GREAT CORMORANTS Phalacrocorax carbo ON FISH PONDS IN SLOVENIA

Milan Vogrin

After the increase of the Great Cormorant Phalacrocorax carbo, and particularly so after 1990, fishermen reported great damages caused by fish-eating birds (Great Cormorant and Grey Heron Ardea cinerea) in some fish ponds in Slovenia. This problem is particularly outspoken in the northeastern part of the country, where both fish ponds and Great Cormorants are most abundant within Slovenia.

Now, the conflict seems to be reaching its summit. Fishermen demand compensation for the losses inflicted by fish-eating birds, mainly Great Cormorants, either from the Slovenian government or from other countries from where Great Cormorants visit Slovenian fish ponds. At the moment, the Great Cormorant enjoys full protection by national law in Slovenia and, consequently, it is not allowed to shoot them, neither on fish ponds or anywhere else. Conservationists in Slovenia are now all trying to find effective ways of preventing Great Cormorants from visiting fish ponds, while not interfering negatively with the presence of other waterbirds breeding or roosting there (e.g. grebes, ducks, coots).

In 1995 the Institute for the Conservation of Natural Heritage of Slovenia (Ministry for Environment) decided to have some field research carried out on fish-eating birds (Great Cormorant and Grey Heron) on some fish ponds in northeastern Slovenia. Five of the most problematical fish ponds in this part of the country: Pernica, Race, Pozeg, Vrbje and Zovnek. Research work lasted for only three months, from December to March. For the moment, the field work has been limited to numerical surveys of both species at each of the five fish ponds. Great Cormorants turned out to be most abundant on the fish ponds of Race and Pozeg on Dravsko polje (Drava field) near the river Drava. In the research report some suggestions for fishermen were included on how to dissuade Great Cormorants and Grey Herons from visiting fish ponds (Vogrin & Sorgo 1995). The problem seems to be the gravest on three of the ponds (Pernica, Race and Pozeg), included in two Landscape Parks. It is also from these three ponds that fishermen have reported the greatest damage due to fish-eating birds in all of the country. The suggestion was made to carry out further investigation into the problem, with special emphasis on feeding ecology, habits near fish ponds and migration routes between rivers and fish ponds.

Fig. 1. Average numbers of Great Cormorants per year on fish pond Race.

8Fig. 2. Average number of Great Cormorants per month on fish pond Race (1983-1994).

A previous study on Great Cormorant numbers (Vogrin, unpubl.) reveals an increase in numbers over the years (Table 1, Fig. 1) as well as the phenology of appearance of the birds per month (Table 2, Fig. 2) at the fish pond of Race. These surveys have been carried for a period of twelve years (1983-1994), of which 1991 turned out to be the year with the highest numbers (Fig. 1). During 26 days out of 50 a total of 986 individuals was seen throughout this year. In 1983, 1985 and 1986, on the other hand, only six cormorants were seen in ten field days. Peak numbers occurred in November during autumn migration, with a pronounced maximum average of 26.3 birds over the entire twelve-year period (Fig. 2). The largest flock ever recorded on Race consisted of 200 individuals. During the winter months Great Cormorants are absent or very scarce at Race (Table 2, Fig. 2).

References

Vogrin M. & Sorgo A. 1995. Siva caplja Ardea cinerea in veliki kormoran Phalacrocorax carbo na ribnikih za vzrejo toplovodnih vrst rib Pernica, Race, Pozeg, Vrbje in Zovnek. Drustvo za opazovanje in proucevanje ptic Slovenije.

Vogrin M. unpubl. Great Cormorant Phalacrocorax carbo on fish pond Race (Slovenia).

Milan Vogrin, Hotinjska cesta 108, 62312 Orehova vas, Slovenia