The apparent recent increase of nematode infection in the European Great Cormorant Phalacrocorax carbo sinensis population reported by Staub & Ruhlé (1996a) is a hoax

Werner Suter

Swiss Federal Institute of Technology Zurich, c/o Institute of Forest, Snow and Landscape Research, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland. e-mail: werner.suter@wsl.ch

Recently Staub & Ruhlé (1996a) reported an apparent increase of nematode infection, assumed to be Contracaecum rudolphii (formerly C. spiculigerum), in Great Cormorants Phalacrocorax carbo sinensis shot at their Swiss winter quarters. They presented a curve showing the infection rate to be zero from 1984/85 to 1990/91 and then quickly rising to 100% between 1991/92 and 1995/96. They did not disclose the origin of their data but simply cited their original report which appeared in a Swiss angling magazine (Staub & Ruhlé 1996b), while their wording left the reader with the impression that they had examined all birds themselves. However, 373 (92%) of the 407 stomachs from the period 1984/85-1991/92 were examined by myself (see Suter 1997). In their original article, Staub & Ruhlé (1996b) refer to them as "data set after W. Suter" and claim an infection rate of zero, although I have never given the authors any data on parasite infection, nor have made such data available to them in any other form.

In fact, the absence of nematodes is simply a fiction produced by the authors. Nearly all cormorant stomachs which I examined since 1984/85 contained nematodes, sometimes up to several hundreds. Because it has been known for decades that the Great Cormorant is a final host for Contracaecum, and because my research focused on foraging ecology, I did not assess parasite load in the early study years but rather noted the rare cases of nematode-free stomachs. However, I assessed parasite numbers in all stomachs that were examined since 1988, and some of these were obtained in earlier years. Infection rates are summarised in Table 1: it is clear that nematode parasites were present in practically all Great Cormorants in Switzerland throughout the 1980s. Therefore, Staub & Ruhlé’s (1996a) Figure 1 is wrong, as are the conclusions drawn from it.

Continuously heavy intestinal parasite loads would have been the normal thing to expect, if the authors had consulted the literature. High infection rates of Great Cormorants by nematodes of the genus Contracaecum (usually rudolphii, sometimes species not properly identified) were already found in the course of the first dietary studies (Niethammer 1938, Madsen & Spärck 1950, Van Dobben 1952), and also in the subsequent decades, when few birds were examined because of the general scarcity of Great Cormorants in western Europe (Vojtechsková 1952, Rysavy 1958 cit. Moravec et al. 1988, Van den Broek & Jansen 1964, Reimer 1969, Køie 1988). Moreover, the high incidence of nematode parasitism by Contracaecum and other ascaroids is a typical and world-wide phenomenon in many piscivorous birds, including Great Cormorant populations in north-eastern North America, Africa and Australasia (Johnston & Mawson 1941, Owre 1962, Hartwich 1964, Huizinga 1971, McOrist 1989).

References

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Werner Suter, Swiss Federal Institute of Technology Zurich, c/o Institute of Forest, Snow and Landscape Research, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland. e-mail: werner.suter@wsl.ch