Title is self explanatory. Do freshwater fish ever accidentally find themselves in brackish or saltwater? How do they deal with that?
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Fish can tell to a degree when the water quality around them is changing, particularly with salinity. When you approach the ocean from the river you usually run into a small area between the two where the water is brackish, not salty enough to be marine but saltier than freshwater. The fish can survive here long enough to turn back, though in floods fish may be washed out to see far enough that they can’t easily return. Saltwater on freshwater fish actually dehydrates them, because the higher salinity outside the body forces water to leave their bodies. Marine and Brackish fish are adapted to deal with this higher salinity however.
And there are a few types of fish that can traverse both. Salmon are one notable fish, and Bull Sharks have been known to swim upriver for long periods of time. A common aquarium fish, the Molly, is also able to adapt to freshwater, brackish, or even marine water so long as they slowly adjust themselves to it.
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