Humans can have a significant impact on fish populations through activities like overfishing, pollution, and habitat destruction. Overfishing can lead to a decline in fish populations, and pollution can harm fish and their habitats. Habitat destruction, like the destruction of wetlands or the building of dams, can also have a negative impact on fish populations. Additionally, climate change can affect fish populations by altering water temperatures, changing ocean currents, and causing sea levels to rise.