Abstract
The benefits to humans of living by the ocean have led many coastal settlements to grow into large, densely populated cities. Large coastal cities have had considerable environmental effects on marine ecosystems through resource extraction, waste disposal, coastal development, and trade and travel routes. While our understanding of the ecological and evolutionary consequences of urbanisation for wildlife in terrestrial systems has received considerable recent attention, the consequences of urbanisation in marine systems are not well known. Using microsatellite datasets associated with published research on marine fish population genetics, we built a global database of genotypic data spanning 75,361 individuals sampled from 73 species at 1085 sample sites throughout the world's oceans. We found that genetic diversity and effective population sizes were significantly lower at marine fish sample sites associated with denser human populations, regardless of species and locality. The loss of genetic diversity near denser human populations indicates habitats near human settlements are less able to support large populations. Small effective population sizes, in turn, dampen the efficiency of natural selection near dense urban settlements. The loss of genetic diversity near cities is concerning for maintaining functioning marine ecosystems and sustainable fisheries. Our work highlights the need to mitigate environmental threats from human activities and focus efforts on sustainable urban planning and resource use to conserve marine biodiversity and sustain coastal fisheries and ecosystems.