Timing of the diet shift from zooplankton to macroinvertebrates and size at maturity determine whether normally piscivorous fish can persist in otherwise fishless lakes

Using a size-structured population model, life-history information and records of piscivores in systems without prey fish, we analysed the role of the timing of shifting from small-to-large invertebrate prey types in regulating piscivore performance, especially under conditions of low availability of prey fish. Large invertebrate prey are generally absent or at low densities in pelagic habitats; consequently, pelagic piscivorous fish species with a poor ability to exploit zooplankton depend on prey fish in order to persist. In contrast, our model shows that abundant large invertebrate prey in the littoral habitat may allow littoral piscivores to persist in the absence of prey fish if they can shift diet from zooplankton to macroinvertebrates early in life. However, if the diet shift from zooplankton to macroinvertebrates is delayed, or density dependence reduces growth rate, the persistence of even littoral piscivorous fish species in the absence of other prey fish is severely constrained. Our results suggest that undergoing an early diet shift from zooplankton to macroinvertebrates may be necessary to reach sizes large enough to enable successful exploitation of the piscivorous niche. These insights can help to understand the persistence of piscivorous fish species, or their absence, in otherwise fishless lakes.