There are some good ideas in these articles. These creatures are very tough and have pretty good adaptability judging by how they are able to live in such a harsh and unforgiving habitat.
How did fish survive the flood?
Snip>>> But what about the fish and other marine creatures? Obviously, they weren’t taken on board the Ark. How could they survive, particularly both fresh and salt water forms? As a matter of fact, most of them didn’t survive. Over 95 percent of all fossils are marine creatures. They died, and are fossilized, by the trillions. Many are buried in great fossil graveyards, tightly packed together, choked with sediments, buried before they had time to decay. Obviously, they didn’t live in the environment in which they died. But how could any have survived?In the complex of events and conditions that made up the Flood, certainly there were pockets of fresh and/or clean water at any one time. Remember, it was raining in torrents, and we can expect that the rain was fairly fresh water. Many studies have shown that waters of various temperatures, chemistries, and sediment loads do not tend to mix; they tend to remain segregated into zones. It would be unlikely for any one area to retain such zones for very long during the tumult of the Flood, but on a worldwide scale, some such segregated zones would have existed at any given time. Furthermore, we don’t know the tolerance levels of pre-Flood fish for sediment, salt, and temperature. Modern fish have a great variety of responses to different environments. Perhaps before the Flood, fish were even more adaptable.
Snip>>>Many of today’s marine organisms, especially estuarine and tidepool species, are able to survive large changes in salinity. For example, starfish will tolerate as low as 16-18 percent of the normal concentration of seawater.<<<