanted in this lake. This species which is rapidly increased more rapidly growth and more become dominant species than pike-perch. Velvet fish (Tinca tinca) were implanted in Egirdir Lake in 1996. But they couldn't become any their population. They were found 10 different species by Balik et al. (2002) in Lake Egirdir. The last implantation in Egirdir Lake was silver fish (Atherina boyeri) while there were caught 80 tons carps, 300 ton pike-perch in 1985; 5.5 ton carp and 4 ton pike-perch Were caught in 2004. Depend on decreasing of the pike-perch's body criterion, economical production of this fishes was also decreased. In this situation of the lake ecosystem is worried about Local government and Fisherman. In this study, some hydrobiological characteristics of the lake were defined and the negative changes of the fish population were also examined.