Updated on April 21, 2016
Fish swimming past a fish window (mostly shad)
Some fish migrate between fresh and salt water in order to spawn. Fish that migrate from freshwater to saltwater and back again (called anadromous) – or the reverse (catadromous) – in order to spawn are known collectively as diadromous fish.
Anadromous fish include American Shad, Striped Bass, and Salmon. They are born in fresh water and swim downstream to the ocean to spend several years feeding and growing (a trip called out-migration) until they reach sexual maturity and then back again to freshwater to mate (a spawning run).
After being born in the river, the young fish seek out brackish water (a combination of fresh water and salty ocean water) that has the best degree of salinity for them to grow in. (The degree of salinity of the water in a certain place is determined by how far up the river the area is.) The area with the best salinity can change somewhat from season to season, but in the Hudson River, for example, young striped bass tend to do best and can therefore be found between Newburgh Bay and the Tappan Zee. Eventually, the juveniles swim out to sea and spend several years eating and growing. When they reach sexual maturity, they head back to their home waters to reproduce.
There are also fish that follow the reverse path: they are born at sea, migrate to fresh water to grow and mature, and then return to sea to spawn. These fish are called catadromous. The American Eel, found on the Atlantic coast, is catadromous: after being born in the Sargasso Sea, juveniles migrate up rivers to grow and mature before migrating back to the Sargasso Sea to spawn.
The spawning run is a time when fish counts are often taken. Some fish counts are done by scientists and professionals and some also rely on volunteers for help.
Some counts use fish windows to help with the counting.
– Roz Cummins