Stress profile in an invasive fish across Europe: Insights from scale cortisol analysis

Biological invasions are a pervasive agent of global change, with some invasive species now present in the environment across large spatial areas. Environmental stressors (e.g., suboptimal temperatures and adverse salinity levels) can result in individuals becoming chronically stressed, with differences in the stress profiles of individuals able to be described using the biomarker of scale cortisol concentration. This biomarker was applied to describe the stress profiles of the globally invasive round goby, Neogobius melanostomus, over a latitudinal gradient, with samples collected from populations in Slovakia, Poland, and Croatia using electrofishing, and from Finland by angling. The Finnish sample comprised the largest individuals and, despite site-specific variations, had the lowest scale cortisol concentrations. The other samples all comprised smaller fish that had significantly higher scale cortisol levels, with no apparent differences between samples, with a lack of clear spatial pattern. These results emphasize the importance of individual traits and the potential effect of local sampling techniques in describing the stress profiles of freshwater fishes.