INCIDENCE OF NEMATODE INFECTION OF
EUROPEAN GREAT CORMORANT Phalacrocorax carbo sinensis POPULATION
APPARENTLY ON THE INCREASE

E. Staub & C. Ruhlé

Stomach examinations of Great Cormorants Phalacrocorax carbo sinensis shot at their Swiss winter quarters throughout the winters of 1984/85 until 1990/91 never revealed the presence of nematodes. However, from the next winter onwards an ever increasing proportion of stomachs examined showed infection by nematodes of up to over 3 cm length, until from 1993/94 onwards virtually all stomachs examined were infected (Fig. 1; Staub & Ruhlé 1996). It would thus seem unlikely that earlier infections of any significant magnitude might have remained undetected. In 1994/95 all nematodes found were indentified as Contracaecum spiculigerum (syn. Contracaecum rudolphii) by Veit et al. (1995), a species that uses cormorants as its main host and fish as intermediate hosts. The fact that birds shot early in winter were already infected indicates that the parasites were probably taken up with their food in the breeding areas, for Swiss wintering birds mainly Denmark and The Netherlands (Reymond & Zuchuat 1995). For the moment it remains unknown whether this recently acquired high percentage of nematode infection among European Great Cormorants in any way affects negatively on either the birds or on the fish populations of their breeding, staging or wintering areas. However, in view of the apparently extremely high incidence of infection, further investigation would certainly be desirable.

Fig. 1. The increase of the incidence of infection of Swiss wintering Great Cormorants by nematodes

References

Reymond A. & Zuchuat O. 1995. Axial migration routes in Cormorants Phalacrocorax carbo passing through or wintering in Switzerland. Ardea 83: 275-280.

Staub E. & Ruhlé C. 1996. Ein Nematode parasitiert neuerdings in den Mägen der europäischen Kormoranpopulation. Schweiz. Fischereiwiss. 13: 1-2.

Veit H., Wunner U. & Wismath P. 1995. Kormoranschäden an der Traun. Fischer & Teichwirt 46: 335-337.

E. Staub & C. Ruhlé, BUWAL, Fisheries Section, Hallwylstrasse 4, CH-3003 Bern, Switzerland

Note from editors: The information presented in this paper suggests that nematode infection among Dutch and Danish breeding Great Cormorants would be a relatively recent phenomenon. However, dissection data from Dutch birds, mostly drowned after entanglement in fishermen's gear in lake IJsselmeer in the period 1980-1990, reveal a proportion of 50% nematode infection among a sample of 46 birds. In the oesophagi and stomachs of these birds a range of one to 180 nematodes was found, with an average of 33.9 nematodes per bird. In spite of the small sample size these data at least show that the incidence of nematode infection among this population may not be as recent as suggested by Staub & Ruhlé (1996), although it certainly may have increased during the 1990s. M.R. van Eerden, K. Koffijberg & M. Platteeuw