Fish in saltwater tend to lose water from their bodies because the surrounding water is saltier than their body fluids. 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. There are migratory species of fish that travel between salt and fresh water. For example, salmon, striped bass, and Atlantic spurgeon spawn in fresh water and mature in salt water.
Eels reproduce in salt water and grow to maturity in fresh water streams and lakes.
So, many of today's species of fish are able to adjust to both fresh water and salt water. There is also evidence of post-flood specialization within a kind of fish. Indeed, most of the families alive today have both fresh and saltwater representatives. This suggests that the ability to tolerate large changes in salinity was present in most fish at the time of the flood. Specialization, through natural selection, may have resulted in the loss of this ability in many species since then.