In 1942 an experiment was started in order to test the possibility of increasing the fertility of a small sea-loch by the addition of nitrate and phosphate fertilizers, with the view of improving the growth rate and yield of fish. It was based on the well-established fact that the phytoplankton organisms are the main producers of organic matter and therefore represent the main source of food supply in the sea, and that nitrogen and phosphorus compounds are among the important factors controlling and limiting plankton production. It was thought that the addition of readily available plant nutrients would result in an increase of plankton production and thus of the food supply of the bottom-living animals which are fed upon by fish. An increase of the bottom fauna might then result in an improved growth rate of the fish population. Normally most marine fish require about four years to reach marketable size, and the object of our experiment was to find out whether the addition of fertilizers would lead to a shortening of that period and to an improvement of the yield in fish per unit area.