Brown trout (Salmo trutta L.) was voluntarily introduced in some rivers of the Kerguelen Islands in the 1950s–1960s. Fish originating from hatcheries rapidly colonized other streams, thanks to the early occurrence of anadromous (i.e., migratory) form. Getting insight into the success of colonization requires investigating fitness-related traits such as growth and reproductive investment. In particular, increased growth and body size—traits that are broadly related to dispersal ability—are predicted on colonization front, to the possible detriment of reproductive ability. We here report such investigation on early data following the first natural reproductions in the founder populations of Kerguelen, from 1971 to 1994, assessing the main characteristic on growth at sea and reproductive investment for both sexes. Our results reveal that growth of sea trout is excellent with individuals fully benefiting from their relatively short period of growth at sea, sizes and weights ranking among the highest recorded to date. During the reproduction period, males lose on average 15–21% of their weight, whereas females lose 18–19% of their weight. Although a trade-off between growth and reproduction may arise at marginal distribution of invasive species, our study indicates that any potential advantages arising from increased growth and therefore potential dispersal ability may not be directly balanced by reduced reproductive investment, with respect to other published studies. Results overall shed light on intrinsic invasiveness of the brown trout in a post-glacial landscape with barely any interaction with human activities.