Effects of CO₂ Ocean Sequestration on Marine Fish

Ocean sequestration of CO₂ has been proposed as a possible measure to retard the increasing rate of the atmospheric CO₂ concentration. Since some negative impacts on marine animals and ecosystems are likely to ensue, we must carefully investigate biological effects of ocean CO₂ sequestration before embarking on this mitigation practice. Considering the expected depths for CO₂ ocean sequestration (> 1,000 m), it is desirable to use deep-sea animals for the experimental assessment of CO₂ ocean sequestration. In addition, experimental protocols preferably mimic environmental conditions at the releasing site: CO₂ concentrations vary due to mixing with surrounding seawater at low temperatures (0–2 °C) and under high pressures. This paper describes our recent experiments to elucidate the effects of high CO₂ on marine fishes. A deep-sea fish Careproctus trachysoma (habitat depth 400–800 m) can be captured alive and be used for in vivo CO₂ exposure experiments. 100% mortality occurred when the fish was exposed to seawater equilibrated with a gas mixture containing 3% CO₂ conditions at 2 °C within 48 h, whereas mortality was never observed when shallow-water fishes (Mustelus manazo, Paralichthys olivaceus and Seriola quinqueradiata) were tested under the same CO₂ conditions but at higher temperatures (17–20 °C). It is currently not clear whether this difference in mortality is due to often presumed high susceptibility of deep-sea organisms to environmental perturbations. Subsequent experiments demonstrated that low water temperature accelerates mortality by CO₂ exposure. Thus, half lethal time decreased from 105h to only 5 h when water temperature was decreased from 26 °C to 20 °C (CO₂ 8.5%, Sillago parvisquamis). Therefore, the high CO₂ susceptibility of C. trachysoma could be solely due to low water temperature. Temporally varying CO₂ conditions resulted in markedly different mortality patterns when compare with mortality recorded under constant CO₂ conditions. Step-wise increases in ambient CO₂ resulted in much lower mortalities than under one-step increases to the same CO₂ levels. Further, a sudden drop of CO₂ from 9–10% CO₂ to air level (0.038%) killed all the surviving fish within a few minutes.