How do fish survive in both saline and fresh water?

04 September 2011


How do fish survive in freshwater and excess salt?


Emma - The thing is, wherever a fish is, in order to survive it needs to keep its blood at a constant salinity or salt level. That optimum level happens to be much less saline than seawater but a lot more saline than freshwater. This ability to regulate like this is called osmoregulation and all fish can osmoregulate. Some fish that live in a saltwater environment, and only live in a saltwater environment, such as tuna, drink the very, very salty seawater. Their kidneys are specially adapted to excrete lots of salt, and even their gills can excrete salt in this way so that they keep their blood salinity levels at an optimum. Freshwater fish are the opposite. Water constantly moves into their bodies from the water so their kidneys excrete lots and lots of water and their gills do also. But, critically, they manage to hang on to salt to keep their blood at the right salinity. But then you have these odd fish called euryhaline fish - like salmon, eels and striped bass - and they have to be able to tolerate huge changes in outside salinity either because of their life cycle, like the salmon which goes upstream into freshwater to spawn, or if they live in an environment like an estuary where the river meets the sea. They're capable of switching from one kind of osmoregulation to another and these sorts of large regulatory changes are controlled by switches in their brain. But it's worth mentioning, they still need quite a bit of time to acclimatise to new salinities while these regulatory mechanisms are switching over.

Chris - Do you think the fish therefore change their behaviour? So if they're swimming down an estuary, would they just naturally adapt because it takes time to swim all the way down the estuary and the water is going to get more and more salty as they do it, or do they loiter in a certain area and expose themselves gently to the rising salt so that they don't literally get a bit of a shock when they go from fresh to really salty?

Emma - I don't know for sure but I should imagine that they "loiter" a bit so they can take the time to switch their system over to the new kind of salinity level.

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