You catch a shrimp in the sea. In the afternoon you decide to release it in the freshwater river you finally found. The shrimps will quickly die. WHY?

Explanation:

They need salt in the water to the tune of 35 grams / kg. If they are thrust into fresh water with virtually no salt, they do not get the natural osmosis process taking place.  They will bloat and die.

Interesting question. Nature has a strange way of making things right. Those salt water shrimps can't over run the fresh water ones, for the reason given above.