Year-to-year fluctuations in seawater temperature are partly responsible for the much slower ups and downs in the abundance of marine fish stocks. This is the conclusion from a worldwide study by Wageningen University & Research, among others. Fish biologists have observed slow fluctuations in fish stocks for centuries, but the causes remained largely unclear. This study shows that fluctuations on timescales of decades are naturally present in fish populations due to their sensitivity to water temperature.