Mild-mannered Manatee asks, “Why can’t fresh water fish survive in the ocean?”

What a great question Mild-mannered Manatee! To answer your question, we need to learn a little bit about osmosis. Osmosis is the movement of water from less salty areas to more salty areas across a membrane. Cells, like the ones in your body, are filled with salt water. These cells have an ideal “saltiness” that they want to maintain–not too salty, not too watery… just right! The cell uses osmosis to keep the right balance of salt and water inside the body. The body of a freshwater fish is used to a less salty environment. If you put a freshwater fish in the ocean, the fish’s body is less salty than the ocean. Because of osmosis, water flows from the less salty fish into the more salty ocean. This makes the cells in the fish way saltier than they are used to and they can’t function. However, there are some fish (like the Atlantic salmon) that can survive in both fresh and salt water! These fish have special adaptations that help them keep the right balance of salt and water in their bodies when they move from freshwater to salt water.

-Kirsten, the fish scientist

This website has some more information on osmosis, as well as some fun experiments!

Main image [Source]: An illustration of an Atlantic salmon.

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