The internal CO 2 threat to fish: high P CO 2 in the digestive tract

Abstract

Our goal was to use novel fibreoptic sensors to make the first direct P CO 2 measurements in the digestive tracts of live freshwater fish (anaesthetized, artificially ventilated, 12°C). P CO 2 levels in gastrointestinal fluids were substantially higher than in blood, and were elevated after feeding. In the carnivorous, gastric rainbow trout, the mean P CO 2 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 P CO 2 values were associated with surprisingly high HC O 3 − concentrations (greater than 40 mmol l −1 ) in the intestinal chyme. Incubations of food pellets with acid or water revealed endogenous P CO 2 generation sufficient to explain gastric P CO 2 in fed trout and anterior intestine P CO 2 in fed goldfish. The impacts of possible equilibration with venous blood draining the tract are assessed. We conclude that fish are already coping with P CO 2 levels in the internal gastrointestinal environment many-fold greater than those of current concern in the external environment for climate change and aquacultural scenarios.