CORMORANTS PHALACROCORAX-CARBO-SINENSIS IN SHALLOW EUTROPHIC FRESH-WATER 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 feedi ng ecology was carried out in two shallow lakes in The Netherlands: la ke Veluwemeer and lake Woldenwijd (3240 and 2600 ha respectively). Inc reasing numbers of Cormorants use these lakes for feeding in the non-b reeding season; the maximum number, usually reached in October or Nove mber, amounted to 1314 in 1991. The effect of Cormorant predation on f ish stock was studied for two reasons: assessing possible damage to co mmercial fishery and assessing the role of the birds in a large-scale biological management programme that has been carried out in lake Wold envijd during the period of study. As part of this programme, fish sto ck in this lake was reduced from 203 to 46 kg/ha. Most important prey species was Ruffe (60% of fish-mass in 1991/92), which has no value to either commercial fishermen or anglers. Perch, Pikeperch, Roach and S melt were found in most samples as well, while Eel, the only species c ommercially caught in the lakes, was hardly found at all. Mean daily i ntakes ranged from 146 to 699 g with highest values in October and Mar ch. 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 potentiall y hazardous to water quality (transparency) the Cormorants seem to sup port biological management. It is discussed whether the larger proport ion of small, shoaling fish after removal of large fish could have enh anced possibilities for mass flock fishing, attracting the birds to la ke Wolderwijd. On the other hand, achieving the long-term goals of bio logical management is likely to reduce the fishing possibilities for C ormorants.