Pikeivory: behavioural mechanisms in northern pike piscivory

Abstract: Optimal foraging and prey preference in top predators may affect population and community dynamics and structure. Also, optimal foraging and prey preference may shift with prevailing conditions, and therefore create changes in the effect of predation. In this thesis, I examine the effects of intraspecific interactions among northern pike on pike predatory behaviour, prey preference, and subsequent trophic effects. Northern pike are gape-size limited, ambush predators that generally choose prey sizes well below their upper limit, and prefer shallow bodied prey before deep. I ascribe these preferences to avoidance of risk of intraspecific interactions, such as kleptoparasitism and cannibalism, since taking large or deep bodied prey leads to longer handling times and increased risk of interaction, and since pike foraging around conspecifics significantly prefer small prey while pike foraging alone do not. Further, optimal prey size for predators can be affected by differences in digestion efficiency between prey sizes, but since northern pike gastric evacuation rates do not differ between equal-mass rations of different prey sizes, I exclude time for digestion in the optimal foraging and risk-avoidance budget in pike. Consequently, as pike forage on prey smaller than predicted by optimal foraging theory, and avoid large and deep prey, those prey enjoy a functional, behaviourally mediated refuge from predation, and the trophic effects of pike predation are decoupled by pike predatory behaviour. Thus, the effects of pike predation on lower trophic levels are affected by size and density dependent risk of interference and cannibalism, which may decrease predator impact, decouple trophic effects, and be a clue to the mechanisms behind e.g. alternative stable states and community dynamics in lakes. For further information on the subject, please visit http://freshwater.ekol.lu.se