KORONEIA, Greece (AP) — Tens of thousands of dead fish have been found on the banks of a lake in a protected nature reserve in northern Greece after high temperatures and drought conditions caused a severe drop in water levels.

State environmental officials said Thursday that the water level at Lake Koroneia has dropped by more than 70% percent in the past three years, to 80 centimeters (31 inches), with the decline triggering the death of carp, sunfish, bleak and other freshwater fish.

Dimitra Bobori, an associate biology professor at the University of Thessaloniki and the lake's senior administrator, said dissolved oxygen concentrations in the water had fallen below a critical level, accelerating the deaths over the last week.