Why can’t Freshwater Fish live in salt water?

This is a very common question Why can’t freshwater fish live in salt water? In this article, I’d like to share a question that you might question yourself. Can saltwater fish adapt in freshwater and vice versa.? Generally, the answer to this question is YES and NO.

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Fish are osmoregulators, meaning that their cells contain different amounts of solutes than the surrounding water.

So…what would happen if osmoregulation fails and the amount of salt concentration is similar between their body and the water? First of all, let’s see what would happen if saltwater fish is a place in a freshwater tank.

As we know, saltwater fish require a water salinity level of approximately 20-30 ppt to survive. A gradual decrease of salinity due to the overflowing river or big storm won’t cause a huge problem since it will not change the salinity drastically.

However, transferring saltwater species who cannot tolerate freshwater into the freshwater environment with the intention to make them survive will definitely bring disaster to the fish. Saltwater fish require sodium (Na+) to survive in order to balance osmotic pressure in their body.

Their body is less salty than the seawater as they are hypotonic, causing the water, which is hypertonic to move out of the gill tissue where it is lost. This means saltwater fish are constantly losing water out of their system through osmosis.

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To balance the concentration of salt in their body, they need to take some salt from the water through the gill down to the digestive tract similarly as drinking water. When placing saltwater fish into the freshwater, the concentration of salt in the freshwater reduce drastically and water will tend to dilute the salt concentration equally in both semi-permeable membrane.

With the shock transition, the bacteria and parasite cannot tolerate the sudden change but the fish are still capable to deal with this adjusted situation for various durations of time depending on species.

Similarly, the Osmoregulation concept could also be applied to freshwater fish place in a saltwater environment. Since freshwater has zero ppt salinity level, the fish living in the environment will have a saltier concentration (more Na+) than the freshwater but less concentrated than the saltwater.

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Therefore, their cell is more hypertonic, and water will keep on coming through the gills and move out through the gill’s cover. So.. they don’t drink water but constantly urinating to release excess water.

When placing freshwater fish in saltwater for a long period than the time limit, the fish will get dehydrated due to the high concentrated salt and insufficient water coming through, their kidney will fail and gradually they will die. On a greater note, fish suffer from bacteria or parasitic infection could be treated with different types of water they live in.

Saltwater fish could be exposed to freshwater baths and vice versa for a short duration of time. This will kill the harmful bacteria/parasite on their body but they will remain to survive out of the treatment.

Although most fish are Osmoregulators, some species could live in both environment and these types of species are called Euryhyaline organisms. They are able to tolerate freshwater, brackish water up to the saltwater environment, which is contradictory to Stenohaline species, the regular species that cannot deal with various salinity levels as what have been discussed earlier.

The species in this group could effectively Osmoregulate across a wide range of salinities. They are able to adapt to both freshwater and saltwater environment through behavioral and physiological.

And their kidney seems to perform well in both environments. The most common example of Euryhyalin species is Salmon. Their life cycle requires them to live in the ocean and travel to the freshwater where they were born to spawn and breed. On the other hand, the eel will do the opposite as they are required to live in the freshwater and spawn in the ocean.

And many other species including Atlantic Stingray, Baramundi, Striped Bass, Lamprey are Euryhyaline species. A unique shark known as Bull Shark is able to thrive in both saltwater and freshwater.

They have been known to travel from the ocean up to the Mississippi River. Although they could live in freshwater, they are only able to reproduce in the ocean as their tolerance to the saltwater only increase with age. This is the reason for why can’t freshwater fish live in salt water

So that’s the best, I could answer this question! If you like to add more.. share them in the comment section.

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