In this thesis I present evidence that the high-seas fishery, in the past, has selected for a non-migratory lifestyle. A time series analysis was performed from the catch statistics of the Icelandic river Haffjardara. The mean weights for multi-seawinter salmon and one-seawinter grilse were plotted over time and were shown to trend downwards for the years 1974-1996. These data suggest that salmon are spending less time at sea and are returning at an earlier age to their natal rivers. The multi-seawinter salmon were harvested at sea before they could reproduce. The data indicate that the salmon have adapted their lifestyle in response to the increased risk of migration. This response has led to grilsefication of the stock.