Behavioural interactions between a threatened native killifish and the alien invasive Eastern mosquitofish

Behavioural interactions between alien invasive fishes and native fishes is considered one of the drivers of native fish decline. However, there are few experimental studies on their behavioural interactions at the individual level. In this study, we investigated the behavioural interactions between the threatened native killifish Valencia letourneuxi, and the alien invasive Eastern mosquitofish Gambusia holbrooki, thought to induce the killifish’s rapid population decline. The aim was to assess aggressive behaviours, disruption in activity, stress levels, boldness, forging efficiency and shelter use by the native fish in the presence of the mosquitofish. Interspecies interactions were assessed in a three-tiered experimental setup, i.e. in an empty arena, in the presence of an artificial cover, and during feeding, using two opponents at each trial. The behaviours recorded and assessed were nips, following, approach, moving/immobile, top/bottom, erratic movement, cover use, feeding and latency to exit. The results showed that the mosquitofish was bolder, followed the native species, spent significantly less time under the cover and consumed food, as opposed to no following by the native species, significantly higher cover use and time immobile, as avoidance behaviours, and zero feeding in the presence of the mosquitofish. More significantly, the native species received direct aggression by the mosquitofish, as opposed to no aggression at all exhibited by it. Future research needs, as well as the conservation implications of our findings are briefly discussed.