PLATTEEUW M, BEEKMAN JH, BOUDEWIJN TJ & MARTEIJN CL (1992) Cormorants Phaiacrocorax carbo in Lake Ketelmeer outside the breeding season: numbers, food choice and food availability. LIMOSA 65 (3): 93-102.

As a consequence of the sharp increase in the NW-European breeding population of Cormorants, the numbers wintering in Dutch freshwater lakes have increased as well. As part of an ecosystem analysis of lake Ketelmeer (fig. 1) their numbers, diet and total food consumption were estimated and compared to the estimated biomass of fish available. From counts on nearby winter night roosts (fig. 2) it was estimated that in the winters (October- March) of 1989/90 and 1990/9166 000 and 114 100 bird-days respectively were spent foraging in the lake. Fish densities in lake Ketelmeer reached a peak in March (fig. 4). Most abundant species were bream, smelt, roach, ruffe and perch. Regurgitate pellets were analysed from two roosts, showing that on fresh weight basis roach, ruffe, perch and (only one site) pikeperch were the main prey species (fig. 5). Bream, even the smaller individuals, were hardly caught, but Cormorants caught more roach than expected. Other species were caught more or less in the same proportion as available (tab. 3). Assuming that each pellet contained fish remains representing the individual daily food intake, it was estimated that total winter fish consumption amounted to c. 23.6 and 40.9 tonnes fresh weight (6.2 and 10.8 kg/ ha) respectively in 1989/90 and 1990/91. This consumption was 8% higher than would be expected from a daily energy expenditure (DEE) of 3x Basal Metabolic Rate (BMR) as proposed for the summer situation. This is probably due to a higher energetic need caused by low water and air temperatures. Most potential prey species are still so abundant that predation pressure (about 11%of the standing stock) does not seem to preclude a further increase in numbers (tab. 5). However, turbid waters hamper the visual detectability of fish, and even in very clear water systems as lake Grevelingen only a maximum of 28-36% of the standing stock can be utilized by birds. In really turbid waters like lake Markermeer the maximum predation pressure by piscivorous birds (5-10% ofthe standing stock) is much lower. Considering that Cormorants and other piscivorous birds wintering at lake Ketelmeer (grebes and mergansers) will take up to 14% of the available fish stock and that lake Ketelmeer is more turbid than lake Grevelingen, it seems likely that maximum carrying capacity for wintering Cormorants is about to be reached. Great Cormorant Phalacrocorax carbo

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