Brown trout (Salmo trutta) is the most widely distributed fish species in freshwater environments. From a limited European natural distribution range, humans successfully introduced the brown trout to freshwater locations on all continents. This was done at a time (before 1900) when knowledge of parasites and pathogens were limited and so the release of fish in various environments could be excused. A favorable factor in earlier day’s transport of living organisms were that transportation took time and often fertilized eggs were the preferred stage to transport. This reduced the probability of unwanted organisms to hike.