Twenty years in the making: Long term population dynamics of an invasive fish in a contaminated ecosystem

Invasive species are a global problem with large ecological and economic costs. A better understanding of how invasive species populations change over time, how these species become integrated into ecosystems, and how their population demographics vary across different environments could help inform management priorities and shape control strategies. For 20 years (2002-2022), we have monitored round goby (Neogobius melanostomus) in Hamilton Harbour, Canada, an industrial harbour and an Area of Concern with high levels of contaminants. We sampled round goby across six sites that vary in contamination levels. We first quantified changes in round goby population demographics and morphology over a twenty-year period and second compared how abundance and other life history trajectories differ between sites of high and low contamination. Round goby abundance, body length, and mass all decreased over the study period. In contrast, body condition, hepatosomatic index, gonadosomatic index (GSI), and the proportion of guarding parental males in the population increased over time. Over the many years of monitoring, there was no clear difference in round goby abundance between sites of high and low contamination, but individuals from sites of high contamination were smaller, lighter, and had larger gonad investment compared to round goby from sites of low contamination. We also found there were fewer guarding parental males at sites of high contamination. Taken together, our results show that population composition can vary dramatically over space and time for an invasive species. Studies like this can inform our understanding on how anthropogenic impacts can influence population dynamics.