Direct and indirect effects of ocean acidification on early lifestages of Atlantic cod and herring

Direct and indirect effects of ocean acidification (OA) were assessed for fish larvae of Atlantic cod and herring. The first study tested the combined effect of end-of-century CO2 concentrations, and elevated temperature, on herring larval performance. Herring larval growth, and survival, was found to decrease in the warmer temperature treatment, which was, most likely, caused by an increased energy demand. Although energy supply was low, no substantial effect of CO2 on herring larval performance was found. The direct effect of the same CO2-levels was tested on Atlantic cod larvae originating from two distinct populations. Cod larvae showed a substantial decrease in survival under end-of-century CO2 concentrations. Based on these results a massive decrease in recruitment of both populations to 8 and 24 % of today’s recruitment was projected. The last study, performed in mesocosms, combined direct physiological and indirect food web effects of OA. Performance of herring larvae was tested within a simulated future ocean food web. The mesocosms enclosed a natural plankton community, with half of the mesocosms set to CO2-concentrations close to the projection for the end-of-century, while the other half was kept as untreated controls. Herring larvae hatched in these environments and exclusively fed on the enclosed prey organisms. Larval survival was increased under elevated CO2 compared to ambient conditions. This effect was related to increased prey abundances from an OA-induced boost in phytoplankton primary production and abundance. However, this positive indirect effect of OA via the food web did no increase larval growth rates. Higher larval abundances in the elevated CO2 treatment may have executed an increased predation pressure, resulting in lower prey availability per larvae. The results of these studies illustrate how the direct effects of projected OA may differ between CO2-tolerant and -sensitive species and how fish larvae may be indirectly affected by OA