Effect of lunar-phased releases on adult recovery of coho salmon from Trinity River and Iron Gate hatcheries in California

Abstract

Previous studies on coho salmon ( Oncorhynchus kisutch) have shown a smolt-associated rise in plasma thyroxine on the new moon closest to the vernal equinox, and that survival of fish transferred to seapens was related directly to the fraction of the thyroxine rise completed before entry into seawater. Heretofore, hatchery releases of coho salmon were conducted at the discretion of the hatchery manager in California within a designated period in spring. A lunar-phased release study of coho salmon was conducted at Trinity River and Iron Gate hatcheries from 1981–1984 in collaboration with the California Department of Fish and Game. Total recovery (adult hatchery return and estimated ocean catch) of the optimal group released on the new moon nearest the vernal equinox was significantly higher than earlier release groups at Trinity River for 1981 and 1984 but significantly higher than the later release groups only in 1981. The results from Iron Gate were less clear because the groups released after the optimal new moon did as well in 1981 and significantly better than the group released on the optimal new moon in 1984. Percent recovery of random release groups prior to this lunar-phased release study were generally lower (3 out of 4) but we cannot ascribe this definitively to lunar-phased releases because of unpredictable environmental and other conditions which can alter year-to-year variations in recoveries. Data for the 1982 and 1983 release groups are not reported because the effects of El Niño drastically reduced the recovery of coho salmon during those 2 years.