Answer by John Mackay and Diane Eager
There are two issues here: was the original water in the sea salty; and if so, how could salt water fish survive in freshwater?
The water in Noah’s Flood came from three main sources.
The first was the water that had been created on day one (Gen 1:1-5), then gathered into ‘one sea’ on the third day of creation (Genesis 1:9). We are not sure how salty this was, but since it had covered the earth since day one, it probably had some salt in it even if it simply dissolved the very soluble salt from the earth surface beneath it.
The second source was the rain that fell for forty days and nights at the beginning of the Flood (Genesis 7:12). All data about rain makes this a very fresh source. The last source was from the breaking open of the “fountains of the great deep” (Genesis 7:11), i.e. water from underground reservoirs erupting onto the earth’s surface due to earthquakes and volcanic activity, which also kicked off Noah’s Flood. This would definitely have contained minerals, especially the more soluble ones such as sodium c (sea salt), since the ‘waters below’ which became ‘fountain water’ had been under ground for some 1600 years after the third day of creation. Whilst these fountains may have varied in their salt content, they would most likely have had the same range of mineral mix as underground aquifers and volcanic emissions do today.
We could add the rivers on the land such as the one flowing out of Eden. Presumably they were freshwater, as Adam and animals would have drunk from them.
Thus, the Flood waters would have been a mix of already existing seawater, rain water and underground water. So, if these varied in salt content and the flood changed the mix, could fish have coped? One author of this section is a fisherman and knows how easily sharks can move from fresh to salt water.
Fish and Salt
If you take most salt water fish and dump them straight into freshwater they will die. This is because their salt balance is upset. A marine fish is constantly off-loading excess salt through its gills and kidneys. A river fish, which lives in a low salt environment, does the opposite – it extracts salt from the environment and conserves whatever salt it gets in its diet.
However, many fish can adjust to different salt levels if the change is gradual (hours to days). We know this because there are fish that spend some of their life cycle in freshwater and some in the salty sea. Salmon and trout are hatched in rivers, migrate into the sea where they spend their adult life and when they are ready to spawn they swim into rivers and lay their eggs. As they make the transition from river to sea and back to river they adjust their salt balance mechanisms from extracting salt to off-loading salt and then back to extracting it. Therefore, these fish, along with fish that live in estuaries where the salt levels vary, are able to detect the saltiness of their surrounding environment and make adjustments very easily.
One of the ways they do this is to change the location of special proteins in the cells of their gills. These proteins are called sodium pumps, because they actively move sodium ions across cell membranes. Sodium is one half of a salt molecule – sodium chloride. By moving the sodium pumps from the inner surface the outer surface of the gill cells (or vice versa) they will change the direction the salt flows, i.e. in or out of the gills.
Some fish (such as goldfish) can make the change very slowly but sudden changes in salt can kill them. There are also present day fish that are unable to make the change from fresh water to sea water (or vice versa), but these have probably lost their ability to adjust their salt pumps and make the other adjustments needed to live in water of differing saltiness. This is a change, but it is not evolution. In fact it is the opposite of evolution as it is degenerate change.
Back to the Flood
It is unlikely that fish would have been rapidly transported from one extreme of saltiness to the other during the flood. Since salt water and freshwater have different densities and do not mix instantly, even during the chaos of Noah’s Flood, there would have been pockets and layers of water that varied from very fresh to very salty. Furthermore, fish in Noah’s day would have been less genetically degenerate and been better able to adjust their salt balance as the levels of salt in their immediate surrounds varied. Therefore, the mix of rain water, sea water and underground water would not have been the same problem then as it is now.
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