Chinook salmon spawning behavior : evidence for size-dependent male spawning success and female mate choice

The spawning behavior of unmanipulated groups of tagged and untagged chinook salmon was observed during three spawning seasons on Bogus Creek, Siskiyou County, California. Individually recognizable male and female chinook salmon were observed daily throughout their lives on the spawning grounds. Male chinook salmon were observed to compete for access to females. Larger than average sized males were observed to be in primary (dominant) status with females much more often (44 percent of observations) than smaller males (8 percent). In 35 observed spawnings, primary males always appeared to spawn, and entered the nest first or at the same time as the first satellite male in 27of the 29 spawnings involving more than one male. Individual large males were observed to spawn as a primary male on three occasions, and may have spawned as a primary male on up to 12 occasions. Female chinook salmon exhibited significantly more aggressive behavior toward smaller males and more courting behavior toward equal sized and larger males. All females spawned in a single redd and almost all females (98 percent) were able to protect their redd from re-excavation by other females to within less than two days of death. The mean lengths of time females occupied their red were and 8.8 days during 1984 and 1985, respectively 7.6 days. Based upon behavioral observations in this study and on published literature for other salmonids, it appears that large, dominant male chinook salmon have a reproductive advantage on the spawning grounds. Thus, the reproductive behavior of male chinook salmon may in part counteract any potential for genetic changes in stock specific maturity schedules due to younger aged male spawners. No such advantage was determined for large female chinook salmon in this study, however. Further study on the fertilization success of primary and satellite males, and on how female size relates to redd site selection, nest depth and red defence is recommended.