Introduction In Estonia, the first colony of great cormorant Phalacrocorax carbo sinensis (L.), newcomer to Estonian fauna, was established in By 2005 already 20 colonies (including Käina Bay, Fig 1) with around nesting pairs were established. Small estuarine fish are often an important source of nutrition for sea birds. To the extent vibrant bird colonies are seen as desirable, it may be necessary to leave the estuarine fish as feed for these colonies. But if the estuarine fish would otherwise be a source of recruitment for commercial fish stocks, we have a conflict between two use values, namely how does the forgone commercial catch measure up against the benefits of having a sufficiently vibrant bird colony? In this study we argue that expanding bird colonies could play a role similar to an overexpanding fishing fleet and decimate the fish stocks on which they feed, to the detriment of the commercial fishery. We report on a study of the effects of the great cormorant in the brackish waters of Estonia. Due to the low salinity, many freshwater species are abundant and have high commercial importance in the north-eastern Baltic Sea. Results and conclusions Gillnet sampling indicated that the abundance (CPUE) of roach, most abundant spawning fish species in Käina Bay in 1995, declined more than 100 times in 1995 – 2005 (Table 2). The number of spawning perch, second most important species, decreased ten-fold from 1995 to 2005 (Table 2). At the same time, abundance of ide (too big to be predated by cormorants) remained roughly the same (Table 2). Since coastal monitoring using standardized methods has been carried out in Väinameri since 1993 every year, there is good background data reflecting the dynamics of fish stocks in this area in general. Comparison of 1995 and 2005 data shows that the abundance of roach and perch in Sarve and Saarnaki has declined around five and two times, respectively (Table 3), which is much less than the dramatic change of more than 100 times (roach) and 5-10 times (perch) in Käina Bay. The decrease of fish also in the Väinameri could be related to the predation by cormorants in the most important spawning ground, the Käina Bay. In the Matsalu reference area ca 40 km from Käina Bay (too far for the cormorants) the differences in CPUE of perch and roach between 1995 and 2005 were rather small. Fishing effort has steadily and significantly decreased in Väinameri during last 15 years. There are no sources of pollution nearby. So, the drastic decline in fish populations of the Käina Bay and in permanent coastal fish monitoring areas in the archipelago sea nearby, indicate that the observed decrease in fish abundance is most likely related to the establishment and explosive increase of the cormorant colony. Daily consumption of cormorants was analysed on the base of pellets and related to the abundance of fish. Our data demonstrates that the invasion of cormorants, which could be encouraged by the existence of nature protection areas like Käina Bay, is especially dangerous in spatially restricted spawning grounds, which have high density of fish during the reproduction period. The establishment of new cormorant colonies could seriously damage the normal functioning of historically important spawning grounds, preventing such areas from providing recruitment to adjacent fisheries. At the same time, the possibility that a new natural predator could impact coastal fish stocks in a way comparable to overfishing, is not always easily acceptable to the environmental administration, which has been the major reason why there have been relatively slow steps to limit the quick population growth of cormorants in Europe. Impact of newly established cormorant colony to fish of the shallow Käina Bay, eastern Baltic Sea Markus Vetemaa 1, Redik Eschbaum 1, Toomas Saat 1 & Rögnvaldur Hannesson 2 1 Estonian Marine Institute, University of Tartu, Vanemuise 46, Tartu, Estonia 2 Norwegian School of Economics and Business Administration, Helleveien 30, 5045 Bergen, Norway. Table 2. Comparative CPUE of fish in the Käina Bay in 1995 and 2005 during five periods in spring/summer. Each row represents the average value of CPUE of two fishing stations. Table 1. Number of cormorants in Käina Bay YearNumber of nests Number of birds ~ ~ ~ ~ ~ ~ ~ no data~ ~ ~5700 Table 3. Comparative CPUE of fish in regular monitoring areas in Väinameri. Study area Väinameri (Moonsund) is a shallow (average depth ca 5 m) archipelago area in Western Estonia. Dominating species are percids and cyprinids, while only two abundant commercially important marine species, herring and garpike, enter the area for spawning in spring. Käina Bay (≈ 9 km2) is situated in the northern Väinameri (see figure). It is a very shallow sea area (average depth < 1 m) almost totally enclosed by the Hiiumaa and Kassari islands. Water exchange with the rest of Väinameri is possible only through two channels in west and east. The bay has several islets. Käina bay with its surroundings is nature protection area, a bird sanctuary. Both commercial and recreational fishing is prohibited. It is (or to be more exact, was...) most important spawning area of freshwater species such as roach, perch, ide, ruffe and crucian carp. In 1995 the state of the fish populations in Käina Bay was much better than in most of other areas of the Väinameri, which was related to its protection status. Material and Methods Fish were sampled with gill nets in Käina Bay in 1995 and Standardised coastal fish monitoring routines for the coastal areas of the northern Baltic Sea were followed. Seven nets (mesh sizes 17, 21.5, 25, 30, 33, 38 and 45 mm knot to knot) were used together in line. The nets were set into the deepest area ( m) of the bay, between Ristlaid and Männaklaid islets in both study years. The nets covered all the water column from bottom to surface; they were set 16:00 – 19:00 p.m. and lifted 8:00 – 11:00 a.m. the following day. Catch per unit of effort (CPUE) data was calculated as the total number of fish caught per station (i.e. 7 nets). For comparison, CPUE data from regular fish monitoring areas in the Väinameri was used, collected from 3 monitoring areas: Sarve (15 km north-east from the Käina Bay), Saarnaki (12 km east) and Matsalu (40 km south-east. The number of breeding pairs of cormorants in the Käina Bay was determined by counting nests and using the correction factor suggested by the relevant literature.