ve correlation between fecundity and size of females. Both males and females die soon after spawning.
Eggs hatch after 8-12 weeks of incubation, the time being inversely related to water temperature. Hatchlings remain in the gravel until their yolk sacs have been absorbed, 4-10 weeks after hatching. Upon emerging, they seek out swallow water along the stream margins. Initially they form schools, but as they grow bigger the schools break up and the juveniles (parr) set up individual territories. The larger parr tend to occupy the heads of pools (Chapman and Bjornn, 1969). As the fish continue to grow, they move into deeper water and expand their territories until, by July and August, they are in deep pools. Territory size for coho is inversely related to the amount of food available.
Between December and February, winter rains results in increased stream flows and by March, following peak flows, fish again feed heavily on insects and crustaceans and grow rapidly. Toward the end of March and the beginning of April they begin to migrate downstream and into the ocean. Out-migration in small California streams typically peaks from mid-April to mid-May, if conditions are favorable. Migratory behavior is related to rising or falling water levels, size of fish, day length, water temperature, food densities and dissolved oxygen levels. At this point, the outmigrants are about one year old and 10-13 centimeters in length. The fish migrate in small schools of about 10-50 individuals. Parr marks are still prominent in the early migrants, but later migrants are silvery, having transformed into smolts.
After entering the ocean, the immature salmon initially remain in near-shore waters close to their natal stream. They gradually move northward staying over the continental shelf. Coho salmon can range widely in the north Pacific. The movements of California fish were poorly known until the Pacific States Marine Fisheries Commissions (PSMFC 1994) Regional Mark Information System. It appears that recent coded wire tag data indicates that at least 65-92% of Californias coho salmon feed in the oceans off our coast. This is also true of Oregons coho as most coho caught off the California coast in ocean fisheries were reared in Oregon streams (natural and hatchery). In 1990 for instance, commercial and recreational fisheries caught 112,600 coho, a number that greatly exceeds the present production capabilities of California populations alone. (A. Baracco, California Department of Fish and Game, personal communication.)
Oceanic coho tend to school together. Although it is not known if the schools are mixed, consisting of fish from different streams, fish from different regions are found in the same general areas. Adult coho salmon are primarily piscivores, but shrimp, crabs and other pelagic invertebrates can be important foods in some areas.
Spawning sites are typically at the head of riffles or tail of pools where there are beds of loose, silt free, course gravel and where cover exists nearby for adults. Unlike other salmon species, coho salmon redds can be situated in substrates composed of up to 10% fines (Emmett, et al, 1991), but in general, spawning success and fry survival are favored by very clean gravel consisting of less then 5% fines (California Department of Fish and Game 1991). Spawning depths are 4-21 inches, with water velocities of 6.5 to 26.2 feet per second (Hassler1987). Optimal temperatures for development of embryos in the gravel are 43-50 degrees F, although eggs and alevins can be found in 40-70 degree F water. Dissolved oxygen levels should be above 8 mg/l for juveniles (Emmett, et al. 1991).
Juveniles prefer deep (greater than 3 feet), well shaded pools with plenty of overhead cover; highest densities are typically associated with logs and other woody debris in the pools or runs. Juveniles require water temperatures that do not exceed 71-77 degrees F for extended time and oxygen and food (invertebrates) levels remain high. Preferred temperatures are 50-59 degrees F (Hassler, 1987); preferred water velocities for juven