Ardea
Official journal of the Netherlands Ornithologists' Union

login


[close window] [previous abstract] [next abstract]

Doornbos G. (1979) Winter food habits of smew (Mergus albellus) on Lake Yssel, the Netherlands: species and size selection in relation to fish stocks. ARDEA 67 (1-2): 42-48
From January through March 1977 a study was made of the numbers, activity, food habits and condition of Smew wintering in the southwest part of Lake IJssel. In this survey the results will be given of the stomach and gullet analyses of 21 Smew shot in this period. The food of Smew in Lake IJssel is composed almost entirely of fish. Only the crustacean Asellus aquatieus may be collected actively during food-seeking by some Smew. The fish remains were identified from study of otoliths and pharyngeal-bones (the latter being useful aids in the Cyprinidae). By making correlation diagrams between length of otolith (resp. pharyngeal-bone) and length and weight of the most frequent fish species of Lake IJssel it was possible to assess the species taken, and the lengths and weights of the fish. The composition of the fish taken by Smew was compared with the supply in Lake IJssel, estimated by surveys by the Rijksinstituut voor Visserijonderzoek. With these data it was also possible to investigate size preference. In Lake IJssel (inclusive Oostvaardersdiep and Markermeer) Smew eat almost entirely smelt (Osmerus eperlanus), with some perch (Perea fluviatilis). Remains of pike-perch (Lucioperea lucioperea) were found too. Neither smelt nor pike-perch were previously known as food species for Smew. Ruffe (Acerina cernua), the most frequent species (on a weight basis) in Lake IJssel is avoided. In areas with little or no smelt, perch is most often eaten and other species in the sequence ruffe, pike-perch, roach (Rutilus rutilus) and pike (Esox lucius). Of the fish taken, 67% belong to the length-classes 5 and 6 cm. Fishes longer than 11 cm are rarely taken. However, remains of two pike-perch of 17 and 18 cm were found. There does not appear to be an active selection for size with regard to smelt and ruffe, the size distribution of the fish taken being similar to the population composition in Lake IJssel, but of perch only the smallest individuals (yearclass 0) are eaten. The equation of Nilsson & Nilsson (1976) is used to estimate the total fish consumption by Smew in Lake IJssel. From January through March 1977 Smew will have eaten about 37 tons of fish, including 28 (76%) tons of smelt. In the entire winter period 1976/1977 they may have consumed about 45-50 tons of fish (including 34-38 tons of smelt) from Lake IJssel. The total smelt consumption by Smew will be less than 1% of the winter population of smelt in Lake IJssel. The Smew accounts for 17 and 11 %, respectively, of the numbers and biomass of the major fish-eating birds on Lake IJssel in January 1977 (Table 3). Because of the relative ease with which fish stocks can be followed and the accuracy attained in the bird census programme, Lake IJssel presents an extremely favourable opportunity for a complete assessment of the impact of the bird community on the food base. I see my work as a pilot study and hope a more comprehensive programme will be forthcoming.


[close window] [previous abstract] [next abstract]