CORMORANTS Phalacrocorax carbo sinensis IN SHALLOW EUTROPHIC FRESHWATER LAKES: PREY CHOICE AND FISH CONSUMPTION IN THE NON-BREEDING PERIOD AND EFFECTS OF LARGE-SCALE FISH REMOVAL

During the period October 1989 April 1992 a study on Cormorant feeding ecology was carried out in two shallow lakes in The Netherlands: lake Veluwemeer and lake Wolderwijd (3240 and 2600 ha respectively). Increasing numbers of Cormorants use these lakes for feeding in the non-breeding season; the maximum number, usually reached in October or November, amounted to 1314 in 1991. The effect of Cormorant predation on fish stock was studied for two reasons: assessing possible damage to commercial fishery and assessing the role of the birds in a large-scale biological management programme that has been carried out in lake Wolderwijd during the period of study. As part of this programme, fish stock in this lake was reduced from 203 to 46 kg/ha. Most important prey species was Ruffe (60% offish-mass in 1991/92), which has no value to either commercial fishermen or anglers. Perch, Pikeperch, Roach and Smelt were found in most samples as well, while Eel, the only species commercially caught in the lakes, was hardly found at all. Mean daily intakes ranged from 146 to 699 g with highest values in October and March. In 1989/90 total consumption in the two lakes was estimated at 3.7 kg/ha. In September 1991 March 1992 however, after fish removal, 80% went to lake Wolderwijd, where smaller fish were taken. Nevertheless, total consumption amounted to 12.5 kg/ha, against 2.1 kg/ha in lake Veluwemeer. As most of the fish consumed belongs to species potentially hazardous to water quality (transparency) the Cormorants seem to support biological management. It is discussed whether the larger proportion of small, shoaling fish after removal of large fish could have enhanced possibilities for mass flock fishing, attracting the birds to lake Wolderwijd. On the other hand, achieving the long-term goals of biological management is likely to reduce the fishing possibilities for Cormorants. lBureau Waardenburg bv, P.O. Box 365, NL-4100 AI Culemborg, The Netherlands. 2Ministry of Transport and Public Works, Institute for Inland Water Management and Waste Water Treatment (RIZA), P.O. Box 17, NL8200 AA Lelystad, The Netherlands.