The species moving upriver 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. 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 upriver and even its young, the elvers, are part of the upriver migration of fish albeit they are the young eels.
Many species moving upriver 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 are what is needed to complete the cycle.
The American shad, whose migrating numbers of fish far outstrip any other species, does not make a bed for its eggs. They are mainstream 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.
The Atlantic salmon needs shallow and smaller streams with small gravel and a guaranteed over-winter flow that is not strong enough to destroy the nest. The stream flow needs to be sufficient to guarantee that the stream will not freeze to the bottom. Bottom ice, better known as anchor ice, will deny the eggs the flowing water with its life-giving oxygen.
Sea lamprey, the species that everyone wrongly loves to hate, need similar bottom and river flow conditions as the Atlantic salmon. The lamprey does not feed when it returns to fresh water so you will not see lamprey marks on our fresh water fish. The lamprey builds a nest as a place to deposit their eggs. Once hatched, the young, called ammocoetes, spend several years drifting downriver when not burrowed in fine sediment on the bottom where they filter-feed on microorganisms.
The striped bass is a fickle visitor in our lower river. What triggers the march of other fish upriver triggers the stripers’ migration but they seldom get beyond the dam at Holyoke, MA. They spawn in fresh to brackish water and only occasionally move farther upriver, beyond where the river is no longer tidal. That point is at Enfield, CT.
Upriver migrating fish either are mechanically lifted above individual dams or navigate their way above the dams using fish ladders, at least on the main river. Once they leave the main river in many tributaries migrating species face limited access to the smaller streams they need for successful spawning due to the dams in those rivers and streams. These dams, usually not retrofitted with fish ladders or other means of passage deny access to needed spawning habitat. That habitat remains cut off from the main river.
Yet the investment in fish passage as far as it goes has had some real impact on the return of our migrating species. Numbers of American shad have gone from zero above Enfield in the 1950s to well over 300,000 in the upper river. The bottleneck at Turners Falls Dam caused by an ill-designed fish ladder system and the Entergy thermal discharge blockage at Vernon are on their way to being resolved through the relicensing now underway at the Turners Falls Dam and the closure of Vermont Yankee later this year.
In order to open further the watershed to our migrating species, the next investment must be to bypass or remove dams that block our long-distance travelers from their necessary spawning habitat. It will be worth the investment.
David Deen is River Steward for the Connecticut River Watershed Council.