Most fish are very sensitive to salinity, and can only survive variations if they are relatively small.
Marine fish are physiologically adapted to salt water, fresh water fishes are adapted to fresh water, and while a few can tolerate the brackish conditions and wild variations of salinity found in estuarine conditions, most cannot survive there for long, or can only survive there during certain periods of their lifecycle.
This begs the question - how did all of the marine fishes survive the sudden deluge of the great flood (assuming it was a deluge of fresh water) - or assuming it was a deluge of salt water of similar salinity to sea water, how did freshwater fishes survive?
for that matter most marine and freshwater animals of all phyla have similar problems with variations in salinity too
Did noah have aquariums on board?
Is there any biblical reference to fish on the ark?
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