Individuals of the invasive round goby face physiological and behavioural trade-offs in their sensitivity to salinity

Invasive species often cause ecological damage, including competition with native species for space, food and survival. One of these invasive species is the round goby (Neogobius melanostomus), a fish introduced into fresh and brackish waters around the world. To understand how round gobies perform if they continue spreading into higher-salinity oceans, we investigated how individuals differ in physiological and behavioural (co)sensitivity to salinity changes. We measured growth, metabolic rates, activity, dispersal and exploration of 59 individuals in the lab after acclimation to salinities ranging from freshwater to seawater (0, 10, 19, 24 and 28 ppt), and quantified trait variation and sensitivity to salinity as reaction norm intercepts and slopes. The fastest-growing individuals at 10 ppt-the salinity the fish were caught at-were also the most active and explorative at this salinity and thus more likely to spread. However, above 15 ppt, these individuals lost their growth advantage and experienced the most severe reductions in aerobic capacity (maximum metabolic rate and aerobic scope). As growth and aerobic capacity are linked to survival and performance in nature, this cross-over at 15 ppt is likely to determine the phenotypic composition of round goby populations across salinities and their continued invasion success and ecological impact.