The decrease in the number of crocodiles in the lake stems from the security vacuum and failure to prevent poachers and illegal fishing in the site, Ahram Online quoted Mohammed Ezzat of the Nature Reserve Authority in Upper Egypt saying.
Only 41 crocodiles were found in part of the lake called Khor Korosko compared to 81 three years ago Ezzat said.
Poachers capture the large crocodiles because their skins fetch high prices he said.
Lake Nasser was created when the Aswan High Dam was built in 1970 and is the largest man-made lake in Africa, the report said.
Egypt recently signed the Convention on International Trade in Endangered Species of Wild Fauna and Flora thereby banning all fishing in the lake.