EXTENSION OF COLOUR RINGING PROGRAMME IN GREAT CORMORANTS Phalacrocorax carbo sinensis
IN THE NETHERLANDS

Stef van Rijn & Menno Zijlstra

Rijkswaterstaat Riza, P.O. Box 17, NL-8200 AA Lelystad, the Netherlands.

It appears that in the western range of the continental European Cormorant population numbers are still levelling off after the spectacular increase in the seventies and eighties (Van Eerden et al. 1995). In the Netherlands, the stronghold of a mere 70% of the population is settled in the IJsselmeer area. In 1998 about 12,000 nests were counted in 4 colonies in the area. After the maximum numbers of about 15-16 thousand breeding pairs in the period 1990-93 and the crash in number in 1994 (Van Eerden & Zijlstra 1994 (CRG, Volume 1) the number has now stabilised to 10-12 thousand pairs in the period 1995-98. The factors which are responsible for this process are subject of study. Recent observations showed a decline in breeding success and a reduction of the number of successful nests every year. In 1995 breeding success was relatively high and in 1998, success was extremely poor. A bigger proportion of nest failures were observed in the seasons of 1997 and 1998 and high mortality rates were recorded in 1996 and 1998. The mortality in 1996 was partly explained by strong conditions of wind (Van Rijn & Platteeuw 1996) and the lower mortality in 1997 could probably be explained because of a reduction of intraspecific competition due to prolonged breeding which lead to a less peaked predation pressure on the fish population (Van Rijn 1998). In this context the very poor success which was recorded in 1998 could possibly be explained by the peaked numbers of breeders in the colonies which resulted in a higher competitive level on the fishing grounds.

It is a reasonable assumption that an increase of the number of breeding pairs is supported by the survival of the surplus of new recruits in the population. The source sink thought makes it acceptable that a radiation of birds in high productive colonies can be expected (source). The output of birds in the IJsselmeer area is, considered to the low breeding success, possibly to low to carry the current population level (sink). If the central question of how and when carrying capacity of the IJsselmeer area is reached has to be solved, exchange of (surplus)birds has to be visible. Colour ringing is a way to achieve this.

The latest change in the Cormorant population is observed in the Waddensea area, situated in the north of the Netherlands. Since 1994 a ground breeding colony is settled on the island Vlieland in the western part of the area. An older colony, named ‘De Hond’ is situated in the eastern site of the area on an artificial island, a small derrick for checking gas and oil resources, where numbers increase especially in the last years. The surface of this location is limited for further growth of numbers of breeding pairs. On the island of Vlieland the numbers of breeding pairs are still increasing since the origin in 1994. At the moment the breeding population of these two groups can be estimated at about 500-600 pairs.

Since 1997 a colour ringing programme has been started in this two colonies. Remarkable differences between breeding success in the population of the IJsselmeer area and the Waddensea area are observed (tabel 1). The first visits to the colony of Vlieland in 1997 and 1998 gave signs of a healthy and viable situation. Survival rates of young are very high in connection with the birds of the IJsselmeer area and condition of chicks on the basis of measurements of biometrics point to a well fed recruitment.

The high breeding success of the Vlieland colony resulted in a relatively high output of recruits in comparison with the ones in the colony of Oostvaardersplassen. For example in the season of 1998 an output (420 nests multiplied with a success of 3.09 young per couple) of 1,298 can be calculated while for Oostvaardersplassen an output (5.225 nests and a success of 0.64) of 3,344 is demonstrated. It can be concluded that in despite of the much lower number of breeding pairs on Vlieland an output of 28% of both colonies accounts for this colony.

In the early nineties some changes appeared in the Haringvliet area, the dammed estuary of the rivers Maas and Waal, where in 1990 a new colony was settled on an artificial shore on the former banks called Ventjagersplaten in the eastern part of the area. In an older colony, situated in a seemingly natural area, called Biesbosch, numbers of breeding pairs especially increased from the mid-eighties. This area is located more to the east but can also be described as a population which is part of the one in the Haringvliet area (Boudewijn & Dirksen). In this two colonies since 1992 a colour ringing programme is carried out and at the moment a stable population of 400-500 pairs is been recorded.

This means that in three important wetlands, distributed all over the country, colour ringing gives us some grip to understand changes in the trends and dynamics of the Cormorant population in the Netherlands. The study areas harbour at least in 75% of the total Dutch population. In five colonies, an annual 400 nestling are being colour ringed. The map, given below, shows the locations of the colonies were a colour ringing project is being performed.

Figure 1. Position of the colonies where nestlings are colour ringed in the Netherlands and the topographical names of the study areas.

In the colony of Oostvaardersplassen birds are colour ringed since 1983. Till 1995 the birds got a white colour ring on the left leg or a yellow ring on the right leg. The inscription on the colour ring is in black with a letter and number, a number and letter or a letter and letter. Sometimes there is a bar (vertical line) between the inscription. For example: AA, AB, 2A, B4, A|A, A|2 etc. are used. You read the code upwards the ring. After 1995, red rings on the left leg with a white inscription are being used. In the 4 other colonies (Biesbosch and Ventjagersplaten, in the Haringvliet area since 1992 and at Vlieland and ‘the Hond’ in the Waddensea area since 1997) birds are being ringed with green colour rings with white inscriptions and the same type of inscription as the used in Oostvaardersplassen. On the other leg, all birds get a metal ring from the Dutch ringing centre.

The plan is to continue the project for at least 3 years. The exchange of birds between populations which can be calculated on the base of identifications of colour ringed birds are used in the stage of writing a final report in which it is intended to make a population model for the study area’s.

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., K. Koffijberg & M. Platteeuw, 1995. Riding on the crest of the wave: possibilities and limitations for a thriving population of migratory Cormorants Phalacrocorax carbo in man-dominated wetlands. Ardea 83(1) 1995 (p. 1-9).

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, September 1996.

Van Rijn, S. 1998. Unusually prolonged breeding in the Great Cormorant Phalacrocorax carbo sinensis population in the IJsselmeer area in the Netherlands in 1997. Cormorant Research Group Bulletin, No. 3, December 1998.

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. Rijkswaterstaat, Directorate Flevoland, Lelystad, pp 53-60.

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