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Fall Chinook (Oncorhynchus tshawytscha) Spawning Escapement Estimate and Age Composition for a Tributary of the Smith River, California—23-Year Analysis


In the fall of 1980, a spawning escapement study was initiated on the West Branch Mill Creek, a major fall chinook salmon (Oncorhynchus tshawytscha) tributary of the Smith River, Del Norte County, California. The purpose of the study was to estimate the relative abundance of spawning fall chinook in a defined study section over a period of more than 20 years as habitat changed. An age composition of the spawning population was also determined. The 23-year analysis of the 2.7 kilometer (1.7 mile) study section of the West Branch Mill Creek provided “minimum” chinook spawning estimates from 1980—2002. The annual spawning chinook estimates ranged from 31–361 total chinook. The 23-year mean for chinook spawners was 151 fish (89 fish per mile). The spawner sampling verified that three distinct chinook runs exist for fall chinook entering the West Branch Mill Creek. Scale sample analysis was used to determine the age composition of West Branch Mill Creek chinook spawners. Age analysis for 22 years (1980 excluded) showed that the overall percentages for female spawners was 53% (4-year olds), 38% (3-year olds), and 9% (5-year olds). The age composition of male spawners showed a high degree of variability throughout the study. Male chinook of age 2, 3 and 4 were dominant annually, but 5- and 6-year-old fish were present in most spawning seasons. All chinook carcasses from which scales were collected were also measured for length (over 1240 samples in 22 years). Decreases in mean length were documented for all age classes for each of the El Niño episodes that occurred during the study (1982–84; 1992–93; 1997–98). The decreases in mean length appeared to carry forward for each cohort’s age class. The total number of chinook redds was tabulated by counting “fresh” redds during weekly spawning surveys. The mean number of redds was 117 for the 23-year period with a mean of 0.9 redds per adult salmon or 1.8 redds per female. The abundance of chinook spawners dramatically increased during the last three years of the study, probably a reflection of improved ocean survival during the recent Pacific Ocean Decadal Oscillation phenomenon.,

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