Foraging behaviour in piscivorous fish: mechanisms and patterns

Abstract: Mechanistic understanding of piscivore foraging at the individual level is important in order to understand and make predictions of piscivore effects at population and community levels. To understand the mechanisms affecting selection and consumption rates each step in the foraging cycle (search, encounter, active choice, attack, capture, ingestion, digestion) should be studied. In my studies of different parts of the foraging cycle I found that: Piscivore search efficiency is severely limited by turbidity along a lake productivity gradient. Encounter rates in natural systems were therefore more negatively affected by increasing turbidity along this gradient than positively affected by the associated increase in prey density. Prey shoaling had large effects on encounter rates in small scale experiments, but incidence of shoaling were low in the natural systems I studied, and therefore of limited importance for encounter rates. In experimental studies on capture success, I found that prey schooling had no effect on predator capture success in pikeperch (Stizostedion lucioperca) and pike (Esox lucius), and only a limited negative effect in perch (Perca fluviatilis, confusion effect). Capture success in pike was higher for sit-and-wait attacks than for pursuit attacks. Piscivores are commonly regarded as opportunistic predators in natural systems, with selective feeding on small prey primarily determined by passive selection, explained by higher capture success for small prey. In a theoretical model, I found that passive processes can generate a wide variety of selectivity patterns. Although passive processes of selectivity are probably important in nature, I found that ring-tailed pike cichlid (Crenicichla saxatilis) and pikeperch actively select prey sizes in experimental situations.The possible importance of active choice still needs to be investigated in nature. Finally, when testing effects of predator density and interspecific interactions, I found that attack frequency, capture success, and consumption rate of pike, pikeperch, and perch were very different in groups of conspecifics compared to when the predators foraged alone. These interaction effects negatively affected consumption rates in pike and led to increased per capita consumption in pikeperch and perch (positive interaction effects).