Breeding Success of Four Male Life-History Types of Spring Chinook Salmon Spawning in an Observation Stream

The relative breeding success of naturally spawning spring Chinook salmon males having different life history strategies was examined in an artificial stream. All the fish originated from the upper Yakima River, Washington State. Seven independent test groups of fish were placed into the stream from 2001 through 2005. Males with four different life history strategies, large anadromous (age-4 and age-5 that spent either two or three years at sea), jacks (age-3 that spent one year at sea), yearling-precocious parr (age-1+ that matured after 18 to 20 months of freshwater residency) and sub-yearling precocious parr (age 0+ that matured after 7 to 8 months of freshwater residency) were used. Their breeding success or ability to produce offspring was estimated by performing DNA-based pedigree assessments. Large anadromous males spawned with the most females and produced the greatest number of offspring per mate. Jacks and yearling-precocious parr spawned with more females than sub-yearling parr. However, jacks, yearling- and sub-yearling precocious parr obtained similar numbers of fry per mate. In the test groups, large anadromous males produced 89%, jacks 3%, yearling precocious 7%, and sub-yearling precocious parr 1% of the fry.  These percentages remained stable even though the proportion of large anadromous males in the test groups ranged from 44 to 88% and tertiary sex ratios varied from 1.4 to 2.4 males per female. Our data suggest that large anadromous males generate most of the fry in natural settings when half or more of the males present on a spawning ground use this life history strategy.