Do invasive freshwater fish species grow better when they are invasive?

A frequent assumption in invasion ecology is that invasive species have enhanced growth rates in their invasive ranges. However, invasions frequently occur in sub-tropical and tropical environments where growth could be higher simply due to climatic conditions rather than novel habitat. In this study, a meta-analysis of growth rates (length-at-age data) was completed for six invasive freshwater fi sh species: common carp Cyprinus carpio , largemouth bass Micropterus salmoides , brown trout Salmo trutta , brown bullhead Ictalurus nebulosus , fl athead catfi sh Pylodictis olivaris and northern snakehead Channa argus . Signifi cant eff ects of climate on growth were observed for all species except common carp, and following normalization of growth for climate eff ects, a range of growth responses between native and invasive populations were revealed. Two species (brown trout, fl athead catfi sh) showed signifi cantly increased growth rates in invasive compared to native ranges, but two species (common carp, largemouth bass) showed signifi cantly faster growth in native ranges, and two other species (northern snakehead, brown bullhead) showed no diff erence in growth rates. No species showed both signifi cantly enhanced growth rates and initial sizes in invasive compared to native ranges. Using the comparative method, countergradient growth variations were apparent for all species within their native ranges and for all but one species in invasive ranges. Invasive populations of freshwater fi sh do not always grow faster when invasive and future studies need to consider growth covariates (e.g. climate and countergradient growth) prior to comparing life-history diff erences between invasive and native populations.