Size-specific food partitioning and growth in benthic whitefish, Coregonus lavaretus (L.), in a subarctic lake

The summer diet of benthic whitefish (Coregonus lavaretus (L.)) was studied in an oligotrophic subarctic lake. The fish were caught with bottom gill net series during two open-water seasons in 1982–1983. Planktonic crustaceans, especially cladocerans, were the most common food of small (< 150 mm) whitefish. The mid-sized fish (150–220 mm) preyed mostly on the larvae and pupae of chironomids, which were also the most frequent prey of larger whitefish during July. Zoobenthos such as molluscs and chironomid larvae were the food most utilized by the largest fish (> 220 mm). The use of zoobenthos increased and that of insect pupae decreased towards the autumn in every size group of fish. Differences were significant within large fish (p < 0.001). Stomach fullness and the frequency of empty stomachs varied markedly from year to year; fluctuations were more pronounced in the larger fish. Between the smallest fish (< 150 mm) and the other size-groups a clear difference was present in the percentage of plankton food (p < 0.001), suggesting the presence of only minimal food competition. Improved growth of fish longer than 150 mm indicated a favorable effect of the diet shift. After the length of 200 mm was attained the diet similarity between size-groups was pronounced. Slow growth was probably caused by intraspecific food competition. After the fish attained the length of 240 mm rapid improvement in growth was observed.