Feeding of fin whales (Balaenoptera physalus) off Iceland - diurnal and seasonal variation and possible rates

Abstract

Information on relative quantities of food remains found in fin whales (Balaenoptera physalus) caught off Iceland in 1967-89 are presented. In 1986-89 attempts were made to quantify these assessments in terms of weight and volumetric capacity of the stomachs. A forestomach visually assessed as full normally contained 5-600 kg of krill, the maximum being 760 kg in the present study (n = 34). The forestomach content was analysed in relation to seasonal and diurnal patterns in feeding activity. Daytime feeding rates declined with the progression of the season, especially in August-September. Pronounced diurnal fluctuations were found in the quantity of forestomach content peaking at 0000-0600 hr, but some feeding activity continued throughout the day. Diurnal variation of food remains found in different parts of the digestive tract suggested that the mean passage time from the forestomach to the fundic chamber was 3-6 hr, and that from the forestomach to the anus around 15-18 hr. The calculated daily feeding rates for fin whales in June-July were between 677 and 1356 kg, assuming 6 and 3 hr evacuation rates in the forestomach, respectively. The latter value fitted better with studies on seasonal fattening of fin whales off Iceland.