Ex situ evaluation of impacts of invasive mosquitofish on the imperiled Barrens topminnow

Central to the protection of native species is an understanding of impacts of actual or potential invasive species and also the mechanisms through which those impacts are mediated. The introduction and spread of western mosquitofish, Gambusia affinis, into spring systems of the Barrens Plateau region of middle Tennessee is a concern for native species such as the Barrens topminnow, Fundulus julisia. We investigated whether mosquitofish might act as predators on early life stages of topminnows as well as affect the physiological well being of adults through aggressive interactions. A short-term, 24-h laboratory study with mosquitofish and topminnows demonstrated the vulnerability of young topminnow life stages to large mosquitofish predation and aggression. Survival of topminnow young, <16 mm total length (TL), was 0% and was attributed to predation by mosquitofish. Survival of juveniles, 20–30 mm TL, was 25%; juveniles mostly succumbed (post 24-h) to injuries inflicted by large mosquitofish. Adult topminnow survival was 100% but adults faced injury risk, primarily during the initial stages of their interaction with large mosquitofish. A long-term, 60-day laboratory study with syntopic and allotopic populations of adult topminnows and mosquitofish failed to detect any negative impacts on topminnows due to coexistence. Survival, growth, and fecundity of adult topminnows syntopic with mosquitofish were not different from the allotopic population, although injury risk in the form of fin damage was greater syntopically. Thus, predation and aggression towards young topminnows may be the primary mechanisms by which western mosquitofish jeopardize the persistence of native Barrens topminnows in the wild. Our results reemphasize the danger to native aquatic biodiversity of unregulated introductions of Gambusia species.