Life History Traits of Invasive Mosquitofish Along Environmental Gradients

Variation of life history traits along spatial gradients is poorly understood in invasive species and particularly in freshwater fish. We examined life history variation in a highly invasive fish (Gambusia holbrooki) along latitudinal and upstream–downstream river gradients and we assessed the effects of age on this variation. Mosquitofish were sampled from the lowest reaches of ten rivers along 6º of latitude in the Mediterranean region and seven sites along the upstream–downstream gradient in three of the rivers. We examined abundance, population structure, size-at-age and other life history traits along these gradients. Lower reaches and lower latitudes resulted in higher reproductive effort and lower body condition. These patterns were nonlinear and strongly depended on age. Independently of size, age groups differed in reproductive effort, in the gonadal weight–size relationship and its variation along spatial gradients. Mean size-at-age did not vary with latitude (so the intra-specific version of Bergmann’s rule does not apply) and in contrast increased upstream in rivers. Our findings suggest that for life history traits of fishes, river longitudinal variation might play a role as important as climate, with often differential effects. Results also illustrate the poor knowledge of spatial variation of many life history traits.