NUMBERS OF NON-BREEDING CORMORANTS AND THEIR TIME BUDGET
ACTIVITY AT EUTROPHIC, SUB-MOUNTAIN RESERVOIR IN SOUTHERN
POLAND

Robert Gwiazda

Karol Starmach Institute of Freshwater Biology Polish Academy of Sciences Krakow, POLAND

e-mail: gwiazda@zbw.pan.krakow.pl

Introduction

The number of breeding pairs of Cormorant Phalacrocorax carbo sinensis rapidly increased in almost all European countries during the last decades. It was caused by total protection, increasing of food base by eutrophication and no limits of breeding places. The same process was observed in Poland where breeding population of this species in 1981 was 1465 pairs (Przybysz et al. 1988) and in 1991 - 7300 pairs (Lindell et al. 1995), mainly in the north-east part of Poland. During passages the Cormorant is observed on lakes, reservoirs and ponds in different parts of Poland but before 1990 in southern part of Poland was not often observed (Tomialojc 1990). The aim of study was to determine the occurrence, numbers and time budget of Cormorants at a sub-mountain reservoir.

Study area, material and methods

The Cormorants were counted at the Dobczyce Reservoir. This is a sub-mountain, eutrophic reservoir in southern Poland (49⁰52’N, 20⁰02’E) created in 1986. The surface area of the reservoir is c. 1000 ha and mean depth c. 11 m. Most shores are quite steep and surrounded with meadows and forest. No sandy islands and large flat shores or shallows occur. Roach Rutilus rutilus, Bream Abramis brama, Bleak Alburnus alburnus and Perch Perca fluviatilis were the dominant species of fish. The breeding population of the Cormorant at the eutrophic, lowland Goczalkowice Reservoir was studied. This is the nearest, known breeding colony of the Cormorant, about 120 km west from the Dobczyce Reservoir and the most southern breeding colony of this species in Poland (Przybysz et al. 1997).

Observations were carried out on the Dobczyce Reservoir in 1997-1999. The Cormorant nests at the Goczalkowice Reservoir were counted in May 1998 and 1999. To compare the number of birds during low and high water level the Mann-Whitney U-test was used. The daily activity of Cormorants was estimated using the focal animal sampling method (Altmann 1974). Totally more than 55 hours of observations were made, mainly in October 1997, August 1998 and September 1999. It was assumed that Cormorants sleep and rest at night (12 hours in autumn). The cost of preening was assumed at 1.2 X BMR, and the cost of courtship activity being 1.5 X BMR. Basal metabolic rate (BMR) is considered equal to the cost of resting. I used an assimilation efficiency of 0.8 for piscivorous birds (Castro et al 1988), a calorific value of Cyprinidae fish for 6.2 kJ/g fresh mass (Cummins, Wuycheck 1971) in order to calculate the average daily intake. The size of consumed fish was analysed from 9 pellets.

Results and Discussion

The Cormorants did not breed at the Dobczyce Reservoir but migrating Cormorants use it as a place for foraging and resting. The breeding colony of the Cormorant at the Goczalkowice Reservoir started in 1986 (Dyrcz et al. 1991). The number of pairs was estimated at 102 in 1998 and 64 in 1999. In the first two years after creation of the Dobczyce Reservoir (1986-1987) the Cormorant was not observed at all, and in the next some years (1988-1990) only one or some individuals of this species was observed. In the period 1991-1994 the number of Cormorants was small with a peak during passage in autumn, further increasing since 1995. In spring and summer the average number was 25.2 (maximum 103 indiv., N=8). Larger flocks were observed in autumn with an average number 232.9 (maximum 470 indiv., N=18). The migration peak occurred in the third decade of September and the first decade of October (310 indiv. in 1997, 470 indiv. in 1998 and 224 indiv. in 1999). The number of birds decreased in November and December. Usually the reservoir is frozen at the end of December till February.

The Cormorants preferred the deeper parts of the reservoir. They were observed in a large (120 ha) shallow bay (depth about 3m) only two times. The Cormorants rested usually on flat and shelter areas, close to the deepest part of the reservoir. When water level of reservoir in autumn was low the number of Cormorants in backwater parts of reservoir had been significantly higher than normal (U=56, p=0.02, N=31). At this time short branches of drowned willows and sandbanks were above water level which were used for resting. A decrease of the reservoir’s depth with about 3-4 m gave better possibility for foraging (higher density of fish).

The Cormorant spent about 90% of the time resting (Table 1). Foraging took only c. 0.8% of the daily time (1.7% of the day). The Cormorant staying at reservoir flew very rare and foraged at reservoir near resting places. To acquire 1 kJ of energy (in food) the Cormorant has to expend c. 0.2 kJ energy (cost of diving). The energy costs of the Cormorant per 24 hours was estimated at c. 1400 kJ (Tab. 1). The average daily intake was calculated at 280 g per bird. The Cormorants dived rare at the Dobczyce Reservoir. This fact can be explain by catching a fish of a big size. The fish from pellets were rather big ranging 8-31 cm in body length (mean - 20.9 cm, SD=7.5, N=11).

Acknowledgements

I am grateful to M. Baran for supplying counts of Cormorants at the Dobczyce Reservoir in autumn 1998 and 1999.

References

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Robert Gwiazda, Karol Starmach Institute of Freshwater Biology Polish Academy of Sciences, Krakow, POLAND
e-mail: gwiazda@zbw.pan.krakow.pl