Effects of ocean warming and acidification on Atlantic cod and Polar cod

Changes in PCO2 and temperature will impact fish populations in future oceans, specifically those living close to their lower or upper thermal limits, such as Atlantic cod (lower) and polar cod (upper) in the sub-arctic Barents Sea. Embryogenesis may represent a bottleneck in this respect since central organ systems supporting homeostasis are not jet fully developed. Embryos are also limited in their ability to compensate for increased metabolic costs possibly associated with acid-base regulation and thermal acclimation. This may lead to trade-offs in resource allocation to vital maintenance functions and developmental processes. Embryos of Atlantic cod and polar cod from the Barents Sea were exposed to factorial combinations of two PCO2 levels (400 and 1100 µatm) and five temperatures (0 to 12 °C for Atlantic cod and 0 to 6 °C for polar cod). In addition to hatching success, we measured oxygen consumption rates (MO2) of embryos as a proxy for metabolic activity, while larval morphology was analysed to assess embryonic growth and resource allocation.In both species, OA exacerbated negative effects of warming on hatching success. MO2 increased with temperature and in response to OA but declined upon extreme warming, especially in combination with OA. This pattern indicates increased energy demand for acclimation to OA and suggests a mechanistic link between oxygen uptake and heat tolerance. The increase in MO2 in response to OA was paralleled by reduced larval size at hatch, while the amount of consumed resources (yolk) remained unaffected.OA has the potential to aggravate effects of thermal stress on embryogenesis in both species tested. Furthermore, our results show that acclimation to OA requires reallocation of limited resources; supporting the idea that energy supply to maintenance functions takes priority over other energy demanding processes such as embryonic growth.