Why can freshwater cause eye problems for seals and sea lions?

In an aquarium setting, keeping seals and sea lions in freshwater can increase their risk of developing eye problems due to the osmotic effect of the water on their eye tissues.1

Because these marine mammals are naturally adapted to saltwater environments, the lower salinity of freshwater creates an osmotic imbalance when it comes into contact with their eyes, which can lead to tissue irritation or damage. While these animals have been kept in freshwater pools, such environments require careful management to mitigate these health concerns.1