How are prey fishes of multiple meals evacuated from the stomach of a piscivorous fish?

The observations that the gastric evacuation of a meal was accelerated after ingestion of an additional meal, whereas the evacuation of the latter meal was slowed down were compared to the predictions of mass-dependent models derived from experiments on evacuation of single meals. In contrast, the present study on cod Gadus morhua fed identical double meals of sandeel Ammodytes tobianus demonstrated that the evacuation of each meal was slowed down relative to the evacuation of a single meal. This fact could be explained by the exposed meal surface area being reduced while the meals were together in the stomach as it was expected by a surface-dependent cylinder interpretation of the square-root model. The cylinder model forecasted accurately the observed evacuation of each meal in the double-meal situation. The simple surface considerations of the model further explained the discrepancies between observed and expected evacuation rates using mass-dependent models, which suggested that the above finding of the previous studies was an artefact. The ability of the cylinder model to predict properly also the evacuation of multiple meals in juvenile coho salmon Oncorhynchus kisutch indicated that the model would apply to gastric evacuation in small-sized predatory fishes as well. This should increase considerably its value as a generic tool for studies on the feeding biology of fishes in the wild. (c) 2007 The Authors Journal compilation. (c) 2007 The Fisheries Society of the British Isles.