Responses of Early Life Stages of Walleye Pollock to Projected Levels of Ocean Acidification

With rising atmospheric CO₂, ocean pH is predicted to decrease 0.3 to 0.5 units by 2100. Several biological consequences of ocean acidification across this range have already been documented in invertebrates and tropical marine fishes. However, little work has been done examining potential responses of the temperate and boreal marine fish species that support much of the worlds fishery production. In this study we examined the responses of walleye pollock to environmental pH at three early life history stages: eggs, larvae, and juveniles. At each stage, groups of fish were reared in the laboratory at ambient (8.05) and 3 reduced pH treatment levels (7.9, 7.6, and 7.2). Walleye pollock eggs were collected from natural spawning of a broodstock maintained at the AFSC laboratory in Newport, Oregon. Eggs were incubated at the 4 pH levels until hatch. At pH 7.2, there was slight delay in the time to hatch of walleye pollock eggs, but there was no significant effect of pH on size at hatch or yolk volume at hatch. Larvae hatched from a separate spawning event were reared for 2 weeks during the pre-feeding stage determine the effect of pH on utilization of yolk reserves. There was no significant difference in the time course of starvation mortality among larvae in the different pH treatments. In a separate experiment, juvenile walleye pollock were captured from nursery grounds and transported to the Newport laboratory. Age-1 fish were reared for 6 weeks with unrestricted rations. There was no significant effect of pH treatment on growth rates or fish condition. Although cortisol levels were generally low at the end of the experiment, there was a non-significant trend toward higher levels in the low pH treatments. Analysis of blood chemistry suggested that fish responded to the reduced environmental pH by increasing the buffering capacity of the blood. In summary, the responses in terms of growth and survival to the pH treatments were minimal, and observed effects were largely restricted to the lowest pH (7.2) treatment. While not exhaustive of potential interactive environmental factors, the experiments to date suggest that walleye pollock are physiologically robust to pH perturbations of the magnitude expected to be observed in the next century. However, the possibility that recruitment and production of this important fishery species may be affected indirectly via effects on the productivity of its invertebrate prey populations has yet to be explored.