The fact that some salmon spend only one year and a little more feeding in the sea before returning to their home river has been clearly demonstrated through scale reading and tagging experiments. These fish are called "grilse". In other words, a grilse usually enters its river of origin after one complete year and part of a summer at sea (A.1+) whereas a fully grown salmon (the backbone of a salmon fishery) usually remains in the sea 25 or 26 months (A.2 or A.2+), 36, 37 or 38 months (A.3 or A.3+), and in some rare cases, 48, 49 or 50 months (A.4 or A.4+) up to 60, 61 or 62 months (A.5 or A.5+) n For the past twenty years, the conservation of Salmo salar L. in Swedish rivers has necessitated the erection of modem smolt rearing stations and the undertaking of expensive management programs elsewhere, in order to compensate for the loss of natural spawning and nursing areas which were destroyed, in one river after another, by the construction of hydro-electric power developments, or simply by the deterioration of the environment.