Effects of size and sex on swimming performance and metabolism of invasive mosquitofish Gambusia holbrooki

In freshwater ecosystems, abiotic factors such as flow regime and water quality are considered important predictors of ecosystem invasibility. The aim of this study was to investigate the critical swimming capacity and metabolism of the eastern mosquitofish, Gambusia holbrooki, focusing on sex and size effects, to evaluate the influence of water flow on its invasive success. Specimens of mosquitofish were captured from the Ter Vell lagoon (L'Estartit, north-eastern Spain) in July 2014, and we measured the critical swimming speed (U-crit) and oxygen consumption of individual fish (30 females and 30 males) using a mini swim tunnel. The mean U-crit of this poeciliid fish was estimated at 14.11cms(-1) (range=4.85-22.26), which is lower than that of many other fishes of similar size and confirms that this species is limnophilic and its invasive success might be partially explained by hydrologic alterations. However, the U-crit and maximal metabolic rate vary markedly with fish size and sex, with males having much higher values for the same body mass, and thus probably being more resistant to strong water flows. Multiple regression models illustrate that multivariate analyses might increase the predictive power and understanding of swimming performance and metabolic traits, compared to results from conventional simple regressions.