Why Do Hatchery and Natural Origin Spring Chinook Salmon Differ in Mean Fitness? Insights from Evaluating Differences in Mate Pair Reproductive Success

Evidence for fitness differences between hatchery and natural origin (HOR, NOR) salmonids has mounted in recent years. Hypotheses that explain these findings include: 1) differences in sexual selection on the spawning grounds, and 2) offspring rearing differences prior to the fry swim up stage. We provide evidence that fitness differences between HOR and NOR spring Chinook salmon on the South Fork McKenzie River, Oregon, USA are partially explained by differences in fork-length. We further investigate findings by evaluating fitness differences among ♀_HOR X ♂_HOR, ♀_HOR X ♂_NOR, ♀_NOR X ♂_HOR, and ♀_NOR X ♂_NOR mate pairs. Mean difference between male and female mate pair length, a parameter used to test for size assortative mating, differs among mate pair types (F₃ = 4.66, p < 0.01). In addition, both male length (e^{0.003 ± 0.001}, t = 3.5, p < 0.001) and mate pair length difference (e^{0.003 ± 0.001}, t = 2.4, p = 0.02) are positively correlated with mate pair reproductive success only among ♀_NOR X ♂_NOR mate pairs. We found no evidence that female length was associated with mate pair reproductive success. Results further support the hypothesis that fitness differences are due to sexual selection on the spawning grounds.