Ontogenetic dietary changes of whitefish larvae: insights from field and experimental observations

Ontogenetic changes in resource use are widespread in many fish species. This study investigated the feeding habits of whitefish (C. lavaretus L.) larvae in Lake Annecy (France) coupled with experimental behavioral studies in order to identify the underlying mechanisms of the ontogenetic shifts in the diet. The predatory behavior of wild larvae, and the escape responses of their zooplankton prey were both videorecorded in experimental tanks under controlled laboratory conditions. Ontogenetic diet patterns showed that young whitefish larvae have a preference for small cyclops, while older larvae selectively predate cladocerans. Our experimental observations showed that the capture success rate also varied in relation to ontogenetic development in fish. Young larvae were more successful in capturing small copepods, whereas old larvae were more successful in capturing Daphnia. In addition, the larvae were able to adjust their predatory behavior (speed, pursuit) according to the swimming pattern of the prey. These observations suggest that the selective predation on cladocerans observed in old larvae is the outcome of both active and passive choices depending on the escape swimming behavior of the prey, and handling time of the predator.