Have you ever wondered why the fish in a fresh tank get shocked and die once they are thrown into the ocean? Saltwater and freshwater are different. They aren’t as similar as they look. The key difference is the salt concentration. I wrote about the difference between the two in my article, “Why Is Sea Water Salty, While River Water Is Fresh?” So what does salt concentration have to do with the death of the fish?
Saltwater fish and freshwater fish have different structures and lifestyles. Generally, fish bodies maintain internal balance through semi-permeable cell membranes, where water can pass, but salts cannot easily move in or out. This leads to osmosis problems. In freshwater, a fish’s body has more salt than the environment. Water constantly enters its body, so it must urinate often to avoid swelling.
In saltwater, the opposite happens. The surrounding water has more salt than the fish’s body, pulling water out of it. Marine fish have to drink seawater and expel excess salt to avoid dehydration. So, when a freshwater fish is dropped into the sea, it rapidly loses water, its organs fail, and it dies. The same process happens to saltwater fish when it’s in freshwater, as it will absorb too much water and its cells can burst.
However, some fish can handle both environments. Euryhaline fish, salmon, bull sharks, and eels are examples of these exceptions. Their organs (like gills and kidneys) are super adaptable, allowing them to switch between environments. There are some types of fish that adapt and change their environments after millions of years based on mutation and natural selection.
Climate change and rising sea levels are pushing saltwater into freshwater ecosystems. Many native species can’t cope, leading to forced migration, collapse of food chains, and even extinction. In the end, salt is the ultimate gatekeeper of aquatic life. Salt decides a fish’s environment. Scientists are exploring genetic engineering to create fish that can survive in all types of water. But should we? Introducing genetically altered “superfish” might disrupt ecosystems in ways we can’t control.
