Vulnerability of early life stage Northwest Atlantic forage fish to ocean acidification and low oxygen

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 4 vital early life-history traits (time-to-hatch, hatching success, post-hatch survival, and growth) of 3 ecologically important estuarine fish species (Menidia beryllina, Menidia menidia, and Cyprinodon variegatus). Offspring were exposed from the egg through the early larval stages to ideal (pH_T [pH total scale] = 7.9, DO [dissolved oxygen] = 9.0 mg l^-1), hypoxic (DO = 1.6-2.5 mg l^-1), acidified (pH_T = 7.4), and hypoxic + acidified (pH_T = 7.4, DO = 1.6-2.5 mg l^-1) conditions. Hypoxia alone significantly delayed hatching of embryos by 1 to 3 d and reduced hatching success of all 3 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 to 45%. Our findings suggest a greater sensitivity of early life estuarine fish to low oxygen compared to low pH conditions, while also demonstrating that the co-occurrence of both stressors can yield both additive and synergistic negative effects on survival and other fitness-related traits. The reduced fitness of forage fish when experiencing acidification and hypoxia may limit the productivity of higher trophic organisms that depend on them as prey.