Emigration of age-0 chinook salmon (Oncorhynchus tshawytscha) smolts from the upper South Umpqua River basin, Oregon, U.S.A.

Abstract

Two rotating smolt traps were used through 4 consecutive years to monitor emigrations of age-0 chinook salmon (Oncorhynchus tshawytscha) from two watersheds of the upper South Umpqua River basin, Oregon, U.S.A. The number of wild smolts moving past the mainstem South Umpqua River trap ranged from 26 455 in 1991 to less than 5000 in 1993. The number of wild smolts passing the Jackson Creek trap ranged from 13 345 in 1991 to 0 in 1993. Higher numbers of wild smolts were significantly (P = 0.003) correlated with higher numbers of prespawning adults counted in index reaches the preceding year. Timing of emigration of smolts was found to be significantly related to stream temperature (P < 0.05) and phase of the lunar cycle (P < 0.05) but not related to changes in discharge (P > 0.05). Median emigration dates, which varied over 9 weeks, were earlier when spring water temperatures were higher. On average, two thirds of yearly smolt runs occurred when the moon was either waning or new, even though these moon phases were present only about half of the time. Significantly (P < 0.05) more fish than expected emigrated past both traps when day length was increasing.