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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No, freshwater fish and saltwater fish cannot live in the same kinds of water. Their bodies are designed to carefully manage the amount of water and salt in them (just like ours are). If they entered water that was too salty – or not salty enough – for them, and they couldn’t swim out, they would die.
I’m not sure it is an accident …
Salmon start in fresh water. They migrate to the ocean. They live there for years, then return to fresh water to spawn and die.
Some rainbow trout can migrate to salt water and “become” (get renamed) to steelhead trout. They spend a couple of years then return to spawn.
They can when there are floods, storm surges, or other special situations, but other than that they avoid salty water as they’re not evolved for it and prolonged contact will kill them.
Certain kinds of fish spawn in one type of water and migrate to the other though.
Catadromous fish spawn in salt water, then move to fresh water for their adult life (eg. certain species of eel). Anadromous (like salmon) do the opposite, they spawn in freshwater and move to salt for their adult life. The overall term for these types of fish is Diadromous.
– https://www.encyclopedia.com/science/news-wires-white-papers-and-books/catadromous-diadromous-and-anadromous-fishes
There’s not a definite line between salty water and fresh water.
Generally rivers will end with a delta, where the tides change the salinity of the water.
There’s some fish that can survive in this environment, but generally they’ll just keep to fresher waters.
Catastrophic events like a flood can cause them to be washed out to sea. Unless they make it back to fresh water quickly, they will die.
Fish can tell when the water around them is increasing in salinity and swim back up toward freshwater. I’m sure some get washed out in floods or similar situations, but most of the time most fish would be able to avoid accidentally winding up in the ocean.
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.
The vast majority wouldn’t, simply because lots of river fish live far up rivers away from the entry point to the oceans. Storm surges could push fish down rivers and into oceans, but it depends again on the distance and location of the flood. If you’re far enough up, it’s more likely a river fish in a flood ends up in a lake, or deposited in a flooded field where it dies once the water recedes.
At any rate, most river fish wouldn’t want to wind up in the ocean, as they normally live in freshwater, and saltwater will slowly kill them. There are examples of fish moving between the two though at different stages of life, and there are also fish who live in estuaries where the rivers meet the oceans, and they can handle a bit of both (it’s usually more salt than fresh at that point though, especially in large wide tidal estuaries).
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