One-to-one spawning

Last week we shared a video of a female Chinook salmon spawning with four males, including a large, dominant male and sneaky, small sized jacks. In contrast, the video above captures a female spawning with a pair of Chinook salmon. While at this second location we witnessed several spawning events between one male and one female. These two spawning behaviors were observed on two different rivers within the same basin and were consistent with our field observations in each river. We were curious about the pattern and decided to look at preliminary data from our fish counting weirs. It turns out, the river in which females are spawning with many males has a greater ratio of males to females (1.6 males to each female), with a large percentage of smaller-sized males (58% of males are jacks). Conversely, in the river where we observed more one-to-one spawning, the ratio of males to females is more even (1.1 males for each female) and a much smaller percentage of males (27%) are jacks. We realize there are other factors such as temporal and spatial variation in spawning behavior that may be in play, but it is still fun to ponder the basis for these two different behaviors.