Some fish live in saltwater. Others thrive in freshwater. But what about the ones who survive in both? Let’s take a look at that fish of legendary migration: the salmon. Hatched in freshwater rivers, salmon then venture out to sea. They journey thousands of saltwater kilometres only to turn round and swim back up the freshwater rivers where they started.
So how do they survive jumping the salty boundary? The first hurdle is trading freshwater hatching grounds for the briny sea. But they don’t just dive into the salt. As they make their way down the rivers to the edge of the sea the freshwater starts to get a bit of salt in it. The closer to the sea, the more the salt.
The salmon dawdles in this brackish water. Taking time to adapt. One of the big adaptations is dealing with dehydration. Sort of. The salty seawater sucks fluid out of the salmon. To understand how this works all you need is a potato and some salt.
Cut a slice off the potato and drop it in bowl of salty water. What happens? The salt draws water out of the potato. The potato shrinks. The more salt you put in the water the smaller the potato gets. It’s dehydrating. And that’s exactly the threat facing the salmon as it enters the salty seas.
To deal with this the salmon starts to drink some of the saltwater. The salty sea will keep pulling water out of the salmon so the fish balances things by drinking water. Topping up its fluids.
When it’s time to return to the freshwater and swim upstream to spawn everything reverses. Heading upstream the salmon stops drinking. The freshwater won’t give the salmon much salt so it needs to conserve what it has. And so the salmon criss-crosses the briny boundary that would kill most fish.
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