Additive and Synergistic Effects of Concurrent Acidification and Hypoxia on Early Life Stages of Three Coastal Forage Fish

Global oceans are undergoing acidification and deoxygenation, yet the concurrent effects of low oxygen and acidification on marine fish are unknown. This study quantified the separate and combined effects of low pH and low oxygen on four early life history traits of three ecologically important estuarine fish species (Menidia beryllina, Menidia menidia, and Cyprinodon variegatus). Offspring were exposed from the egg through the early larval stages to control, hypoxic, acidified, and hypoxic + acidified conditions. Hypoxia alone significantly delayed hatching of embryos by one to three days and reduced hatching success of all three species by 24 to 80%. Acidification alone significantly depressed the survival of M. beryllina. Acidification and hypoxia had an additive negative effect on survival of M. beryllina, a seasonal, synergistic negative effect on survival of M. menidia, and no effect on survival of C. variegatus. Acidification and hypoxia had an additive negative effect on length of larval M. beryllina, while hypoxia alone significantly reduced length of M. menidia and C. variegatus from 15–45%. Our findings suggest a greater sensitivity of early life estuarine fish to low oxygen compared to low pH conditions, while also demonstrating both additive and synergistic negative effects on survival and other fitness-related traits.