The effect of carbon dioxide on growth of juvenile Atlantic cod Gadus morhua L.

A trial was undertaken to investigate how exposure to graded hypercapnia affected the growth performance of juvenile (15-80 g) Atlantic cod. Juveniles were grown at 20‰ salinity and 10 °C for 55 days under three hypercapnic regimes: low (2 ± 0.9 mg L−1 CO2, 0.6 mm Hg, 1000 μatm), medium (8 ± 0.5 mg L−1 CO2, 2.8 mm Hg, 3800 μatm) and high CO2 exposure (18 ± 0.2 mg L−1 CO2, 6.3 mm Hg, 8500 μatm). All water quality parameters were within the range of what might normally be considered acceptable for good growth, including the CO2 levels tested. Weight gain, growth rate and condition factor were substantially reduced with increasing CO2 dosage. The size-specific growth trajectories of fish reared under the medium and high CO2 treatments were approximately 2.5 and 7.5 times lower (respectively) than that of fish in the low treatment. Size variance and mortality rate was not significantly different amongst treatments, indicating that there was no differential size mortality due the effects of hypercapnia, and the CO2 levels tested were within the adaptive capacity of the fish. In addition, an analysis was carried out of the test CO2 concentrations reported in three other long-term hypercapnia experiments using marine fish species. The test concentrations were recalculated from the reported carbonate chemistry conditions, and indicated that the CO2 concentration effect threshold may have been overestimated in two of these studies. Our study suggests that juvenile Atlantic cod are more susceptible to the chronic effects of environmental hypercapnia than other marine fish examined to date.