Human activity is altering the dynamics of populations through effects on fecundity, mortality and migration. An increased abundance of three-spined stickleback (Gasterosteus aculeatus) in the Baltic Sea has been attributed to a human-caused decline of top predators. However, recent research indicates that a top-down effect cannot fully explain the population growth, but the contribution of a bottom-up effect has not been investigated. Yet, anthropogenic eutrophication has increased algae biomass at the spawning sites of the stickleback and, thus, the abundance of benthic prey. We investigated if increased fecundity could have contributed to the population growth of the stickleback by analysing a two decade time series of stickleback abundance, fecundity, and body size at three spawning sites. The results show an increase in the proportion of gravid females in the populations, which correlates with the population growth. In particular, the proportion of gravid females late in the spawning season has increased, which indicates enhanced food intake at the sites during the spawning season. Thus, a bottom-up effect could have contributed to the growth of the populations by increasing the number of egg clutches females produce. These results stress the importance of considering both bottom-up and top-down processes when investigating the mechanisms behind human impact on population dynamics.