Biological characteristics and population status of anadromous salmon in southeast Alaska.

Life History Coho salmon typically return to the outside waters of southeast Alaska in July and August and enter freshwater in September and October. The direction of the return migration generally moves from northwest to southeast along the coast. Relatively little is known about the migration routes used by specific coho salmon stocks (Schmidt 1988). Migratory pathways that have been studied indicate some stocks follow a direct route (e.g., Politofski Lake) and others take complicated paths to their natal stream (e.g., Kegan Lake; Shaul and others 1984, 1986). In general, run timing is earliest in outer coast stocks on islands and latest in northern mainland stocks (Van Alen and Wood 1986). Most spawning occurs in late October and November; however, some runs return as early as mid-July and adults may hold for several months before spawning. Spawning areas tend to have groundwater seepage or high intergravel flow rates, and small low-gradient tributaries are commonly used. Hatching of eggs occurs in about 50 days. Fry typically emerge from the gravel in April. In southeast Alaska, all juveniles spend at least 1 year rearing in freshwater before emigration, and most juveniles rear in freshwater for 2 years. This schedule contrasts with coho salmon stocks in southern British Columbia and the Pacific Northwest, where most juveniles spend 1 year in freshwater. The availability of suitable rearing habitat is considered to be the limiting factor for most coho salmon populations. Optimum rearing habitat for coho juveniles is typically found in small streams, beaver ponds, off-channel areas of river flood plains, and lakes (Bryant 1985a, 1985b; Elliott and Reed 1974; Gray and Marriott 1986). These areas all feature a complex habitat structure with abundant aquatic and bank cover (McMahon 1983). Coho smolt typically migrate in May and early June. The mean length of age 2.0 smolts in southeast Alaska is nearly 100 mm. Most coho salmon in southeast Alaska spend about 16 months in saltwater before returning to spawn. Individuals that spend 28 months in the ocean are rare. Adult body length in escapements averages 618 mm for males and 634 mm for females. Coho salmon from northern stocks typically are larger than those from southern stocks. This difference may reflect the later timing of the run for northern stocks, which provides additional opportunity for growth in the summer of return (Van Alen and Wood 1986). Survival of rearing juveniles to return as adults, based on coded-wire-tag studies of 13 coho stocks, is 5.6 percent (SE = 0.2 percent) and includes both the late segment of freshwater rearing and oceanic life history stages. The estimated mean ocean survival rate, from smolt emigration to adult return, is 10 percent (SE = 0.69 percent) for six coho stocks in southeast Alaska, when all years for which data are available are included in the average. During the late 1980s, however, estimated ocean survival averaged 14 percent (Shaul and others 1991). Jacks are males that smolt in late spring and return to spawn in the fall, after spending only 4 to 5 months in saltwater. The proportion of jacks present in spawning runs has been determined for few stocks in southeast Alaska. Coho salmon are a valuable resource in the commercial, sport, and subsistence fisheries of southeast Alaska. In 1987, 1.5 million coho were harvested in southeast Alaska fisheries (Wood 1992). The $17-million value of this harvest was the highest single-species contribution to the total economic value of all salmon species