Emigration of age-0 chinook salmon (Oncorhynchus tshawytscha) smelts from the upper South Umpqua River basin, Oregon, USA

Two rotating smelt traps were used through 4 consecutive years to monitor e migrations of age-0 chinook salmon (Oncorhynchus tshawytscha) from two wate rsheds of the upper South Umpqua River basin, Oregon, U.S.A. The number of wild smelts moving past the mainstem South Umpqua River trap ranged from 26 455 in 1991 to less than 5000 in 1993. The number of wild smelts passing t he Jackson Creek trap ranged from 13 345 in 1991 to 0 in 1993. Higher numbe rs of wild smelts were significantly (P = 0.003) correlated with higher num bers of prespawning adults counted in index reaches the preceding year. Tim ing of emigration of smelts was found to be significantly related to stream temperature (P < 0.05) and phase of the lunar cycle (P < 0.05) but not rel ated to changes in discharge (P > 0.05). Median emigration dates, which var ied over 9 weeks, were earlier when spring water temperatures were higher. On average, two thirds of yearly smelt runs occurred when the moon was eith er waning or new, even though these moon phases were present only about hal f of the time. Significantly (P < 0.05) more fish than expected emigrated p ast both traps when day length was increasing.