Fish Survival and the Flood
How did fresh and/or salt water fish survive the flood? This only seems like a challenge to explain because we assume the past is like today. The flood reshaped (wrecked) our world and explains what we see today. God included it in the Bible so we would know why our world is the way it is. Before the flood, there was one land mass and a single water mass. Genesis tells us the land was gathered together. Secular scientists believe in a single land mass, Pangea, as well.
There is no reason to believe the ocean was salty before the flood. During and after the flood, the ocean began to become salty due to all the volcanic activity and erosion (Ice Age melting). The fish not in the ocean stayed the same while ocean fish began to adapt to the salinity increase. We see lots of examples showing creatures were designed to be able to adapt to their surroundings. DNA also tells us that freshwater fish share DNA with both fresh and salty origins. Salmon can go back and forth. There are brackish water fish that can go back and forth between fresh and salt. This is all excellent evidence to support the Biblical explanation.
Additional details available here:
https://answersingenesis.org/the-flood/how-could-fish-survive-the-genesis-flood/
Quickly moving a fish adapted for fresh or salt to the other and having it die, does not prove in any way that fish we see today could not have all been freshwater fish in the past. If you quickly move short hair Australian dogs (dingo) into the wild of northern Canada, they would die. We understand this adaptation, so no big questions arise. Fish adapting to salt is just another adaption.
Keep in mind, adaption and evolution are not remotely the same. Adaption is a loss of genetic information. Take dogs for example. If you breed out all the genes that create hair, you get a hairless dog which has lost the genetic information for hair. We have never seen a single example of adding information to DNA which is required for evolution. We can breed a hairless dog with a hairy dog. The puppy might have hair but no DNA was added to the puppy that was not already present in one of the parents.
Darwin’s famous finches used to “prove” evolution ironically are an example of adaptation not evolution. Darwin did not know about genetics and falsely assumed adaption was evidence for evolution. Secular scientists know and agree adaption is not evolution but continue to teach that it is to indoctrinate kids with the religion of evolution.