Invasive mosquitofish become more aggressive in the presence of native pike young-of-year: implications for native predator recruitment

Invasive species often exhibit aggressive behaviour, boldness, and high foraging activity, which contribute to their establishment success and impact on native ecosystems. The mosquitofish (Gambusia holbrooki), one of the world’s most invasive fish species, is known for its aggressive nature, which threatens the survival of native species. Lake littoral zones, critical for juvenile fish development, are particularly vulnerable to such invasions. This study investigates the interactions between mosquitofish and young-of-the-year (YOY) cisalpine pike (Esox cisalpinus), a native top predator reliant on these habitats. We conducted a controlled laboratory experiment over five days, observing behavioural interactions across different density treatments. Both species exhibited high levels of intraspecific aggression, with mosquitofish forming dominance hierarchies, where a single dominant individual accounted for over 84% of interactions. In interspecific treatments, mosquitofish aggression intensified, leading to the direct mortality of ~92% of pike through fin nipping. This aggression was considered primarily social, aimed at maintaining dominance and competing for food, while pike aggression was driven by cannibalistic predation. The vulnerability of pike to mosquitofish aggression was linked to fin morphology, as mosquitofish targeted the caudal fin, impairing pike mobility and survival. Although laboratory conditions lack the complexity of natural ecosystems, the findings highlight the potential impact of mosquitofish on native species, particularly at early life stages. Enhanced aggression during interspecific interactions may be a key mechanism behind mosquitofish dispersal and invasion success. While the effects observed here may be less pronounced in natural environments, the ability of mosquitofish to dominate and outcompete native species, combined with their adaptability and rapid reproduction, poses a significant ecological threat.