Effect of predation on the low-density dynamics of vendace: significance of the functional response

During the past 20 years, there have been prolonged vendace (Coregonus albula) recessions in several Finnish lakes. Hypotheses have been proposed that predation by brown trout (Salmo trutta m. lacustris) or perch (Perca fluviatilis) on young-of-the-year vendace could prevent the recovery of the vendace stocks from a low-density state. In this study, dynamic modelling was applied to examine the effect of predation, assuming a dome-shaped spawning stock–recruitment relationship for vendace, type II or III functional responses to predation by brown trout and perch, and a constant rate of fishing. The results showed that the form of the functional response is crucial in determining the significance of the predation on vendace stocks that have a steep dome-shaped stock–recruitment relationship. In all cases, however, predation by perch had more effect than that by brown trout, probably due to perch occupying the pelagic zone when the vendace stock is sparse. This may make the mortality of vendace increase with decreasing population density (depensatory mortality) at certain density levels.