The length of residence of juvenile Fall Chinook salmon in Sixes River, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9z903242m

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  • This study was designed to provide life history information about juvenile fall Chinook salmon, Oncorhynchus tshawytscha (Walbaum), in a small coastal river by 1) documenting the length of residence of the juveniles throughout the river, 2) exploring several factors possibly influencing their length of residence, and 3) assessing the relative importance of freshwater and estuarine rearing areas for producing returning spawners. The juveniles were followed from their emergence in the spawning streams to their entry into the ocean. Most information on the length of residence of the juveniles was obtained by seining and trapping at various times and locations in the river. Spawning was distributed throughout the tributary streams, but most egg deposition occurred in Dry Creek. Most fish spawned from November to January. Fry emerged from the gravel from March to May. Newly emerged fry moved downstream from the spawning areas in large numbers at night. Based on experimental studies of juvenile behavior, this movement apparently resulted from emergence at night and lack of visual orientation of the fry during darkness. Downstream movement was reduced during increased light levels (daylight or moonlight). This initial movement of fry is thought to assure rapid dispersal of juveniles throughout the river without extensive energy costs of dispersal by a social mechanism. Many juveniles remained in fresh water until early summer. Most then entered the estuary, possibly because of the influence of high temperature in the main river. A small number of fish continued to reside in the cool spawning tributaries. Detailed studies in 1969 showed that juveniles began entering the estuary in spring, but large increases in the population did not occur until June. During the period of increasing abundance, many juveniles were also captured in the ocean surf. The population level in the estuary peaked at about 145,000 fish during July and August and then declined to a low level in autumn. The rate of growth of the juveniles was reduced for three months during the period of high population abundance. Population density is hypothesized as a major cause of the depressed rate of growth of the juveniles. After the population declined in late summer, growth of juveniles again improved. Following the autumn freshets, most fall Chinook salmon remaining in the estuary and those in the cool spawning streams entered the ocean. A few fish from the tributary populations remained in fresh water through the winter and migrated to the ocean as yearlings the following spring. Based on variation in the length of residence of the juveniles in fresh water and the estuary, five types of life histories were defined. Scale patterns from these types were distinguished and returning spawners from the 1965 brood sorted into the various types. The type-3 fish, those remaining in fresh water until early summer and then remaining for a period of improved growth in the estuary, represented about 90 percent of the returning spawners. Based on the return of these type-3 fish, freshwater and estuarine rearing were concluded to be about equally important to fall Chinook salmon in Sixes River.
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