One study on coho salmon found that 75% of jacks passed on their genes, whereas only 58% of older, larger males were successful at reproducing.
During mating, male salmon frequently cross from side-to-side just downstream from the female as she prepares her redd (nest). From this position, the male can drive off competing males and detect odors indicating the female’s readiness to lay her eggs so that he can fertilize them. If multiple males are present, they will typically line up in order of dominance with the most dominant male closest to the female. However, jacks (young male fish that typically return to the rivers as two-year olds) are smaller and can often sneak in close to the female to pass on their genes.
The fish in this video are fall-run Chinook salmon spawning in a California tributary.