The combined effects of ocean warming and ocean acidification on Pacific cod (Gadus macrocephalus) early life stages

The eastern North Pacific is simultaneously experiencing ocean warming (OW) and ocean acidification (OA), which may negatively affect fish early life stages. Pacific cod (Gadus macrocephalus) is an economically and ecologically important species with demonstrated sensitivity to OW and OA, but their combined impacts are unknown. Through a ~ 9-week experiment, Pacific cod embryos and larvae were reared at one of six combinations of three temperatures (3, 6, 10 °C) and two CO₂ levels (ambient: ~ 360 μatm; high: ~ 1560 μatm) in a factorial design. Both embryonic and larval mortality were highest at the warmest temperature. Embryonic daily mortality rates were lower under elevated CO₂ and there was no effect of CO₂ level on larval daily mortality rates. Growth rates of young larvae (0 to 11 days post-hatch) were faster at warmer temperatures and at high CO₂ levels, but growth during the 11–28 days post-hatch interval increased by temperature alone. The condition of larvae decreased with age, but less markedly under high CO₂ levels. However, at 6 °C, fish incubated in ambient CO₂ remained in higher condition than fish in the high CO₂ treatment throughout the experiment. Overall, temperature had the greater influence on Pacific cod early life stages across each measurement endpoint, while CO₂ effects were more modest and inconsistent. Subtle developmental differences in larval Pacific cod could be magnified later in life and important in the context of recruitment. These results show the complexity of stage- and trait-specific responses to and value of investigating the combined effects of co-occurring climatic stressors.