In spring, the Connecticut River becomes a highway. Moving along this migration artery are fish stimulated to do so by the urge to spawn and extend the life of their species to the next generation. Water temperature, hours of sunlight, flow level and some say the taste of springtime water triggers these organisms and sends them doggedly up river.
The species moving up river are the American Shad, Atlantic salmon, Blueback Herring, Gizzard Shad, Sea Lamprey and Striped Bass. This is quite an array of fish and some species are moving in substantial numbers.
For the most part fish need shallower fresh water that is quieter, slower flowing in order to spawn successfully then they could ever find in the ocean. The ocean is a great place to find food and grow into a spawning adult but it is hard to find mates and harder still for the eggs to be successful. Over the eons, these species have become reliant on fresh water habitat in order to be successful.
Therefore, the instinct is up river, except of course for the American Eel that goes as an adult out to the Sargasso Sea to spawn. However, it is the only outlier in this parade of fish moving up river and even its young, the elvers are part of the upriver migration of fish albeit they are the young not the adults eels.
Many species, alewives and blueback herring on the move up river head into headwater streams or ponds. The water levels, river bottom, flow composition and in some cases, the particular size of the river bottom gravels is what is needed to complete the cycle.
The American shad whose migrating numbers of fish far outstrip any other species, do not make a redd for its eggs. They are mainstem free river spawners, with males surrounding a gravid female and when she releases the eggs, the males release their milt. The fertilized eggs are slightly heavier than water, just below buoyant so they drift with the current and eventually find the bottom where they gestate.