Influence of post-ejaculatory reproductive senescence and ovarian fluid on sperm quality and reproductive success in a wild population of Chinook salmon, Oncorhynchus tshawytscha

Reproductive senescence (i.e., the decline in reproductive function associated with increasing cellular age) negatively impacts sperm cell performance, which becomes particularly evident when ejaculate is stored for extended periods of time following collection for supplementation programs. In light of this relationship, female reproductive fluids have been shown to modulate ejaculate quality by enhancing sperm performance across taxa. Chinook salmon ( Oncorhynchus tshawytscha ), a free spawning species, provided an ideal opportunity to study senescent-related interactions in a wild-population that remains reliant on artificial reproduction to maintain population targets and genetic diversity, thanks to their external fertilization mode of reproduction allowing for facilitated experimental manipulation. In this study, ejaculate, ovarian fluid, and eggs were collected from wild spawning Chinook salmon. Ejaculate was stored and sperm quality was assessed at multiple binned time points post-collection (0.5 h, 2.5 h, 4.5 h, 6.5 h) using automated sperm analysis, with two separate activation treatments (river water or a 25% ovarian fluid solution) used at each time point. We measured sperm quality traits (sperm velocity, linearity, longevity, and motility) then conducted fertilization trials at 4.5 h post-collection (reflecting the typical time required to transport gametes from river to hatchery) using a split clutch in-vitro fertilization design to assess activation treatment effects on reproductive success metrics. We found that velocity and linearity significantly declined as a function of post-collection time while longevity and motility were unaffected; however, all traits were significantly improved when activated by the ovarian fluid solution as compared to river water. Fertilization and hatching success did not differ between activation treatments during fertilization trials. These findings provide novel insights into post-meiotic reproductive senescence as it relates to sperm quality and the influence of ovarian fluid, informing management practices with the goal of optimizing artificial reproduction protocols. • Sperm quality traits, namely velocity and linearity, declined over post-collection time • Sperm motility and longevity were unrelated to post-collection time • Ovarian fluid improved all sperm quality traits despite senescent declines • Fertilization success and hatching success were unrelated to ovarian fluid usage during fertilization