apply to Atlantic salmon as well. Although strays probably have lower reproductive success than fish that are returning to their native streams, they do provide some potential for new genetic combinations—important for the salmon’s evolutionary potential in the face of changing environments—and perhaps more importantly, they allow for recolonization of streams if a local population disappears. In some ways, a metapopulation structure can be likened to the structure of a large tree. A few branches can be lost without serious damage, but if only a few branches survive with little communication among them, the tree’s survival is in doubt.
The complex physiological transition to salt water at the smolt stage requires suites of behavioral adaptations for navigation and avoidance of predators, including seals, cormorants, and striped bass, and for finding marine foods, including invertebrates and fish. Survival of smolt to 2SW stage would have to be about 2% (based on Baum’s estimate of 90 smolts produced per female) to maintain a steady population. Decrease in either freshwater or oceanic survival would cause a decline of Maine’s wild salmon populations.
The anadromous pattern, with some repeat spawning, means that counting the fish returning to a stream gives information only on a small part of the population. The rest of the population is either in the river as fry, parr, or smolts or at sea preparing to return. In addition, salmon have overlapping rather than discrete generations. The presence of early maturing males (precocious parr) tends to buffer the population somewhat against random variation among the anadromous (adult) male spawners each generation (Martinez et al. 2000, Garcia-Vazquez et al. 2001). Repeat spawning is important because of the increased egg production of older females and their success in the face of natural selection.
Finally, for Atlantic salmon populations to have colonized and survived for extensive periods near the southern limit of the species’ range (currently Maine), they probably had to acquire adaptations to the distinct physical and environmental challenges of local waters. Local adaptations, promoted by strong homing and strong selection pressures, are known for salmon populations throughout the world.