Introduced, invaded and forgotten: allopatric and sympatric native snow trout life-histories indicate brown trout invasion effects in the Himalayan hinterlands

After 150 years of introduction of the brown trout Salmo trutta in Himalaya, the native species’ response to this globally pervasive invader, is still unknown. Here, we investigate the effects of invasion of brown trout on native snow trout Schizothorax richardsonii, one of the most primitive species that co-evolved with the Himalayan orogeny. We contrast two natural river systems which harbour snow trout in the absence (allopatry) and presence (sympatry) of brown trout. We put forth that sympatric snow trout adapted to a ‘fast’ life history with maturation at a smaller length, greater fecundity and smaller egg diameter to cope with brown trout invasion. However, investment in a reproductive-somatic trade off was evident with a disrupted size structure and reduced abundance as compared to the allopatric population. Although the fast life history adaptations of snow trout might increase their competitive ability with invasive brown trout, trading off the somatic fitness in the process seemingly acts as a deterrent to longevity. We attribute the plastic responses of snow trout to their plausible inherent potential of sustenance and recovery from high invasion pressures. The popularity of brown trout as a sport fish in Himalaya however poses extraneous propagule pressure on the snow trout, which warrants quantification through future research.