Gut check: Researchers uncover high levels of CO2 in digestive tracts of fish

Researchers have made the first direct measurements of CO2 levels in the digestive tract of live fish — uncovering levels much higher than the fish would experience under climate change scenarios or aquaculture.

“There’s a lot of debate regarding how elevated water CO2 levels impact the survival of fish,” says University of British Columbia (UBC) zoologist Chris Wood, who led the study. “But our research indicates fish routinely tolerate much higher internal CO2 levels in their gastrointestinal fluids, especially after feeding.”

Wood and UBC colleague Junho Eom found that CO2 levels in the digestive tract of fasted rainbow trout were two to four times greater than CO2 levels in their blood. In fed trout, the difference increased to five to 10 times.

The digestive tract levels are 10 to 50 times higher than those of concern in the external water environment under climate change scenarios, and several times higher than those currently generated by intensive aquaculture operations.

Scientists are investigating how increasing ocean acidification and temperatures resulting from global change will impact the survival of fish. Elevated water CO2 levels could impact their ability to identify food, find shelter and avoid predators.

“Given how interconnected the intestinal and blood systems are in fish—and all animals—teasing out the impact of these elevated gut CO2 levels is critical,” says Wood. "What we don't know for sure is whether those high gut C02 levels get into the blood stream, where they can impact performance. And if not, what mechanisms does the fish use to prevent it?"

The discovery that CO2 levels spike after feeding on commercial food pellets could also have implications for aquaculture.

“Industry can design aquaculture feeds to raise or lower gut CO2 levels after feeding,” said Wood. “Very likely, this will affect feed conversion efficiency and productivity."

Wood and Eom used state-of-the-art fibre-optic sensors to make the measurements. It’s the first time researchers have been able to make direct C02 measurements in the digestive tracts of live freshwater fish.

The research was published this week in the Proceedings of Royal Society B.