Competing Predators and Prey: Juvenile Bottlenecks in Whole-Lake Experiments

In size-structured populations, prey have the potential to restrict the recruitment of their predator by decreasing the growth rate of young predators through interspecific competition, a situation referred to as a “competitive juvenile bottleneck.” Two mechanisms have been advanced by which a decrease in growth may increase the mortality of young predators: (1) increased time during which juvenile predators are susceptible to gape-limited predation and (2) increased susceptibility to starvation. To study the effects of competition from a prey fish (roach, Rutilus rutilus) on the recruitment of a piscivorous fish (perch, Perca fluviatilis), roach were introduced in two of four small adjacent unproductive lakes inhabited by natural populations of perch. We thereafter studied the diet, growth, and survival of the new-born cohorts of perch during a 13-mo period. Growth and survival of larval perch were not affected by roach whereas during the latter part of the growing season, growth and condition of young-of-the-year (YOY) perch were negatively affected by roach. The growth retardation of YOY perch in roach-treatment lakes coincided with a shift in diet from free-swimming zooplankton to benthic cladocerans and chironomids, which in turn was related to a stronger decline in pelagic zooplankton resources in these lakes. The small size and poor condition of YOY perch in the roach-treatment lakes resulted in almost all these perch dying from starvation during the winter and subsequent spring. There was a tendency for there to be higher densities of YOY perch in the roach-treatment lakes, possibly due to a strong die-off of adult perch in these lakes that in turn may have reduced cannibalism in these lakes. The results from an enclosure experiment on intra-cohort competition among YOY perch, however, suggested that the discrepancy in growth of YOY perch between the lakes was mainly due to direct competition from roach for the zooplankton resource. Temporal variation in competition intensity and in duration of periods for growth and energy gain (e.g., summer) vs. periods of only energy loss (e.g., winter) are suggested to have major effects on juvenile predator growth and subsequent recruitment to the adult stage through competitive juvenile bottlenecks, and thus to affect overall community dynamics. The relative importance of size-dependent winter starvation mortality compared to mortality caused by gape-limited predators as a mortality agent on juveniles is hypothesized to increase with latitude. Whole-lake experiments lend themselves both to testing the importance in natural systems of mechanisms previously identified at small spatial and short temporal scales, and also to identifying important mechanisms that cannot be studied at smaller scales.