Gastric evacuation in mackerel: the effects of meal size, prey type and temperature

Gastric evacuation experiments, performed with groups of Atlantic mackerel Scomber scombrus and three different prey species: sprat Sprattus sprattus , sandeel Ammodytes marinus and krill Meganectyphanes norwegicus , revealed a strong temperature effect on gastric evacuation ( Q ₁₀=4.l) and a curvilinear evacuation with a shape intermediate between the square root and the exponential model. From the experiments with krill, where no internal tagging was possible, the medians and upper quartiles of the stomach content distributions were analysed instead of individual data. From these experiments the evacuation rate of krill was 60% higher than that of sandeel. An additional simulation study confirmed that the use of medians results in more accurate estimates of the evacuation rates when compared to the arithmetic means.