PLATTEEUW M & BEEKMAN JH (1994) Disturbance of waterbirds by ships on lakes Ketelmeer and IJsselmeer. LIMOSA 67 (1): 27-33.

During two consecutive days in winter 1990/91, observations were made on disturbance of waterbirds by ships (Fig. I) at lake Ketelmeer. These ship-based observations only involved the determination of the specific critical disturbance distance, defined as the distance within which at least 20% of a certain species exhibits abnormal behaviour, e.g. alertness, swimming or diving away from the approaching ship or flying off. Many waterbirds may be found resting close to the polderdikes bordering the lakes. Since a shipping lane is planned for lake Ketelmeer at 440-560 m from the SW dike, the distances of the resting Aythya ducks from the dikes have been recorded as well. Of the two piscivorous species, Great Crested Grebe and Goosander, critical disturbance distances were estimated at 300 and well over 300 m respectively (Tab. 1,2). Two diurnal benthos (mainly Zebra Mussel Dreissena polymorpha) feeders, Coot and Goldeneye, showed markedly different reactions. While the first species hardly reacted at all to the approach or passing of the ship, the latter flew off at distances of up to 500 m or more. The critical distances were estimated at less than 100 m for the Coot and at possibly 1000 m for the Goldeneye (Tab. I, 2). The three nocturnal musselfeeders Pochard, Scaup and Tufted Duck were disturbed at distances of 300, little over 400 and well over 400 m respectively (Tab. 1, 2). It is argued that disturbance of feeding birds may have a more markedly negative impact on the individual energy budgets of the birds than disturbance of resting birds, since both the loss of feeding time and the extra energy expenditure must be compensated for. A diurnal forager as vulnerable to disturbance as the Goldeneye may well be limited in its possibilities to exploit lake Ketelmeer's mussel resources by the frequent passing of ships at distances below its critical range. On the other hand, the confident Coot can exploit any musselbank. The feeding pressure of all musselfeeders on the local zebra mussel stock seems to be so severe, that any increase in intra- or interspecific interference, due to the need of compensate for extra energy losses, may lead to a lower rather than a higher food intake. Since both Pochard and Tufted Duck may be resting at distances of up to 300 m from the nearest polder dike and the planned shipping route will be at 440-560 mfrom the southwestern dike (offering shelter from the prevailing winds), many ships will pass at distances well below their critical disturbance distance. However, the concentration of all ship movements within a confined area leads to more predictable ship activity (important for habituation) and to less pressure on other zones within the lake. Therefore, it is considered unlikely that the lake will be abandoned by resting Aythya ducks as a consequence of the projected shipping route. Great Crested Grebe Podiceps cristatus Goosander Mergus merganser Eurasian Coot Fulica atra Common Goldeneye Bucephala clangula Tufted Duck Aythya fuligula Common Pochard Aythya ferina Greater Scaup Aythya marila

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