Answer 1:
In order to survive, a fish needs to keep its
blood at a constant salinity or salt level, and the
optimum level is less salty than seawater but a
lot more salty than freshwater. Fish are
“osmoregulators” which means they can
actively control salt concentrations in their
bodies using
different strategies depending on the species and
its environment.
Most fish are restricted to
either salt or fresh water and cannot survive in
water with a different salt concentration than they
are adapted to. This is because the methods
they use for maintaining the right level of
salinity in
their body only works in the environment they’ve
evolved in. For example, tuna can only live in
saltwater because their kidneys and gills
excrete excess salt from the water they drink. On the
other hand, a freshwater fish like a catfish
excretes lots of water from its kidneys and gills
while
retaining salt.
However, some fish can osmoregulate across a broad
range of salinities. Fish that can do this are
called “euryhaline” and they either live in
places where salinities change dramatically (like
estuaries), or else they migrate between
freshwater and marine environments (e.g.,
salmon).
Euryhaline species can switch back and forth
between freshwater and marine water whenever
they want, while others only do it once or twice
during certain phases of their life cycle. | 
