Of the 42 species of fish found in Scottish fresh waters, only half have arrived by natural colonisation. Native species include allis shad, brown trout, European eel and river lamprey. Scottish rivers support one of the largest Atlantic salmon resources in Europe, with nearly 400 rivers supporting genetically distinct populations. Five fish species are considered 'late arrivals' to Scotland, having colonised by natural means prior to 1790. They are the northern pike, roach, stone loach, European perch, and minnow. Rarer native species include the endemic "Salvelinus killinensis" and the powan, the latter found in only two locations and under threat from introduced ruffe and the Arctic charr. The latter may have been the first fish species to re-enter fresh waters when the last ice age ended, and about 200 populations exist.