Simulating fish population responses to elevated CO2: a case study using winter flounder

Scaling experimentally derived effects of CO2 on marine fauna to population responses is critical for informing management about potential ecological ramifications of ocean acidification. We used an individual-based model of winter flounder to extrapolate laboratoryderived effects of elevated CO2 assumed for early life stages of fish to long-term population dynamics. An offspring module with detailed hourly to daily representations of spawning, growth, and mortality that incorporates potential elevated CO2 effects was linked to an annual time-step parent module. We calibrated the model using a 40 yr Reference simulation (1977-2016) that included gradual warming and then performed 'Retrospective' simulations that assumed a suite of elevated CO2 effects by changing fertilization rate, mortality rate of embryos due to developmental malformations, larval growth rate, and size-at-settlement. 'Recovery' simulations that started at low population size were then used to further explore possible interactions between the effects of CO2 and warming on population productivity. Warming had a major negative effect on the simulated winter flounder population abundance, and reduced larval growth had the largest single impact among the CO2 effects tested. When a combination of the assumed CO2 effects was imposed together, average annual recruitment and spawning stock biomass were reduced by half. In the Recovery simulations, inclusion of CO2 effects amplified the progressive decrease in population productivity with warming. Our analysis is speculative and a first step towards addressing the need for extrapolating from laboratory effects of ocean acidification to broader, ecologically relevant scales.