8.12.2000

Trends in Finland's cormorant population

Pekka Rusanen, Markku Mikkola-Roos and Timo Asanti
Finnish Environment Institute, PO Box 140, 00251 Helsinki, Finland

The range of the cormorant (Phalacrocorax carbo sinensis) around the Baltic Sea has been rapidly spreading northwards since the early 1980s. The first signs of this phenomenon in Finland were a sudden increase in the numbers of cormorants on migration passing over or resting around the Gulf of Finland during the early 1990s. This was due to the establishment and growth of two cormorant colonies in Russia on islands in the Gulfjust a few kilometres from the Finnish border. By 1995 a total of over 1,000 pairs were nesting in these colonies, and the number has continued to rise steeply ever since.

Finland's breeding population small but burgeoning

The cormorant only bred for the first time in Finland in 1996, but at the moment the breeding population is growing rapidly. Based on experiences in other parts of the Baltic, this increase can be expected to continue for at least the next 10-15 years. By the summer of 2000 the breeding population had grown to 336 pairs, double the previous year's figure, with birds nesting in four colonies, two of which were new.

The Finnish Environment Institute's research project

In spring 1998, the Finnish Environment Institute's Nature and Land Use Division set up a field research project to monitor cormorant breeding sites. This research examines factors such as the effect of the cormorants on the local vegetation and other nesting birds, as well as the cormorants' own nutrition, feeding areas and breeding success. Each summer most of the fledglings are ringed.

So far the environmental impact of the cormorant colonies on their surroundings has been limited. In Finland, cormorants nest in the outer archipelago, on treeless skerries where there are no threatened plant species. No adverse effects on other breeding birds have been observed as yet, but even the country's largest cormorant colony still only contained 229 pairs in the summer of 2000.

During the breeding season 67% of the food remnants left by adult birds consisted of roach (Rutilus rutilus) and 24% of perch (Perca fluviatilis), and there were no traces of salmon or trout, for instance. The fish caught by the cormorants were most typically 18-23 cm long (less than 150 g).

Research to continue

It is important to continue with the annual monitoring of trends in Finland's cormorant population as it is currently at a very dynamic stage in its growth. More comprehensive data on the cormorants' diet during the breeding season is also required, as with the population growing so rapidly, this factor is central to discussions on the status of the species in Finland. The impacts on other birds and plants at breeding sites should also be monitored further, as there is little comparable data available from other parts of the birds' Baltic range. The cormorant is currently a protected species in Finland all year round.