How Could Saltwater and Freshwater Fish Survive the Flood?
Related Questions: Why didn't the hot, salty, subterranean water kill all freshwater fish during the flood? How did saltwater fish survive before the flood? Were preflood fish adapted to salt water or fresh water?
Chemistry of Body Fluids in Fish. Blood and other body fluids of almost all fish, freshwater and saltwater, have surprisingly similar chemistry. Their blood's salinity, for example, is somewhere between that of fresh water and salt water. Actually, its concentration is about one-third that of normal seawater, not just for salt (NaCl) but for many other substances.1 For reasons that will soon be apparent, a typical preflood sea probably had a small salt content, as if you mixed two parts of fresh water with one part of seawater. However, just as oceans and seas today have variations in salt content, variations probably existed in and among preflood seas—perhaps large variations.
Living things have many marvelous, semipermeable membranes that allow some liquids or gases to pass through, but not others. For example, capillary walls are semipermeable membranes. Oxygen in our lungs can pass through capillary walls and mix with our blood, but blood does not normally pass through those walls. Substances that can pass through the membrane (such as oxygen) will, on balance, go from the higher concentration (in the lungs) to the lower concentration (in the blood). This is called osmosis.
Fish have a water problem. Freshwater fish have greater salinity in their blood (less concentration of water) than is in the water they swim in, so water seeps into their blood by osmosis. To correct this problem, freshwater fish seldom drink, and their kidneys secrete a watery urine. Conversely, saltwater fish have less salinity in their blood than is in their saline environment, so osmosis forces water from their bodies. Their kidneys pump out so little water that saltwater fish seldom urinate.
Mixing. During the flood, fish would have tried to stay in the most comfortable regions of the volume of water that was their preflood habitat. Salty, subterranean water, erupting onto the earth's surface, would not have rapidly mixed with the less salty preflood seas. In fact, the larger a preflood sea, the slower it mixed and diffused, and the better it insulated its fish from muddy, hot, salty currents during the flood.2 Besides, preflood seas would have tended to "float" on the denser, muddier, saltier water.
In one 55-gallon experiment, a layer of freshwater floated on a typical layer of seawater. Several freshwater fish, salt-water fish, and other organisms placed in the tank lived in their respective environments for 30 days. The fish even made brief excursions into the more hostile environment.3 No doubt fresh water and salt water would mix at increasingly slower rates per unit volume if the experiment were scaled up to the size of a global flood.
Natural Selection. After 150 days (according to Genesis 8:3), flood waters began to drain into newly formed ocean basins. Fish trapped in continental basins were the potential ancestors of our freshwater fish. Rainfall over the next several decades diluted the salt concentration in most postflood lakes.4 Natural selection eliminated fish in each generation that could not tolerate the dechning salinity. Those that could, had less competition for resources and could reproduce their tolerance for lower salinities. Because fish reproduce frequently and profusely, limited variations in each generation allowed rapid adaptation in their ability to control the water in their bodies. This is micro evolution, not macro evolution. No new organs were needed.
Meanwhile, fish that ended up in the new oceans either had to tolerate slowly increasing salinity or face extinction. Survivors became our saltwater fish. Those unable to adapt are now extinct. (This largely explains why marine animals have experienced the most extinctions.) Some fish, the best-known being salmon, are adapted to both fresh water and salt water. Wider salinity tolerances, such as those of salmon, may have existed before the flood.
Design. The ability over many generations to adapt to changing environments is a wonderful feature designed into all life. Without this capability, extinctions would be more common, and life would eventually cease— beginning, perhaps, near the bottom of the food chain. But adaptation has never produced macroevolution.
THIS IS ALL A REDUNDANT QUESTION AND THEORY BECAUSE OF THE MISTAKEN IDEA THAT NAOH'S FLOOD WAS GLOBAL IN NATURE. IT WAS NOT! IT WAS REGIONAL. SEE THE FULL IN-DEPTH STUDY ON THIS WEBSITE CONCERNING NOAH'S FLOOD.
THE WORLDWIDE FLOOD IS GENESIS 1:2. IN THAT VIOLENT FLOOD ALL LIVING CREATURES ON LAND AND SEA WERE DESTROYED; HENCE GOD HAD TO CREATE AGAIN ALL THE LAND AND SEA CREATURES AS WE ARE TOLD IN GENESIS CHAPTER ONE.