Behavioural and Physiological Responses of a Native Iberian Minnow Species to Heatwaves in the Presence of Invasive Species

Freshwater ecosystems are increasingly exposed to the dual pressures of rising temperatures and biological invasions, leaving native fish populations particularly vulnerable. In Portugal, 67 invasive fish species have been recorded in freshwater habitats—53 in mainland systems—with new introductions occurring roughly every 2 years. Concurrently, heatwaves, defined as abnormally hot periods lasting from days to months, have increased across Mediterranean regions such as Portugal. This study experimentally simulated a heatwave to investigate the combined effects of elevated temperature and invasive fishes— Gambusia holbrooki (Girard, 1859) and Lepomis gibbosus (Linnaeus, 1758)—on the behaviour and physiology of the Iberian endemic Squalius alburnoides (Steindachner, 1866). Experiments reproduced three 6‐day thermal phases (17°C → 26°C → 17°C), representing typical autumn conditions and an extreme heatwave scenario in temporary Mediterranean rivers, where fish are confined to summer pools. With G. holbrooki present, S. alburnoides exhibited a marked rise in heterospecific aggression during and after the heatwave. This behavioural shift was accompanied by increased serotonergic turnover (5‐HIAA/5‐HT ratio), suggesting neurophysiological modulation of social stress. Conversely, exposure to L. gibbosus resulted in more attacks suffered by S. alburnoides after the heatwave, without clear physiological responses. Overall, our results show that heatwave effects on native fish are strongly context‐dependent, varying with specificities of the invasive species involved. These findings highlight how both climate warming and biological invasions can interact to reshape social dynamics in freshwater communities, with potential consequences for native species survival.