The findings in this dissertation support the Grilsefication Hypothesis (McDonald 2001) and demonstrate a profound shift in Atlantic Salmon Life History phenomena due to commercial fishing pressure on Atlantic salmon in Iceland. A time series analysis, 1974-2008, was performed from the rod-catch data of the Icelandic rivers Haffjardara, Haukadalsa, Hofsa, Laxa in Adaldal, Midfjardara, and Vatnsdalsa. The proportions of migratory, multi-sea winter salmon and non-migratory one-sea winter grilse were plotted over time and show a shift that favors grilse. Additionally, the mean weights for both salmon and grilse are declining. These data suggest that we are seeing a profound life history shift in Atlantic salmon from iteroparity to semelparity, and a non-migratory life history change favored over a migratory one. The data indicate that Atlantic salmon in Iceland have adapted key life history parameters in response to the increased cost of migration. This response to the commercial exploitation in the Western Greenland fishery has led to grilsefication of the stock.