Catadromy

The freshwater eels
For years, European eels were extremely popular food fish and the subject of a bit of a mystery. Although they were fished in rivers all over Europe, few, if any, of spawning age were ever found. It was not until much later that scientists realized that these eels were swimming over 4,000 miles to the Sargasso Sea to do their spawning. After maturing in fresh water rivers for nine to fifteen years, European eels and their American counterparts undertake an impressive migration across thousands of miles of ocean to reach their spawning grounds near the Bermuda Islands. Once the young eels hatch, they drift with the Gulf Stream until some signal tells them it's time to get off, and they head up freshwater rivers in the United States and Europe to begin the freshwater phase of their lives

World's smallest
A catadromous goby just happens to be the world's smallest animal with a backbone. The dwarf pygmy goby, Pandaka pygmaea, boasts a modest overall length of 11 millimeters. That's just 13/32 of an inch--about the width of your pinkie nail.

Gobies and galaxiids
Gobies are an incredibly successful group of fishes that have not only conquered salt and freshwater but have even moved out onto the land. The famed mudskippers move from pool to pool looking for food, some even climbing up into trees. Several goby species have penetrated the freshwater habitats of several tropical islands but always return to the ocean to spawn. These gobies do not go through physical changes when switching from one habitat to the next. They have an amazing tolerance for very low salinities.

Galaxiids are distant relatives of salmon, sort of like backward cousins. These fish look like skinny trout with no scales and all the fins pushed back towards the tail. They are found exclusively in the Southern Hemisphere. The galaxiids spawn in tidal grasses and estuaries. As adults they only grow to about six inches.