The role of gastric evacuation rate in handling time of equal-mass rations of different prey sizes in northern pike

In a laboratory experiment, northern pike Esox lucius gastric evacuation rates did not differ between equal-mass rations of small and large prey. In a comparison between intermediate and large prey, the pike were unable to fit two intermediate prey completely into the stomach at the same time, resulting in two consecutive evacuations, and changes in patterns of gastric evacuation. Thus. total gastric evacuation time was not affected by prey size composition in equal-mass rations. but patterns in evacuation rate may depend on the size ratio between predator and prey. Cumulative manipulation time between strike and complete swallowing of prey differed between equal-mass rations of small. intermediate and large prey, in that small prey took the shortest time to manipulate. Pike had problems striking and redirecting intermediate prey to swallow them head first, and the manipulation times for intermediate prey were as long as for large prey. Since an increased time manipulating prey in the mouth increases risk of predation and intraspecific interactions in Dike, it is concluded that risks associated with long manipulation times, and not only energy per total handling time, determine prey value and prey size preference in this piscivore. (C) 2000 The Fisheries Society of the British Isles