The internal CO₂ threat to fish: high PCO₂ in the digestive tract

Our goal was to use novel fibreoptic sensors to make the first direct PCO₂ measurements in the digestive tracts of live freshwater fish (anaesthetized, artificially ventilated, 12°C). PCO₂ levels in gastrointestinal fluids were substantially higher than in blood, and were elevated after feeding. In the carnivorous, gastric rainbow trout, the mean PCO₂ in various parts of the tract increased from 7-13 torr (1 torr = 0.1333 kPa) during fasting to 20-41 torr after feeding, relative to arterial levels of 3.5-4 torr. In the agastric, omnivorous goldfish, the mean gut levels varied from 10-13 torr in fasted animals to 14-18 torr in fed animals, relative to arterial levels of 5-7 torr. These elevated PCO₂ values were associated with surprisingly high [Formula: see text] concentrations (greater than 40 mmol l^-1) in the intestinal chyme. Incubations of food pellets with acid or water revealed endogenous PCO₂ generation sufficient to explain gastric PCO₂ in fed trout and anterior intestine PCO₂ in fed goldfish. The impacts of possible equilibration with venous blood draining the tract are assessed. We conclude that fish are already coping with PCO₂ levels in the internal gastrointestinal environment many-fold greater than those of current concern in the external environment for climate change and aquacultural scenarios.