Diet composition uncertainty determines impacts on fisheries following an oil spill

•Exposure to oil hydrocarbons can impact food web structure.•We applied an uncertainty analysis to the Gulf of Mexico Atlantis ecosystem model.•We simulated variability in catch and biomass under diet composition uncertainty.•We built a statistical emulator using simulated biomass and catch outputs.•The computationally-efficient emulator explored the complete parameter space.•Parameter uncertainty in diet composition affected the determination of oil effects.