dian border but today are found only in Maine, from the lower Kennebec River to the Canadian border. The populations have declined drastically, from perhaps half a million adults returning to U.S. rivers each year in the early 1800s to about 1,000 in 2000.
Salmon spawn in fresh water, where the young hatch and grow for a year or 2 before migrating to sea. At sea, they grow faster in the rich marine environment and then return to the rivers where they hatched (called natal streams) to spawn. Most fish die after spawning, but some return to the ocean, and some of those return to spawn again. Adults return to their natal streams; only about 2% stray to other (usually nearby) streams.
The occasional straying is probably important evolutionarily, because it allows recolonization of a stream if the local population dies out and provides for small infusions of new genetic material for evolutionary adaptation to changing conditions. Their homing provides an opportunity for the salmon to adapt to environmental conditions in their natal streams. This complex life-history pattern makes salmon vulnerable to environmental disruptions both at sea and in fresh water. It also complicates the understanding of the genetic makeup of salmon populations because of the relationship between local adaptations and exchange of genetic material through occasional straying.
Augmentation of wild populations of Maine salmon with hatchery releases began in the early 1870s. At first, young fish were obtained from Lake Ontario, and then the Craig Brook Hatchery, using eggs from Penobscot River fish in Maine, was the stocking source. By the 1920s, Canadian eggs were being used, followed in the 1940s by eggs from the Machias, Penobscot, and Dennys rivers of Maine. In the 1950s and 1960s some eggs of Canadian origin were used again, but by the late 1960s, eggs from Maine’s Machias, Narraguagus, and Penobscot rivers were used. Fish reared in hatcheries derived from Penobscot River fish were used until late 1991, when the practice of river-specific stocking was adopted. The protocol used since involves catching young, actively feeding fish (parr) in the river, rearing them to maturity in the hatchery, mating them, and releasing the resulting fry into the rivers before they start to feed.
In addition to stocking, which at least until 1992 added to rivers many fish (and eggs) whose genotypes did not reflect adaptation to the local environment, aquaculture (farming) of Atlantic salmon began in Maine in the 1980s,