Diurnal periodicity in the activity of the common sole, solea vulgaris quensel

Abstract

1. 1. The diurnal rhythm in the trawl catch of Solea vulgaris Quensel gave rise to this investigation into the diurnal activity rhythm of the fish. 2. 2. Periodicity in the food intake of the Sole in its natural habitat was studied by analyses of the contents of the intestines. Food intake appears to have a maximum during the night and a minimum during the day. It is argued that this reflects a diurnal periodicity in locomotory activity. 3. 3. In the aquarium, periodicity in the activity of the Sole was measured directly. Soles touched small lead balls suspended in the tank; the number of contacts was recorded automatically. Activity appears to be zero during the day, maximal during the night, thus confirming observations in the natural habitat. 4. 4. The changes in behaviour during this diurnal cycle are described. An active and inactive phase are distinguished; the characteristic behaviour patterns between the phases (the Omega-jump and digging-in) were studied. 5. 5. The diurnal activity cycle of the Sole is adjusted to the light cycle via the releasing mechanisms of the behaviour patterns studied. Some experiments were carried out in which the activity was recorded in continuous darkness, continuous light and in a twelve hours light cycle. 6. 6. The possible functions of the behaviour described are discussed. 7. 7. The mechanism behind the daily periodicity in the Sole catch by trawl is discussed. It is argued that the main difference between day and night in the catch per hour is brought about by a periodicity in accessability of the fish; a hypothesis is put forward to explain the differences in the catch per hour during the night.