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Biology and Evolution of Atlantic Salmon

NATURAL HISTORY OF ATLANTIC SALMON IN MAINE

Anadromy

Atlantic salmon, like their Pacific cousins, are anadromous: they begin their lives in fresh water, where the young grow to several inches in length, and then migrate to the sea, where they grow more rapidly and become sexually mature after 1, 2, or 3 years1 (Baum 1997). Maine’s Atlantic salmon exhibit two run timings that are in part influenced by genetic factors. “Early run” adults enter fresh water between May and mid-July, and “late-run” adults enter fresh water later in the summer. Some rivers have only early or late-runs, large rivers have both. Unlike their Pacific cousins, which always die after spawning, some Atlantic salmon survive spawning and return to sea, either soon after spawning or the following spring (1–6% according to Baum 1997). Some of these fish might spawn again.

Homing

When salmon return to rivers from the sea to spawn, they return to their natal stream (the stream where they hatched). Straying, or returning to an-

1  

Fish that return after 1 year are termed 1SW (one sea-winter) fish; 2SW and 3SW mean fish that spend two and three winters at sea, respectively. Fish that spend multiple winters at sea are called multi-sea-winter (MSW) fish.



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Genetic Status of Atlantic Salmon in Maine: Interim Report from the Committee on Atlantic Salmon in Maine 2 Biology and Evolution of Atlantic Salmon NATURAL HISTORY OF ATLANTIC SALMON IN MAINE Anadromy Atlantic salmon, like their Pacific cousins, are anadromous: they begin their lives in fresh water, where the young grow to several inches in length, and then migrate to the sea, where they grow more rapidly and become sexually mature after 1, 2, or 3 years1 (Baum 1997). Maine’s Atlantic salmon exhibit two run timings that are in part influenced by genetic factors. “Early run” adults enter fresh water between May and mid-July, and “late-run” adults enter fresh water later in the summer. Some rivers have only early or late-runs, large rivers have both. Unlike their Pacific cousins, which always die after spawning, some Atlantic salmon survive spawning and return to sea, either soon after spawning or the following spring (1–6% according to Baum 1997). Some of these fish might spawn again. Homing When salmon return to rivers from the sea to spawn, they return to their natal stream (the stream where they hatched). Straying, or returning to an- 1   Fish that return after 1 year are termed 1SW (one sea-winter) fish; 2SW and 3SW mean fish that spend two and three winters at sea, respectively. Fish that spend multiple winters at sea are called multi-sea-winter (MSW) fish.

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Genetic Status of Atlantic Salmon in Maine: Interim Report from the Committee on Atlantic Salmon in Maine other stream, occurs, but only occasionally. For example, Penobscot River salmon show more than 98% fidelity to the home stream (Baum 1997). Distribution Atlantic salmon are distributed from about 40° N, northward, on both sides of the Atlantic Ocean. In Europe, wild salmon breed from western Russia to Iceland and south to northern Spain; on the western side, wild breeding populations are found from Labrador to Maine. These distributions have been influenced by geological changes, including ice ages (MacCrimmon and Gots 1979). Populations in the United States probably date from the end of the last ice age 10,000 years ago. Atlantic salmon were probably present in all watersheds from the Hudson River in New York north to the St. Croix River, the Canadian border (Kendall 1935). Atlantic salmon once occupied 34 rivers and streams in Maine (Beland 1984). Today, wild Atlantic salmon populations in the United States are found only in Maine,2 from the lower Kennebec River in the southwest to the Canadian border. Maine’s salmon take part in extensive marine migrations, including movements to the waters off western Greenland (Friedland 1994), where they become a small portion of a large mixed-stock complex of salmon from both European and North American sources. Unlike Atlantic salmon populations across the Canadian border from Maine, where 1SW fish are common among spawning adults, about 94% of adults returning to Maine are 2SW fish (USASAC 1999). Thus, the average body size of Maine adults is larger than Canadian adults. Because spawning populations of Maine salmon include several age groups (especially 2SW and 3SW adults but also precocious mature parr3), there is considerable exchange of genetic material across age classes (cohorts). Atlantic salmon spawn in freshwater streams on both sides of the Atlantic (Figure 2). The oceanic fishery in recent years has been concentrated to the west of Greenland, where fish from North America and Europe are encountered (Reddin andFriedland 1999). Natural movement (straying) of spawning 2   Many Atlantic salmon have escaped from farms off the west coast of North America and concern has been expressed about their becoming established there (e.g., Volpe et al. 2001). Although adult Atlantic salmon have been observed in streams there, they are not wild and have not become established. 3   Parr are young salmon growing in fresh water. Precociously mature parr are young males that spawn without first migrating to the sea.

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Genetic Status of Atlantic Salmon in Maine: Interim Report from the Committee on Atlantic Salmon in Maine fish from Europe to North America, or the reverse, is extremely limited (Reddin et al.1984). Reproduction Spawning occurs in autumn, and the eggs develop in gravel nests that are dug by the female. Because Maine’s females are large 2SW fish, they deposit about 7,200 eggs. The fry emerge in mid-May and grow into parr. Vertical bars on the parr camouflage them, providing protection from predators. The territorial parr attempt to occupy portions of the river that allow rapid growth on insect fauna. Most parr remain for 2 years in freshwater, before becoming smolts and migrating to the ocean. Occasionally, parr will mature in the stream5 and can have some success in fertilizing eggs. Survival from the egg to the smolt stage is estimated to be 1.25% (Bley and Moring 1988, Baum 1997), and thus a rough calculation from Baum’s data suggests that an average of 90 smolts are produced by a wild Maine 2SW female. CONSEQUENCES OF SALMON LIFE HISTORY The salmon life-history pattern is of enormous import for the evolution, survival, and current problems of Atlantic salmon. Because the fish migrate upstream to spawn, they are visible and a source of food and recreation to many people, even those who do not live by the sea. Because salmon spend time in both ocean and freshwater environments, they are subjected to the vagaries of two systems that are only loosely connected. This pattern leads to a kind of double jeopardy and complicates understanding of the factors that affect their populations. The pattern of homing to their natal streams leads to a variety of local adaptations, including the timing of spawning runs, growth rates, and other life-history features (e.g., Gharrett and Smoker 1993, NRC 1996, and Smoker et al. 1998 for Pacific salmon species; Kendall 1935, Saunders 1981, Heggberget et al. 1986, Verspoor et al. 1991, Kincaid et al. 1994, Webb and McLay 1996, Hutchings and Jones 1998, and Nielsen 1998 for Atlantic salmon; and Allendorf and Ryman 1987 and Taylor 1991 for both Atlantic and Pacific salmon). The occurrence of at least some straying allows the development of what geneticists call a metapopulation structure. The importance of the metapopulation structure for the survival of Pacific salmon species was discussed in an earlier NRC report (NRC 1996), and the conclusions

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Genetic Status of Atlantic Salmon in Maine: Interim Report from the Committee on Atlantic Salmon in Maine FIGURE 2 Map showing the wild Atlantic salmon’s range in 2000 and its known migration routes. Source: The Status of Wild Atlantic Salmon: A River by River Assessment. Reprinted with permission from World Wildlife Fund—U.S., Copyright 2001.

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Genetic Status of Atlantic Salmon in Maine: Interim Report from the Committee on Atlantic Salmon in Maine 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.