Life history variation of invasive mosquitofish (Gambusia holbrooki) along a salinity gradient

The mosquitofish (Gambusia holbrooki) is among the most invasive fish worldwide, with well documented ecological impacts on ecosystem functioning and several native taxa. Although it has been suggested that salinity limits the invasive success of mosquitofish, there are few studies on how salinity affects its ecology. We examined differences in life history traits of mosquitofish at 19 sampling sites from fresh (438 μS/cm; ca. 0.2‰ of salinity) to polysaline waters (35.1 mS/cm; ca. 23‰). Salinity affected the density and life history traits of both male and female mosquitofish; density showed a unimodal response to salinity, stronger for females and thus affecting the sex ratio. On the other hand, the endemic Aphanius iberus was only present in polysaline and eusaline waters, where mosquitofish were generally absent. Mosquitofish increased reproductive investment (higher gonadal mass) in oligosaline waters, at the cost of somatic condition in females. The increase in reproductive investment also implied an increase in total mass in contrast to somatic mass, demonstrating that the latter is a better indicator of condition. Females from higher salinity waters presented more embryos in advanced developmental stages, indicating earlier reproduction than in fresh water. After correction for brood and fish sizes, females from saltier environments presented heavier embryos (independently of maturation state). In summary, salinity mainly affected mosquitofish females which, despite presenting earlier maturation and higher reproductive investment, reduced their condition and density, hence supporting the hypothesis that salinity limits mosquitofish invasive success.