The Bitter Lakes are two very salty ( hypersaline ) lakes about mid-way in the canal . At first , this blocked the migration of species from the Red Sea to the Mediterranean . The Bitter Lakes , which are hypersaline natural lakes that form part of the canal , blocked the migration of Red Sea species into the Mediterranean for many decades , but as the salinity of the lakes gradually equalized with that of the Red Sea , the barrier to migration was removed , and plants and animals from the Red Sea have begun to colonize the eastern Mediterranean .