Theory and analyses of fisheries datasets indicate that harvesting can alter population structure  and destabilize nonlinear processes, which increases population fluctuations. We conducted a factorial experiment on the population dynamics of Daphnia magna in relation to size-selective harvesting and stochasticity of food supply. Harvesting and stochasticity treatments both increased population fluctuations. Timeseries analysis indicated that fluctuations in control populations were nonlinear, and nonlinearity increased substantially in response to harvesting. Both harvesting and stochasticity induced population juvenescence, but harvesting did so via depletion of adults whereas stochasticity increased the abundance of juveniles. A fitted fisheries model indicated that harvesting shifted populations towards higher reproductive rates and larger-magnitude damped oscillations that amplify demographic noise. These findings provide experimental evidence that harvesting increases nonlinearity of population fluctuations and that both harvesting and stochasticity increase population variability and juvenescence.