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.

The dependence of plant production in the sea on the presence of nutrient salts, especially nitrate and phosphate, and on illumination has been repeatedly demonstrated. The object of the present work was to find primarily whether, and to what degree, plant and animal production could be increased by adding fertilizers to an enclosed body of sea water under otherwise natural conditions.

The plankton in Loch Craiglin was sampled at Station I, the deepest part of the loch. Samples were taken at intervals of two or three weeks, and since, as is mentioned below, the sampling of zooplankton was unreliable for part of 1942, the results for that year are referred to only briefly and the more complete results of 1943 given in detail.

The density of the animal population in any area of the sea bottom must be closely related to the food supply available. An increase in food may be expected to lead to more rapid growth and to more prolific breeding, and a consequent increase in the number of animals sustained on a given area of the bottom.

The amount of food available to the “basic” bottom fauna, consisting of plankton, detritus and bottom microflora (cf. Hunt, 1925), depends upon the amounts of plant nutrients in the water, which limit photosynthetic activity in the presence of sufficient light. The addition of fertilizers to an enclosed area like Loch Craiglin (Orr, 1947) might be expected to lead to a greater food supply, and thus to a greater density of the bottom fauna.