Here's a good cooking tip: if you want to make
tasty, crispy french fries from scratch, soak your
potatoes in salt water. You might be wondering
what that has to do with salt water fish surviving
in fresh water, but it'll become clear soon.
When you soak the potatoes in salt water, the
potato loses water and so when you throw your
french fries in the deep fryer, they come out nice
and crispy. This happens through a process that
goes by the scientific term "osmosis". For
now, osmosis can be thought of simply as water
trying to move from an area that has less salt to
an area that has more salt. Potatoes in salt
water is one example of osmosis. Osmosis happens
all the time in everyday life, and fish use
osmosis to maintain a careful balance of salt and
water in their body.
A fish that lives in freshwater will drink
water, but fish have more salt in their bodies
than the freshwater has. Osmosis says wants to
get to an area of less salt to an area of more
salt, so the freshwater will try to get into the
fish. That means the fish has to get rid of the
water in the same way that we do: using the
bathroom! The opposite is true for saltwater fish.
Saltwater has more salt in it than the amount of
salt that is in a fish. So water will try to go
from the fish (area of less salt) to the saltwater
(area of more salt). That means a saltwater fish
has to continue to drink a lot of water but
doesn't have to use the bathroom as much.
So, what happens when a saltwater fish gets
put into freshwater? The direction of water
due to osmosis is reversed. Instead of water going
from the fish to its environment, the fish begins
to take in lots of water - much more water than
its used to. It is drinking more water than it
can get rid of, and the result is not good.
Poor fish. The chemicals in the fish's body, which
keep it alive and well, are not balanced
correctly, and the fish will soon die.
The opposite is true for freshwater fish.
In saltwater, they get too much salt and end up
dehydrated. The same thing can happen to any
other animal if they drink too much water or if
they don't drink enough water (but don't worry,
it's almost impossible to do this on accident.)
But with fish, their drinking is controlled by
their environment, so a saltwater fish in
freshwater can be really bad.
As a side note, there are some types of aquatic
life that survive in freshwater and saltwater.
Salmon is a good example. But even salmon need
a little time to get used to the change in
saltiness. They can't just dive into the ocean
straight from a freshwater stream. It's a gradual
change. If you want to learn more about these
kinds of animals, you can search for
"euryhaline", which just means animals that
like a wide range of salty environments.
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