Oxidative stress in Pacific salmon (Oncorhynchus spp.) during spawning migration.

The energetic and physiological challenges of spawning migrations in semelparous Pacific salmon (Oncorhynchus spp.) have been well characterized. However, the accompanying costs associated with oxidative stress during this nonfeeding migration and the potential connection to senescence have not been explored. Oxidative stress is caused by an imbalance between free radical production and absorption, leading to irreparable cellular damage that accumulates over time and contributes to senescence. The objective of this study was to determine whether oxidative stress occurs during migration between river entrance and spawning for maturing pink salmon (Oncorhynchus gorbuscha), a semelparous species. Samples of plasma, liver, heart, brain, red muscle, and white muscle were collected from individual pink salmon at both the beginning and the end of the freshwater migration and then assayed for antioxidant capacity as well as for oxidative DNA damage. Antioxidant capacity and DNA damage changed between sites on a tissue-specific basis, demonstrating that oxidative stress may be experienced differentially between tissues. Consistent with our prediction, DNA damage was higher and antioxidant capacity lower in plasma (an integrative measure of body condition) and heart tissue at the spawning grounds compared with river entrance. The increased oxidative stress of these tissues is correlated with the senescence and deterioration associated with a semelparous reproductive strategy. However, similar changes were not seen in liver, red muscle, or white muscle. More surprisingly, the antioxidant capacity was higher and DNA damage was lower in the brains of spawning migrants at the spawning grounds than at river entrance. The latter results highlight the importance of tissue-specific variability in understanding the role that oxidative stress may play in spawning migration success.