<p>Nonlinear dynamics readily occur in natural ecosystems and can drive irregular population fluctuations through oscillations, chaos and alternative stable states. However, the effects of anthropogenic changes, such as to demography and the climate, on nonlinearity of population fluctuations are unknown. We evaluated the extent and magnitude of nonlinearity and its environmental and life history correlates in 243 recruitment and 266 spawner time series of 143 marine fish species, worldwide. Here we show that temperature variation amplifies nonlinearity in recruitment and spawner biomass, while life history mediates the degree of nonlinearity for the latter, dampening it in slow-lived species. Nonlinearity was shown by 81% of populations and correlated with the magnitude of fluctuations. These nonlinear dynamics were low dimensional and causally forced by temperature in 69% of populations with the probability of forcing increasing for recruits in variable-temperature environments and fast-lived spawners. Our results challenge assumptions of stable dynamics and sustainable yield common to fisheries management, and suggest that nonlinear fluctuations of fish populations are magnified by size-selective fisheries and environmental variability from global climate change.</p>