You may or may not know it, but the blood plasma of all vertebrates
higher than the jawless fishes is always stable at 9 parts per thousand
of salt. The body of any animal needs devices to keep this in a stable
way, especially those animals living in water. Those animals living in
freshwater, where the salt levels are maybe one third of those of blood
plasma have a slight problem, but the fishes that live in the sea have
an immense problem with this, as they need to drink continuously to
osmoregulate and therefore they have advanced mechanisms in their
kidneys which enable this.

There is no missing link here, by the way, no animals that are between
normal saline and isotonic with the water they live in. It is either
one or the other. And this is a massive gap with absolutely no
explanation by evolution.

In discussion with a noted creationist professor in talk.origins, it
pretty much transpired that if vertebrates evolved as required for the
theory of evolution to be true, they must have done so in fresh water,
or at any rate water much fresher than today's seas.

Either the sea used to be at much lower saline levels - which is not
consistent with the view that when the earth was younger the seas were
like a chemical soup that allowed life to appear in the first place,
and have been getting progressively less salty - or life evolved beyond
the Agnathans in the rivers. And it seems equally unlikely that fishes,
the dominant life forms of the sea, evolved in only the tiny 5% of
water that is not sea water, and no competing higher life forms emerged
in the other 95% which had produced everything up to that point.

Neither version makes any sense. But it makes much more sense out of
the idea that the sea was created unsalty, and became so after the
flood, which was associated with great tectonic upheavals. That is
which freshwater fishes did not die out in the flood.

It also explains why migratory fishes such as the salmon, need to go to
freshwater to breed. They are a living clue of this. Eels appear, at
face value, to do the reverse, and go all the way to the Sargasso sea
to breed, but in fact they do this in deep water next to subterranean
vents which pump out into the sea exceptionally clean water. They then
are covered by a leaf-like sheath in which protection they cross the
salt water, and they can shed this when they hit freshwater. They are
freshwater fishes that become saltwater later, over the course of one
lifetime, giving us a living clue that has survived thousands of years
to today's date as to what happened after the flood, when systems
suddenly appeared that enabled fishes to cope with all the salts that
emerged when the continents suddenly moved in the time of Peleg.

But a fish that has gone into the saltwater system at the full 2.7% (I
am not talking about the brackish systems like the Baltic pike) can
never go back - the adult eels do not return again from the Sargasso
sea, (In fact the congers are a type of eel that simply decided to stay
in the marine system for good, and do not bother sending their young
into the river systems, although they also migrate to near freshwater
vents to breed) and they also die in the process, because they have to
go into a freshwater environment that exists in some places in very
deep water near to these vents. In this, they are like another heroic
migratory fish, the salmon, which swims upstream and dies because it
cannot readjust to the freshwater system.

This is a natural parable of the resurrection body also. The
resurrection body can cope with these old circumstances only briefly.
It is intended to be used by us in an eternity in comparison to which
our earthly life is like a river to an ocean. So our Lord Jesus Christ
did not remain long after the Resurrection, but went out of this world
into heaven. Likewise, when He returns with the shout and the final
trumpet call and wakes the dead, and we are changed in the twinkling of
an eye, as we can read in Thessalonians, we are caught up and meet the
Lord in the air, together with the dead in Christ who are resurrected
and gathered (it doesn't matter if their earthly atoms were sprinkled
to the four winds) and we are then able to fly and disappear together
with him from this world into Paradise.

As with the migratory fishes, this world was needed to produce the
offspring, for sexual reproduction, for going forth and multiplying,
filling the earth and populating eternity, where people are not
reproducing sexually any more, but are as the angels of heaven, as our
Lord taught us. The salmon and the eel take to a limited system to
reproduce, but that is not the be-all-and-end-all if their existence.
They are constructed so as to journey in their lives over a vast ocean.
And we are made to glorify God, and enjoy him forever. Not in this
sinful flesh, but in redeemed resurrection bodies, eternal-oceangoing
bodies, free from carnality and veniality: "when we see Him, we shall
be like Him".