Boldness associated with higher erythrocyte levels and a preference for higher temperatures can support the spread of European catfish to novel and warmer ecosystems

Coping styles are the consistent physiological and behavioural responses of animals to stressors and, more generally, to variability in social and natural environments. Individuals can be characterized as proactive or reactive on the basis of their coping style, and their behavioural and physiological characteristics differ. Proactive, i.e., more bold, exploratory and stress-resistant individuals, spread more easily and prefer higher temperatures; however, available data regarding this natural stressor are available only from laboratory experiments. Indeed, data regarding the characteristics of invasive species in the wild are rare. In this study, we analysed the relationships among the boldness, exploratory behaviour, activity, physiological parameters, and temperature preferences of the European catfish, a large invasive species that destabilizes ecosystems outside its native environment. As expected, bold individuals presented greater erythrocyte counts, lower baseline cortisol levels, and increased exploratory behaviour. In a riverine environment, bold fish preferred warmer zones, reflecting their heightened metabolic demands and increased activity levels. Boldness, the number of erythrocytes and a preference for higher temperatures were key traits in both the field and experimental environments. However, the relationships among the parameters differed, suggesting behavioural adaptability to varying environmental conditions. In our study, the observed physiological and behavioural traits tended to increase the invasive potential of European catfish in warmer regions.