UNUSUALLY PROLONGED BREEDING IN THE GREAT CORMORANT Phalacrocorax carbo sinensis POPULATION IN THE IJSSELMEER AREA IN THE NETHERLANDS IN 1997

Stef van Rijn

Institute for Inland Water Management and Wastewater Treatment RIZA, P.O. Box 17, NL-8200 AA Lelystad, The Netherlands

Bureau Waardenburg b.v., P.O. Box 365, NL-4100 AJ Culemborg, The Netherlands

In the colony of Oostvaardersplassen, at the moment one of the largest colonies in western Europe, a colour ringing programme has been carried out since 1983 (Van Eerden & Zijlstra 1988). Every year 150-275 young are being ringed in order to provide enough data about the population dynamics of the subspecies in the study area.

In 1997 around 11,000 nests were counted over four colonies in the IJsselmeer area. This is less than last year when more than 12,000 nests were recorded. It would seem that some stabilising factors are working on the population. It is not easy to investigate the exact relationship between breeding success, weather, the attainable fish stock and age distribution of the breeding population. However, at least part of the changes in breeding success are known to be the result of changes in the availability of food caused by the lower transparency of the water column due to wind (Van Eerden & Zijlstra 1991, Van Eerden & Voslamber 1995). Another factor is the breeding density, also known to be a potentially negative factor for reproductive success (Van Eerden & Zijlstra 1991, Bregnballe 1996).

Data from the breeding area are confined to Oostvaardersplassen in the breeding season of 1997, where about 160 individual birds were recognised. The plastic colour-rings, engraved with two letter codes, are readable within a distance of 700 m if allowed by weather conditions. Colour-ring reading in the colony took place from the second week of March till the beginning of September. Throughout this entire period the colony was being visited at least during three afternoons each week by at least one or two observers. Ringing of young at the nests has been taking place at the same locations each year and starts at the moment that quite a big number of birds have reached the age of 30 days. On average the first chick ringing visit occurs in the first week of June. However in 1997 the first ringing session had to be planned as early as the third week of May, which has only happened before in 1991. However, a lot of nests remained unoccupied in this early and quite a few breeding attempts were unsuccessful.

After the very cold and dry period from December 1996 to January 1997 with recorded temperatures of at least -15 ºC, the winter ended with a very soft but windy month of February and still a lot of ice covering the cormorants’ foraging range in the IJsselmeer area. March and April showed many days with strong northerly to westerly winds. In May and June quite a lot of rainfall was recorded. Periods with relatively good weather conditions were rather short. These relatively short spans of favourable circumstances may very well have been one of the causes of an exceptionally prolonged breeding season among the Great Cormorants in 1997.

In August there were still a lot of nests where young were being raised. Many pairs must have put off the egg laying and tried again when weather condition allowed them to. In the plots where chicks had to be colour-ringed there were still dozens of unringed young up till as late as July. By the end of that month it was still possible to have a last ringing session to finish the plastic rings prepared for the year. Thus, 1997 was the year in which chicks have been ringed over the longest period of time ever since the onset of the ringing programme in 1983 (Fig. 1).

Figure 1. Number of days between the first visit and the last visit of ringing chicks of Cormorants Phalacrocorax carbo sinensis in the colony of Oostvaardersplassen in the period 1983-1997.

The consequence of the prolonged breeding season of the birds caused difficulties in determining breeding success. In former seasons breeding success was recorded by counting young in the nests which could be planned in one day. In an average year this took place in the period just after the ringing of the chicks, when young were visible in the majority of the nests. In mid-June, when the first young had fledged, a lot of nests still contained eggs or little chicks. The breeding success, as recorded in the traditional way, provided a large underestimation as compared to real values. Proportions of successful nests throughout the season had to be considered in order to estimate a more realistic breeding success.

This way, breeding success in the colony of Oostvaardersplassen was estimated at 1.2 young per nest. In the other three IJsselmeer colonies the re-calculated breeding success varied between 0.7 and 1.2 young per nest. On average an output of about 11,000 young can be calculated. A part of this number usually dies just before or after fledging. The year 1997 can be described as a year with very low chick mortality, in contrast to last year (1996) when at least 20% was found dead before fledging and only another 20% was seen alive afterwards (van Rijn & Platteeuw 1996). In 1997 at least 68% has been seen alive just after fledging and only about 5% of the ringed birds were found dead before.

Figure 2. Fate of Great Cormorant Phalacrocorax carbo sinensis chicks ringed at Oostvaardersplassen in 1997 (n= 250).

It is tentatively suggested that the prolonged breeding season of Great Cormorants of the IJsselmeer population in 1997 may have been an important factor for the higher survival of chicks. Because of a reduction of intraspecific competition, and thereby a more evenly distributed predation pressure on the fish resources, during the period of fledging of young, food conditions for recent fledglings may have been more favourable than usual during the period of starting their independent life.

It will be very interesting to monitor the effect of the prolonged breeding season of 1997 on long term survival of young from spring and summer 1997.

References

Bregnballe, T. 1996. Udviklingen i bestanden af Mellemskarv i Nord- og Mellemeuropa 1960-1995. Dansk Or. Foren. Tidsskr. 90: 15-20.

Van Eerden, M.R. & M. Zijlstra 1988. Aalscholvers Phalacrocorax carbo met kleurringen uit de Oostvaardersplassen. Limosa 61: 57-60.

Van Eerden, M. R. & B. Voslamber 1995. Mass fishing by Cormorants Phalacrocorax carbo sinensis at lake IJsselmeer, The Netherlands: a recent and successful adaptation to a turbid environment. Ardea 83: 199-212.

Van Rijn, S. & M. Platteeuw 1996. Remarkable fledgling mortality at the largest Great Cormorant Phalacrocorax carbo sinensis colony in the Netherlands. Cormorant Research Group Bulletin, No. 2: 30-35.

Zijlstra, M. & M.R. Van Eerden 1991. Development of Cormorants Phalacrocorax carbo in the Netherlands till 1989. In: M.R. van Eerden & M. Zijlstra (eds.): Proceedings workshop 1989 on Cormorants Phalacrocorax carbo: 53-60. Rijkswaterstaat Directorate Flevoland, Lelystad.

Stef van Rijn, Rijkswaterstaat RIZA, P.O. Box 17, NL-8200 AA Lelystad, The